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Message-Id: <20180809052328.27942-5-me@tobin.cc>
Date: Thu, 9 Aug 2018 15:23:28 +1000
From: "Tobin C. Harding" <me@...in.cc>
To: Daniel Borkmann <daniel@...earbox.net>,
Alexei Starovoitov <ast@...nel.org>
Cc: "Tobin C. Harding" <me@...in.cc>, Jonathan Corbet <corbet@....net>,
"David S. Miller" <davem@...emloft.net>,
Kees Cook <keescook@...omium.org>,
Andy Lutomirski <luto@...capital.net>,
Will Drewry <wad@...omium.org>, linux-doc@...r.kernel.org,
netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: [PATCH bpf-next 4/4] docs: Remove filter.txt from the tree
We just split up the content of filter.txt into three RST files. We can
remove filter.txt now.
Remove filter.txt
Signed-off-by: Tobin C. Harding <me@...in.cc>
---
Documentation/networking/filter.txt | 1480 ---------------------------
1 file changed, 1480 deletions(-)
delete mode 100644 Documentation/networking/filter.txt
diff --git a/Documentation/networking/filter.txt b/Documentation/networking/filter.txt
deleted file mode 100644
index 1fe4adf9c4c6..000000000000
--- a/Documentation/networking/filter.txt
+++ /dev/null
@@ -1,1480 +0,0 @@
-Linux Socket Filtering aka Berkeley Packet Filter (BPF)
-=======================================================
-
-Introduction
-------------
-
-Linux Socket Filtering (LSF) is derived from the Berkeley Packet Filter.
-Though there are some distinct differences between the BSD and Linux
-Kernel filtering, but when we speak of BPF or LSF in Linux context, we
-mean the very same mechanism of filtering in the Linux kernel.
-
-BPF allows a user-space program to attach a filter onto any socket and
-allow or disallow certain types of data to come through the socket. LSF
-follows exactly the same filter code structure as BSD's BPF, so referring
-to the BSD bpf.4 manpage is very helpful in creating filters.
-
-On Linux, BPF is much simpler than on BSD. One does not have to worry
-about devices or anything like that. You simply create your filter code,
-send it to the kernel via the SO_ATTACH_FILTER option and if your filter
-code passes the kernel check on it, you then immediately begin filtering
-data on that socket.
-
-You can also detach filters from your socket via the SO_DETACH_FILTER
-option. This will probably not be used much since when you close a socket
-that has a filter on it the filter is automagically removed. The other
-less common case may be adding a different filter on the same socket where
-you had another filter that is still running: the kernel takes care of
-removing the old one and placing your new one in its place, assuming your
-filter has passed the checks, otherwise if it fails the old filter will
-remain on that socket.
-
-SO_LOCK_FILTER option allows locking of the filter attached to a socket.
-Once set, a filter cannot be removed or changed. This allows one process to
-setup a socket, attach a filter, lock it then drop privileges and be
-assured that the filter will be kept until the socket is closed.
-
-The biggest user of this construct might be libpcap. Issuing a high-level
-filter command like `tcpdump -i em1 port 22` passes through the libpcap
-internal compiler that generates a structure that can eventually be loaded
-via SO_ATTACH_FILTER to the kernel. `tcpdump -i em1 port 22 -ddd`
-displays what is being placed into this structure.
-
-Although we were only speaking about sockets here, BPF in Linux is used
-in many more places. There's xt_bpf for netfilter, cls_bpf in the kernel
-qdisc layer, SECCOMP-BPF (SECure COMPuting [1]), and lots of other places
-such as team driver, PTP code, etc where BPF is being used.
-
- [1] Documentation/userspace-api/seccomp_filter.rst
-
-Original BPF paper:
-
-Steven McCanne and Van Jacobson. 1993. The BSD packet filter: a new
-architecture for user-level packet capture. In Proceedings of the
-USENIX Winter 1993 Conference Proceedings on USENIX Winter 1993
-Conference Proceedings (USENIX'93). USENIX Association, Berkeley,
-CA, USA, 2-2. [http://www.tcpdump.org/papers/bpf-usenix93.pdf]
-
-Structure
----------
-
-User space applications include <linux/filter.h> which contains the
-following relevant structures:
-
-struct sock_filter { /* Filter block */
- __u16 code; /* Actual filter code */
- __u8 jt; /* Jump true */
- __u8 jf; /* Jump false */
- __u32 k; /* Generic multiuse field */
-};
-
-Such a structure is assembled as an array of 4-tuples, that contains
-a code, jt, jf and k value. jt and jf are jump offsets and k a generic
-value to be used for a provided code.
-
-struct sock_fprog { /* Required for SO_ATTACH_FILTER. */
- unsigned short len; /* Number of filter blocks */
- struct sock_filter __user *filter;
-};
-
-For socket filtering, a pointer to this structure (as shown in
-follow-up example) is being passed to the kernel through setsockopt(2).
-
-Example
--------
-
-#include <sys/socket.h>
-#include <sys/types.h>
-#include <arpa/inet.h>
-#include <linux/if_ether.h>
-/* ... */
-
-/* From the example above: tcpdump -i em1 port 22 -dd */
-struct sock_filter code[] = {
- { 0x28, 0, 0, 0x0000000c },
- { 0x15, 0, 8, 0x000086dd },
- { 0x30, 0, 0, 0x00000014 },
- { 0x15, 2, 0, 0x00000084 },
- { 0x15, 1, 0, 0x00000006 },
- { 0x15, 0, 17, 0x00000011 },
- { 0x28, 0, 0, 0x00000036 },
- { 0x15, 14, 0, 0x00000016 },
- { 0x28, 0, 0, 0x00000038 },
- { 0x15, 12, 13, 0x00000016 },
- { 0x15, 0, 12, 0x00000800 },
- { 0x30, 0, 0, 0x00000017 },
- { 0x15, 2, 0, 0x00000084 },
- { 0x15, 1, 0, 0x00000006 },
- { 0x15, 0, 8, 0x00000011 },
- { 0x28, 0, 0, 0x00000014 },
- { 0x45, 6, 0, 0x00001fff },
- { 0xb1, 0, 0, 0x0000000e },
- { 0x48, 0, 0, 0x0000000e },
- { 0x15, 2, 0, 0x00000016 },
- { 0x48, 0, 0, 0x00000010 },
- { 0x15, 0, 1, 0x00000016 },
- { 0x06, 0, 0, 0x0000ffff },
- { 0x06, 0, 0, 0x00000000 },
-};
-
-struct sock_fprog bpf = {
- .len = ARRAY_SIZE(code),
- .filter = code,
-};
-
-sock = socket(PF_PACKET, SOCK_RAW, htons(ETH_P_ALL));
-if (sock < 0)
- /* ... bail out ... */
-
-ret = setsockopt(sock, SOL_SOCKET, SO_ATTACH_FILTER, &bpf, sizeof(bpf));
-if (ret < 0)
- /* ... bail out ... */
-
-/* ... */
-close(sock);
-
-The above example code attaches a socket filter for a PF_PACKET socket
-in order to let all IPv4/IPv6 packets with port 22 pass. The rest will
-be dropped for this socket.
-
-The setsockopt(2) call to SO_DETACH_FILTER doesn't need any arguments
-and SO_LOCK_FILTER for preventing the filter to be detached, takes an
-integer value with 0 or 1.
-
-Note that socket filters are not restricted to PF_PACKET sockets only,
-but can also be used on other socket families.
-
-Summary of system calls:
-
- * setsockopt(sockfd, SOL_SOCKET, SO_ATTACH_FILTER, &val, sizeof(val));
- * setsockopt(sockfd, SOL_SOCKET, SO_DETACH_FILTER, &val, sizeof(val));
- * setsockopt(sockfd, SOL_SOCKET, SO_LOCK_FILTER, &val, sizeof(val));
-
-Normally, most use cases for socket filtering on packet sockets will be
-covered by libpcap in high-level syntax, so as an application developer
-you should stick to that. libpcap wraps its own layer around all that.
-
-Unless i) using/linking to libpcap is not an option, ii) the required BPF
-filters use Linux extensions that are not supported by libpcap's compiler,
-iii) a filter might be more complex and not cleanly implementable with
-libpcap's compiler, or iv) particular filter codes should be optimized
-differently than libpcap's internal compiler does; then in such cases
-writing such a filter "by hand" can be of an alternative. For example,
-xt_bpf and cls_bpf users might have requirements that could result in
-more complex filter code, or one that cannot be expressed with libpcap
-(e.g. different return codes for various code paths). Moreover, BPF JIT
-implementors may wish to manually write test cases and thus need low-level
-access to BPF code as well.
-
-BPF engine and instruction set
-------------------------------
-
-Under tools/bpf/ there's a small helper tool called bpf_asm which can
-be used to write low-level filters for example scenarios mentioned in the
-previous section. Asm-like syntax mentioned here has been implemented in
-bpf_asm and will be used for further explanations (instead of dealing with
-less readable opcodes directly, principles are the same). The syntax is
-closely modelled after Steven McCanne's and Van Jacobson's BPF paper.
-
-The BPF architecture consists of the following basic elements:
-
- Element Description
-
- A 32 bit wide accumulator
- X 32 bit wide X register
- M[] 16 x 32 bit wide misc registers aka "scratch memory
- store", addressable from 0 to 15
-
-A program, that is translated by bpf_asm into "opcodes" is an array that
-consists of the following elements (as already mentioned):
-
- op:16, jt:8, jf:8, k:32
-
-The element op is a 16 bit wide opcode that has a particular instruction
-encoded. jt and jf are two 8 bit wide jump targets, one for condition
-"jump if true", the other one "jump if false". Eventually, element k
-contains a miscellaneous argument that can be interpreted in different
-ways depending on the given instruction in op.
-
-The instruction set consists of load, store, branch, alu, miscellaneous
-and return instructions that are also represented in bpf_asm syntax. This
-table lists all bpf_asm instructions available resp. what their underlying
-opcodes as defined in linux/filter.h stand for:
-
- Instruction Addressing mode Description
-
- ld 1, 2, 3, 4, 10 Load word into A
- ldi 4 Load word into A
- ldh 1, 2 Load half-word into A
- ldb 1, 2 Load byte into A
- ldx 3, 4, 5, 10 Load word into X
- ldxi 4 Load word into X
- ldxb 5 Load byte into X
-
- st 3 Store A into M[]
- stx 3 Store X into M[]
-
- jmp 6 Jump to label
- ja 6 Jump to label
- jeq 7, 8 Jump on A == k
- jneq 8 Jump on A != k
- jne 8 Jump on A != k
- jlt 8 Jump on A < k
- jle 8 Jump on A <= k
- jgt 7, 8 Jump on A > k
- jge 7, 8 Jump on A >= k
- jset 7, 8 Jump on A & k
-
- add 0, 4 A + <x>
- sub 0, 4 A - <x>
- mul 0, 4 A * <x>
- div 0, 4 A / <x>
- mod 0, 4 A % <x>
- neg !A
- and 0, 4 A & <x>
- or 0, 4 A | <x>
- xor 0, 4 A ^ <x>
- lsh 0, 4 A << <x>
- rsh 0, 4 A >> <x>
-
- tax Copy A into X
- txa Copy X into A
-
- ret 4, 9 Return
-
-The next table shows addressing formats from the 2nd column:
-
- Addressing mode Syntax Description
-
- 0 x/%x Register X
- 1 [k] BHW at byte offset k in the packet
- 2 [x + k] BHW at the offset X + k in the packet
- 3 M[k] Word at offset k in M[]
- 4 #k Literal value stored in k
- 5 4*([k]&0xf) Lower nibble * 4 at byte offset k in the packet
- 6 L Jump label L
- 7 #k,Lt,Lf Jump to Lt if true, otherwise jump to Lf
- 8 #k,Lt Jump to Lt if predicate is true
- 9 a/%a Accumulator A
- 10 extension BPF extension
-
-The Linux kernel also has a couple of BPF extensions that are used along
-with the class of load instructions by "overloading" the k argument with
-a negative offset + a particular extension offset. The result of such BPF
-extensions are loaded into A.
-
-Possible BPF extensions are shown in the following table:
-
- Extension Description
-
- len skb->len
- proto skb->protocol
- type skb->pkt_type
- poff Payload start offset
- ifidx skb->dev->ifindex
- nla Netlink attribute of type X with offset A
- nlan Nested Netlink attribute of type X with offset A
- mark skb->mark
- queue skb->queue_mapping
- hatype skb->dev->type
- rxhash skb->hash
- cpu raw_smp_processor_id()
- vlan_tci skb_vlan_tag_get(skb)
- vlan_avail skb_vlan_tag_present(skb)
- vlan_tpid skb->vlan_proto
- rand prandom_u32()
-
-These extensions can also be prefixed with '#'.
-Examples for low-level BPF:
-
-** ARP packets:
-
- ldh [12]
- jne #0x806, drop
- ret #-1
- drop: ret #0
-
-** IPv4 TCP packets:
-
- ldh [12]
- jne #0x800, drop
- ldb [23]
- jneq #6, drop
- ret #-1
- drop: ret #0
-
-** (Accelerated) VLAN w/ id 10:
-
- ld vlan_tci
- jneq #10, drop
- ret #-1
- drop: ret #0
-
-** icmp random packet sampling, 1 in 4
- ldh [12]
- jne #0x800, drop
- ldb [23]
- jneq #1, drop
- # get a random uint32 number
- ld rand
- mod #4
- jneq #1, drop
- ret #-1
- drop: ret #0
-
-** SECCOMP filter example:
-
- ld [4] /* offsetof(struct seccomp_data, arch) */
- jne #0xc000003e, bad /* AUDIT_ARCH_X86_64 */
- ld [0] /* offsetof(struct seccomp_data, nr) */
- jeq #15, good /* __NR_rt_sigreturn */
- jeq #231, good /* __NR_exit_group */
- jeq #60, good /* __NR_exit */
- jeq #0, good /* __NR_read */
- jeq #1, good /* __NR_write */
- jeq #5, good /* __NR_fstat */
- jeq #9, good /* __NR_mmap */
- jeq #14, good /* __NR_rt_sigprocmask */
- jeq #13, good /* __NR_rt_sigaction */
- jeq #35, good /* __NR_nanosleep */
- bad: ret #0 /* SECCOMP_RET_KILL_THREAD */
- good: ret #0x7fff0000 /* SECCOMP_RET_ALLOW */
-
-The above example code can be placed into a file (here called "foo"), and
-then be passed to the bpf_asm tool for generating opcodes, output that xt_bpf
-and cls_bpf understands and can directly be loaded with. Example with above
-ARP code:
-
-$ ./bpf_asm foo
-4,40 0 0 12,21 0 1 2054,6 0 0 4294967295,6 0 0 0,
-
-In copy and paste C-like output:
-
-$ ./bpf_asm -c foo
-{ 0x28, 0, 0, 0x0000000c },
-{ 0x15, 0, 1, 0x00000806 },
-{ 0x06, 0, 0, 0xffffffff },
-{ 0x06, 0, 0, 0000000000 },
-
-In particular, as usage with xt_bpf or cls_bpf can result in more complex BPF
-filters that might not be obvious at first, it's good to test filters before
-attaching to a live system. For that purpose, there's a small tool called
-bpf_dbg under tools/bpf/ in the kernel source directory. This debugger allows
-for testing BPF filters against given pcap files, single stepping through the
-BPF code on the pcap's packets and to do BPF machine register dumps.
-
-Starting bpf_dbg is trivial and just requires issuing:
-
-# ./bpf_dbg
-
-In case input and output do not equal stdin/stdout, bpf_dbg takes an
-alternative stdin source as a first argument, and an alternative stdout
-sink as a second one, e.g. `./bpf_dbg test_in.txt test_out.txt`.
-
-Other than that, a particular libreadline configuration can be set via
-file "~/.bpf_dbg_init" and the command history is stored in the file
-"~/.bpf_dbg_history".
-
-Interaction in bpf_dbg happens through a shell that also has auto-completion
-support (follow-up example commands starting with '>' denote bpf_dbg shell).
-The usual workflow would be to ...
-
-> load bpf 6,40 0 0 12,21 0 3 2048,48 0 0 23,21 0 1 1,6 0 0 65535,6 0 0 0
- Loads a BPF filter from standard output of bpf_asm, or transformed via
- e.g. `tcpdump -iem1 -ddd port 22 | tr '\n' ','`. Note that for JIT
- debugging (next section), this command creates a temporary socket and
- loads the BPF code into the kernel. Thus, this will also be useful for
- JIT developers.
-
-> load pcap foo.pcap
- Loads standard tcpdump pcap file.
-
-> run [<n>]
-bpf passes:1 fails:9
- Runs through all packets from a pcap to account how many passes and fails
- the filter will generate. A limit of packets to traverse can be given.
-
-> disassemble
-l0: ldh [12]
-l1: jeq #0x800, l2, l5
-l2: ldb [23]
-l3: jeq #0x1, l4, l5
-l4: ret #0xffff
-l5: ret #0
- Prints out BPF code disassembly.
-
-> dump
-/* { op, jt, jf, k }, */
-{ 0x28, 0, 0, 0x0000000c },
-{ 0x15, 0, 3, 0x00000800 },
-{ 0x30, 0, 0, 0x00000017 },
-{ 0x15, 0, 1, 0x00000001 },
-{ 0x06, 0, 0, 0x0000ffff },
-{ 0x06, 0, 0, 0000000000 },
- Prints out C-style BPF code dump.
-
-> breakpoint 0
-breakpoint at: l0: ldh [12]
-> breakpoint 1
-breakpoint at: l1: jeq #0x800, l2, l5
- ...
- Sets breakpoints at particular BPF instructions. Issuing a `run` command
- will walk through the pcap file continuing from the current packet and
- break when a breakpoint is being hit (another `run` will continue from
- the currently active breakpoint executing next instructions):
-
- > run
- -- register dump --
- pc: [0] <-- program counter
- code: [40] jt[0] jf[0] k[12] <-- plain BPF code of current instruction
- curr: l0: ldh [12] <-- disassembly of current instruction
- A: [00000000][0] <-- content of A (hex, decimal)
- X: [00000000][0] <-- content of X (hex, decimal)
- M[0,15]: [00000000][0] <-- folded content of M (hex, decimal)
- -- packet dump -- <-- Current packet from pcap (hex)
- len: 42
- 0: 00 19 cb 55 55 a4 00 14 a4 43 78 69 08 06 00 01
- 16: 08 00 06 04 00 01 00 14 a4 43 78 69 0a 3b 01 26
- 32: 00 00 00 00 00 00 0a 3b 01 01
- (breakpoint)
- >
-
-> breakpoint
-breakpoints: 0 1
- Prints currently set breakpoints.
-
-> step [-<n>, +<n>]
- Performs single stepping through the BPF program from the current pc
- offset. Thus, on each step invocation, above register dump is issued.
- This can go forwards and backwards in time, a plain `step` will break
- on the next BPF instruction, thus +1. (No `run` needs to be issued here.)
-
-> select <n>
- Selects a given packet from the pcap file to continue from. Thus, on
- the next `run` or `step`, the BPF program is being evaluated against
- the user pre-selected packet. Numbering starts just as in Wireshark
- with index 1.
-
-> quit
-#
- Exits bpf_dbg.
-
-JIT compiler
-------------
-
-The Linux kernel has a built-in BPF JIT compiler for x86_64, SPARC, PowerPC,
-ARM, ARM64, MIPS and s390 which can be enabled through CONFIG_BPF_JIT. The JIT
-compiler is transparently invoked for each attached filter from user space
-or for internal kernel users if it has been previously enabled by root:
-
- echo 1 > /proc/sys/net/core/bpf_jit_enable
-
-For JIT developers, doing audits etc, each compile run can output the generated
-opcode image into the kernel log via:
-
- echo 2 > /proc/sys/net/core/bpf_jit_enable
-
-Example output from dmesg:
-
-[ 3389.935842] flen=6 proglen=70 pass=3 image=ffffffffa0069c8f
-[ 3389.935847] JIT code: 00000000: 55 48 89 e5 48 83 ec 60 48 89 5d f8 44 8b 4f 68
-[ 3389.935849] JIT code: 00000010: 44 2b 4f 6c 4c 8b 87 d8 00 00 00 be 0c 00 00 00
-[ 3389.935850] JIT code: 00000020: e8 1d 94 ff e0 3d 00 08 00 00 75 16 be 17 00 00
-[ 3389.935851] JIT code: 00000030: 00 e8 28 94 ff e0 83 f8 01 75 07 b8 ff ff 00 00
-[ 3389.935852] JIT code: 00000040: eb 02 31 c0 c9 c3
-
-When CONFIG_BPF_JIT_ALWAYS_ON is enabled, bpf_jit_enable is permanently set to 1 and
-setting any other value than that will return in failure. This is even the case for
-setting bpf_jit_enable to 2, since dumping the final JIT image into the kernel log
-is discouraged and introspection through bpftool (under tools/bpf/bpftool/) is the
-generally recommended approach instead.
-
-In the kernel source tree under tools/bpf/, there's bpf_jit_disasm for
-generating disassembly out of the kernel log's hexdump:
-
-# ./bpf_jit_disasm
-70 bytes emitted from JIT compiler (pass:3, flen:6)
-ffffffffa0069c8f + <x>:
- 0: push %rbp
- 1: mov %rsp,%rbp
- 4: sub $0x60,%rsp
- 8: mov %rbx,-0x8(%rbp)
- c: mov 0x68(%rdi),%r9d
- 10: sub 0x6c(%rdi),%r9d
- 14: mov 0xd8(%rdi),%r8
- 1b: mov $0xc,%esi
- 20: callq 0xffffffffe0ff9442
- 25: cmp $0x800,%eax
- 2a: jne 0x0000000000000042
- 2c: mov $0x17,%esi
- 31: callq 0xffffffffe0ff945e
- 36: cmp $0x1,%eax
- 39: jne 0x0000000000000042
- 3b: mov $0xffff,%eax
- 40: jmp 0x0000000000000044
- 42: xor %eax,%eax
- 44: leaveq
- 45: retq
-
-Issuing option `-o` will "annotate" opcodes to resulting assembler
-instructions, which can be very useful for JIT developers:
-
-# ./bpf_jit_disasm -o
-70 bytes emitted from JIT compiler (pass:3, flen:6)
-ffffffffa0069c8f + <x>:
- 0: push %rbp
- 55
- 1: mov %rsp,%rbp
- 48 89 e5
- 4: sub $0x60,%rsp
- 48 83 ec 60
- 8: mov %rbx,-0x8(%rbp)
- 48 89 5d f8
- c: mov 0x68(%rdi),%r9d
- 44 8b 4f 68
- 10: sub 0x6c(%rdi),%r9d
- 44 2b 4f 6c
- 14: mov 0xd8(%rdi),%r8
- 4c 8b 87 d8 00 00 00
- 1b: mov $0xc,%esi
- be 0c 00 00 00
- 20: callq 0xffffffffe0ff9442
- e8 1d 94 ff e0
- 25: cmp $0x800,%eax
- 3d 00 08 00 00
- 2a: jne 0x0000000000000042
- 75 16
- 2c: mov $0x17,%esi
- be 17 00 00 00
- 31: callq 0xffffffffe0ff945e
- e8 28 94 ff e0
- 36: cmp $0x1,%eax
- 83 f8 01
- 39: jne 0x0000000000000042
- 75 07
- 3b: mov $0xffff,%eax
- b8 ff ff 00 00
- 40: jmp 0x0000000000000044
- eb 02
- 42: xor %eax,%eax
- 31 c0
- 44: leaveq
- c9
- 45: retq
- c3
-
-For BPF JIT developers, bpf_jit_disasm, bpf_asm and bpf_dbg provides a useful
-toolchain for developing and testing the kernel's JIT compiler.
-
-BPF kernel internals
---------------------
-Internally, for the kernel interpreter, a different instruction set
-format with similar underlying principles from BPF described in previous
-paragraphs is being used. However, the instruction set format is modelled
-closer to the underlying architecture to mimic native instruction sets, so
-that better performance can be achieved (more details later). This new
-ISA is called 'eBPF' or 'internal BPF' interchangeably. (Note: eBPF which
-originates from [e]xtended BPF is not the same as BPF extensions! While
-eBPF is an ISA, BPF extensions date back to classic BPF's 'overloading'
-of BPF_LD | BPF_{B,H,W} | BPF_ABS instruction.)
-
-It is designed to be JITed with one to one mapping, which can also open up
-the possibility for GCC/LLVM compilers to generate optimized eBPF code through
-an eBPF backend that performs almost as fast as natively compiled code.
-
-The new instruction set was originally designed with the possible goal in
-mind to write programs in "restricted C" and compile into eBPF with a optional
-GCC/LLVM backend, so that it can just-in-time map to modern 64-bit CPUs with
-minimal performance overhead over two steps, that is, C -> eBPF -> native code.
-
-Currently, the new format is being used for running user BPF programs, which
-includes seccomp BPF, classic socket filters, cls_bpf traffic classifier,
-team driver's classifier for its load-balancing mode, netfilter's xt_bpf
-extension, PTP dissector/classifier, and much more. They are all internally
-converted by the kernel into the new instruction set representation and run
-in the eBPF interpreter. For in-kernel handlers, this all works transparently
-by using bpf_prog_create() for setting up the filter, resp.
-bpf_prog_destroy() for destroying it. The macro
-BPF_PROG_RUN(filter, ctx) transparently invokes eBPF interpreter or JITed
-code to run the filter. 'filter' is a pointer to struct bpf_prog that we
-got from bpf_prog_create(), and 'ctx' the given context (e.g.
-skb pointer). All constraints and restrictions from bpf_check_classic() apply
-before a conversion to the new layout is being done behind the scenes!
-
-Currently, the classic BPF format is being used for JITing on most 32-bit
-architectures, whereas x86-64, aarch64, s390x, powerpc64, sparc64, arm32 perform
-JIT compilation from eBPF instruction set.
-
-Some core changes of the new internal format:
-
-- Number of registers increase from 2 to 10:
-
- The old format had two registers A and X, and a hidden frame pointer. The
- new layout extends this to be 10 internal registers and a read-only frame
- pointer. Since 64-bit CPUs are passing arguments to functions via registers
- the number of args from eBPF program to in-kernel function is restricted
- to 5 and one register is used to accept return value from an in-kernel
- function. Natively, x86_64 passes first 6 arguments in registers, aarch64/
- sparcv9/mips64 have 7 - 8 registers for arguments; x86_64 has 6 callee saved
- registers, and aarch64/sparcv9/mips64 have 11 or more callee saved registers.
-
- Therefore, eBPF calling convention is defined as:
-
- * R0 - return value from in-kernel function, and exit value for eBPF program
- * R1 - R5 - arguments from eBPF program to in-kernel function
- * R6 - R9 - callee saved registers that in-kernel function will preserve
- * R10 - read-only frame pointer to access stack
-
- Thus, all eBPF registers map one to one to HW registers on x86_64, aarch64,
- etc, and eBPF calling convention maps directly to ABIs used by the kernel on
- 64-bit architectures.
-
- On 32-bit architectures JIT may map programs that use only 32-bit arithmetic
- and may let more complex programs to be interpreted.
-
- R0 - R5 are scratch registers and eBPF program needs spill/fill them if
- necessary across calls. Note that there is only one eBPF program (== one
- eBPF main routine) and it cannot call other eBPF functions, it can only
- call predefined in-kernel functions, though.
-
-- Register width increases from 32-bit to 64-bit:
-
- Still, the semantics of the original 32-bit ALU operations are preserved
- via 32-bit subregisters. All eBPF registers are 64-bit with 32-bit lower
- subregisters that zero-extend into 64-bit if they are being written to.
- That behavior maps directly to x86_64 and arm64 subregister definition, but
- makes other JITs more difficult.
-
- 32-bit architectures run 64-bit internal BPF programs via interpreter.
- Their JITs may convert BPF programs that only use 32-bit subregisters into
- native instruction set and let the rest be interpreted.
-
- Operation is 64-bit since on 64-bit architectures pointers are also
- 64-bit wide and we want to pass 64-bit values in/out of kernel functions.
- 32-bit eBPF registers would otherwise require us to define a register-pair
- ABI, thus we would not be able to use a direct eBPF register to HW register
- mapping and JIT would need to do combine/split/move operations for every
- register in and out of the function, which is complex, bug prone and slow.
- Another reason is the use of atomic 64-bit counters.
-
-- Conditional jt/jf targets replaced with jt/fall-through:
-
- While the original design has constructs such as "if (cond) jump_true;
- else jump_false;", they are being replaced into alternative constructs like
- "if (cond) jump_true; /* else fall-through */".
-
-- Introduces bpf_call insn and register passing convention for zero overhead
- calls from/to other kernel functions:
-
- Before an in-kernel function call, the internal BPF program needs to
- place function arguments into R1 to R5 registers to satisfy calling
- convention, then the interpreter will take them from registers and pass
- to in-kernel function. If R1 - R5 registers are mapped to CPU registers
- that are used for argument passing on given architecture, the JIT compiler
- doesn't need to emit extra moves. Function arguments will be in the correct
- registers and BPF_CALL instruction will be JITed as single 'call' HW
- instruction. This calling convention was picked to cover common call
- situations without performance penalty.
-
- After an in-kernel function call, R1 - R5 are reset to unreadable and R0 has
- the return value of the function. Since R6 - R9 are callee saved, their state
- is preserved across the call.
-
- For example, consider three C functions:
-
- u64 f1() { return (*_f2)(1); }
- u64 f2(u64 a) { return f3(a + 1, a); }
- u64 f3(u64 a, u64 b) { return a - b; }
-
- GCC can compile f1, f3 into x86_64:
-
- f1:
- movl $1, %edi
- movq _f2(%rip), %rax
- jmp *%rax
- f3:
- movq %rdi, %rax
- subq %rsi, %rax
- ret
-
- Function f2 in eBPF may look like:
-
- f2:
- bpf_mov R2, R1
- bpf_add R1, 1
- bpf_call f3
- bpf_exit
-
- If f2 is JITed and the pointer stored to '_f2'. The calls f1 -> f2 -> f3 and
- returns will be seamless. Without JIT, __bpf_prog_run() interpreter needs to
- be used to call into f2.
-
- For practical reasons all eBPF programs have only one argument 'ctx' which is
- already placed into R1 (e.g. on __bpf_prog_run() startup) and the programs
- can call kernel functions with up to 5 arguments. Calls with 6 or more arguments
- are currently not supported, but these restrictions can be lifted if necessary
- in the future.
-
- On 64-bit architectures all registers map to HW registers one to one. For
- example, x86_64 JIT compiler can map them as ...
-
- R0 - rax
- R1 - rdi
- R2 - rsi
- R3 - rdx
- R4 - rcx
- R5 - r8
- R6 - rbx
- R7 - r13
- R8 - r14
- R9 - r15
- R10 - rbp
-
- ... since x86_64 ABI mandates rdi, rsi, rdx, rcx, r8, r9 for argument passing
- and rbx, r12 - r15 are callee saved.
-
- Then the following internal BPF pseudo-program:
-
- bpf_mov R6, R1 /* save ctx */
- bpf_mov R2, 2
- bpf_mov R3, 3
- bpf_mov R4, 4
- bpf_mov R5, 5
- bpf_call foo
- bpf_mov R7, R0 /* save foo() return value */
- bpf_mov R1, R6 /* restore ctx for next call */
- bpf_mov R2, 6
- bpf_mov R3, 7
- bpf_mov R4, 8
- bpf_mov R5, 9
- bpf_call bar
- bpf_add R0, R7
- bpf_exit
-
- After JIT to x86_64 may look like:
-
- push %rbp
- mov %rsp,%rbp
- sub $0x228,%rsp
- mov %rbx,-0x228(%rbp)
- mov %r13,-0x220(%rbp)
- mov %rdi,%rbx
- mov $0x2,%esi
- mov $0x3,%edx
- mov $0x4,%ecx
- mov $0x5,%r8d
- callq foo
- mov %rax,%r13
- mov %rbx,%rdi
- mov $0x2,%esi
- mov $0x3,%edx
- mov $0x4,%ecx
- mov $0x5,%r8d
- callq bar
- add %r13,%rax
- mov -0x228(%rbp),%rbx
- mov -0x220(%rbp),%r13
- leaveq
- retq
-
- Which is in this example equivalent in C to:
-
- u64 bpf_filter(u64 ctx)
- {
- return foo(ctx, 2, 3, 4, 5) + bar(ctx, 6, 7, 8, 9);
- }
-
- In-kernel functions foo() and bar() with prototype: u64 (*)(u64 arg1, u64
- arg2, u64 arg3, u64 arg4, u64 arg5); will receive arguments in proper
- registers and place their return value into '%rax' which is R0 in eBPF.
- Prologue and epilogue are emitted by JIT and are implicit in the
- interpreter. R0-R5 are scratch registers, so eBPF program needs to preserve
- them across the calls as defined by calling convention.
-
- For example the following program is invalid:
-
- bpf_mov R1, 1
- bpf_call foo
- bpf_mov R0, R1
- bpf_exit
-
- After the call the registers R1-R5 contain junk values and cannot be read.
- An in-kernel eBPF verifier is used to validate internal BPF programs.
-
-Also in the new design, eBPF is limited to 4096 insns, which means that any
-program will terminate quickly and will only call a fixed number of kernel
-functions. Original BPF and the new format are two operand instructions,
-which helps to do one-to-one mapping between eBPF insn and x86 insn during JIT.
-
-The input context pointer for invoking the interpreter function is generic,
-its content is defined by a specific use case. For seccomp register R1 points
-to seccomp_data, for converted BPF filters R1 points to a skb.
-
-A program, that is translated internally consists of the following elements:
-
- op:16, jt:8, jf:8, k:32 ==> op:8, dst_reg:4, src_reg:4, off:16, imm:32
-
-So far 87 internal BPF instructions have been implemented. 8-bit 'op' opcode
-field has room for new instructions. Some of them may use 16/24/32 byte
-encoding. New instructions must be a multiple of 8 bytes to preserve backward
-compatibility.
-
-Internal BPF is a general purpose RISC instruction set. Not every register and
-every instruction are used during translation from original BPF to new format.
-For example, socket filters are not using 'exclusive add' instruction, but
-tracing filters may do to maintain counters of events, for example. Register R9
-is not used by socket filters either, but more complex filters may be running
-out of registers and would have to resort to spill/fill to stack.
-
-Internal BPF can used as generic assembler for last step performance
-optimizations, socket filters and seccomp are using it as assembler. Tracing
-filters may use it as assembler to generate code from kernel. In-kernel usage
-may not be bounded by security considerations, since generated internal BPF code
-may use an optimised internal code path and may not be being exposed to user
-space. Safety of internal BPF can come from a verifier (TBD). In such use cases
-as described, it may be used as safe as the instruction set.
-
-Just like the original BPF, the new format runs within a controlled environment,
-is deterministic and the kernel can easily prove that. The safety of the program
-can be determined in two steps: first step does depth-first-search to disallow
-loops and other CFG validation; second step starts from the first insn and
-descends all possible paths. It simulates execution of every insn and observes
-the state change of registers and stack.
-
-eBPF opcode encoding
---------------------
-
-eBPF is reusing most of the opcode encoding from classic to simplify conversion
-of classic BPF to eBPF. For arithmetic and jump instructions the 8-bit 'code'
-field is divided into three parts:
-
- +----------------+--------+--------------------+
- | 4 bits | 1 bit | 3 bits |
- | operation code | source | instruction class |
- +----------------+--------+--------------------+
- (MSB) (LSB)
-
-Three LSB bits store instruction class which is one of:
-
- Classic BPF classes: eBPF classes:
-
- BPF_LD 0x00 BPF_LD 0x00
- BPF_LDX 0x01 BPF_LDX 0x01
- BPF_ST 0x02 BPF_ST 0x02
- BPF_STX 0x03 BPF_STX 0x03
- BPF_ALU 0x04 BPF_ALU 0x04
- BPF_JMP 0x05 BPF_JMP 0x05
- BPF_RET 0x06 [ class 6 unused, for future if needed ]
- BPF_MISC 0x07 BPF_ALU64 0x07
-
-When BPF_CLASS(code) == BPF_ALU or BPF_JMP, 4th bit encodes source operand ...
-
- BPF_K 0x00
- BPF_X 0x08
-
- * in classic BPF, this means:
-
- BPF_SRC(code) == BPF_X - use register X as source operand
- BPF_SRC(code) == BPF_K - use 32-bit immediate as source operand
-
- * in eBPF, this means:
-
- BPF_SRC(code) == BPF_X - use 'src_reg' register as source operand
- BPF_SRC(code) == BPF_K - use 32-bit immediate as source operand
-
-... and four MSB bits store operation code.
-
-If BPF_CLASS(code) == BPF_ALU or BPF_ALU64 [ in eBPF ], BPF_OP(code) is one of:
-
- BPF_ADD 0x00
- BPF_SUB 0x10
- BPF_MUL 0x20
- BPF_DIV 0x30
- BPF_OR 0x40
- BPF_AND 0x50
- BPF_LSH 0x60
- BPF_RSH 0x70
- BPF_NEG 0x80
- BPF_MOD 0x90
- BPF_XOR 0xa0
- BPF_MOV 0xb0 /* eBPF only: mov reg to reg */
- BPF_ARSH 0xc0 /* eBPF only: sign extending shift right */
- BPF_END 0xd0 /* eBPF only: endianness conversion */
-
-If BPF_CLASS(code) == BPF_JMP, BPF_OP(code) is one of:
-
- BPF_JA 0x00
- BPF_JEQ 0x10
- BPF_JGT 0x20
- BPF_JGE 0x30
- BPF_JSET 0x40
- BPF_JNE 0x50 /* eBPF only: jump != */
- BPF_JSGT 0x60 /* eBPF only: signed '>' */
- BPF_JSGE 0x70 /* eBPF only: signed '>=' */
- BPF_CALL 0x80 /* eBPF only: function call */
- BPF_EXIT 0x90 /* eBPF only: function return */
- BPF_JLT 0xa0 /* eBPF only: unsigned '<' */
- BPF_JLE 0xb0 /* eBPF only: unsigned '<=' */
- BPF_JSLT 0xc0 /* eBPF only: signed '<' */
- BPF_JSLE 0xd0 /* eBPF only: signed '<=' */
-
-So BPF_ADD | BPF_X | BPF_ALU means 32-bit addition in both classic BPF
-and eBPF. There are only two registers in classic BPF, so it means A += X.
-In eBPF it means dst_reg = (u32) dst_reg + (u32) src_reg; similarly,
-BPF_XOR | BPF_K | BPF_ALU means A ^= imm32 in classic BPF and analogous
-src_reg = (u32) src_reg ^ (u32) imm32 in eBPF.
-
-Classic BPF is using BPF_MISC class to represent A = X and X = A moves.
-eBPF is using BPF_MOV | BPF_X | BPF_ALU code instead. Since there are no
-BPF_MISC operations in eBPF, the class 7 is used as BPF_ALU64 to mean
-exactly the same operations as BPF_ALU, but with 64-bit wide operands
-instead. So BPF_ADD | BPF_X | BPF_ALU64 means 64-bit addition i.e.
-dst_reg = dst_reg + src_reg
-
-Classic BPF wastes the whole BPF_RET class to represent a single 'ret'
-operation. Classic BPF_RET | BPF_K means copy imm32 into return register
-and perform function exit. eBPF is modeled to match CPU, so BPF_JMP | BPF_EXIT
-in eBPF means function exit only. The eBPF program needs to store return
-value into register R0 before doing a BPF_EXIT. Class 6 in eBPF is currently
-unused and reserved for future use.
-
-For load and store instructions the 8-bit 'code' field is divided as:
-
- +--------+--------+-------------------+
- | 3 bits | 2 bits | 3 bits |
- | mode | size | instruction class |
- +--------+--------+-------------------+
- (MSB) (LSB)
-
-Size modifier is one of ...
-
- BPF_W 0x00 /* word */
- BPF_H 0x08 /* half word */
- BPF_B 0x10 /* byte */
- BPF_DW 0x18 /* eBPF only, double word */
-
-... which encodes size of load/store operation:
-
- B - 1 byte
- H - 2 byte
- W - 4 byte
- DW - 8 byte (eBPF only)
-
-Mode modifier is one of:
-
- BPF_IMM 0x00 /* used for 32-bit mov in classic BPF and 64-bit in eBPF */
- BPF_ABS 0x20
- BPF_IND 0x40
- BPF_MEM 0x60
- BPF_LEN 0x80 /* classic BPF only, reserved in eBPF */
- BPF_MSH 0xa0 /* classic BPF only, reserved in eBPF */
- BPF_XADD 0xc0 /* eBPF only, exclusive add */
-
-eBPF has two non-generic instructions: (BPF_ABS | <size> | BPF_LD) and
-(BPF_IND | <size> | BPF_LD) which are used to access packet data.
-
-They had to be carried over from classic to have strong performance of
-socket filters running in eBPF interpreter. These instructions can only
-be used when interpreter context is a pointer to 'struct sk_buff' and
-have seven implicit operands. Register R6 is an implicit input that must
-contain pointer to sk_buff. Register R0 is an implicit output which contains
-the data fetched from the packet. Registers R1-R5 are scratch registers
-and must not be used to store the data across BPF_ABS | BPF_LD or
-BPF_IND | BPF_LD instructions.
-
-These instructions have implicit program exit condition as well. When
-eBPF program is trying to access the data beyond the packet boundary,
-the interpreter will abort the execution of the program. JIT compilers
-therefore must preserve this property. src_reg and imm32 fields are
-explicit inputs to these instructions.
-
-For example:
-
- BPF_IND | BPF_W | BPF_LD means:
-
- R0 = ntohl(*(u32 *) (((struct sk_buff *) R6)->data + src_reg + imm32))
- and R1 - R5 were scratched.
-
-Unlike classic BPF instruction set, eBPF has generic load/store operations:
-
-BPF_MEM | <size> | BPF_STX: *(size *) (dst_reg + off) = src_reg
-BPF_MEM | <size> | BPF_ST: *(size *) (dst_reg + off) = imm32
-BPF_MEM | <size> | BPF_LDX: dst_reg = *(size *) (src_reg + off)
-BPF_XADD | BPF_W | BPF_STX: lock xadd *(u32 *)(dst_reg + off16) += src_reg
-BPF_XADD | BPF_DW | BPF_STX: lock xadd *(u64 *)(dst_reg + off16) += src_reg
-
-Where size is one of: BPF_B or BPF_H or BPF_W or BPF_DW. Note that 1 and
-2 byte atomic increments are not supported.
-
-eBPF has one 16-byte instruction: BPF_LD | BPF_DW | BPF_IMM which consists of
-two consecutive 'struct bpf_insn' 8-byte blocks and is interpreted as single
-instruction that loads 64-bit immediate value into a dst_reg.
-
-Classic BPF has similar instruction: BPF_LD | BPF_W | BPF_IMM which loads
-32-bit immediate value into a register.
-
-eBPF verifier
--------------
-The safety of the eBPF program is determined in two steps.
-
-First step does DAG check to disallow loops and other CFG validation.
-In particular it will detect programs that have unreachable instructions
-(though classic BPF checker allows them).
-
-Second step starts from the first insn and descends all possible paths.
-It simulates execution of every insn and observes the state change of
-registers and stack.
-
-At the start of the program the register R1 contains a pointer to context
-and has type PTR_TO_CTX.
-If verifier sees an insn that does R2=R1, then R2 has now type
-PTR_TO_CTX as well and can be used on the right hand side of expression.
-If R1=PTR_TO_CTX and insn is R2=R1+R1, then R2=SCALAR_VALUE,
-since addition of two valid pointers makes invalid pointer.
-(In 'secure' mode verifier will reject any type of pointer arithmetic to make
-sure that kernel addresses don't leak to unprivileged users)
-
-If register was never written to, it's not readable:
- bpf_mov R0 = R2
- bpf_exit
-will be rejected, since R2 is unreadable at the start of the program.
-
-After kernel function call, R1-R5 are reset to unreadable and
-R0 has a return type of the function.
-
-Since R6-R9 are callee saved, their state is preserved across the call.
- bpf_mov R6 = 1
- bpf_call foo
- bpf_mov R0 = R6
- bpf_exit
-is a correct program. If there was R1 instead of R6, it would have
-been rejected.
-
-load/store instructions are allowed only with registers of valid types, which
-are PTR_TO_CTX, PTR_TO_MAP, PTR_TO_STACK. They are bounds and alignment checked.
-For example:
- bpf_mov R1 = 1
- bpf_mov R2 = 2
- bpf_xadd *(u32 *)(R1 + 3) += R2
- bpf_exit
-will be rejected, since R1 doesn't have a valid pointer type at the time of
-execution of instruction bpf_xadd.
-
-At the start R1 type is PTR_TO_CTX (a pointer to generic 'struct bpf_context')
-A callback is used to customize verifier to restrict eBPF program access to only
-certain fields within ctx structure with specified size and alignment.
-
-For example, the following insn:
- bpf_ld R0 = *(u32 *)(R6 + 8)
-intends to load a word from address R6 + 8 and store it into R0
-If R6=PTR_TO_CTX, via is_valid_access() callback the verifier will know
-that offset 8 of size 4 bytes can be accessed for reading, otherwise
-the verifier will reject the program.
-If R6=PTR_TO_STACK, then access should be aligned and be within
-stack bounds, which are [-MAX_BPF_STACK, 0). In this example offset is 8,
-so it will fail verification, since it's out of bounds.
-
-The verifier will allow eBPF program to read data from stack only after
-it wrote into it.
-Classic BPF verifier does similar check with M[0-15] memory slots.
-For example:
- bpf_ld R0 = *(u32 *)(R10 - 4)
- bpf_exit
-
-is an invalid program.
-
-Though R10 is correct read-only register and has type PTR_TO_STACK
-and R10 - 4 is within stack bounds, there were no stores into that location.
-
-Pointer register spill/fill is tracked as well, since four (R6-R9)
-callee saved registers may not be enough for some programs.
-
-Allowed function calls are customized with bpf_verifier_ops->get_func_proto()
-The eBPF verifier will check that registers match argument constraints.
-After the call register R0 will be set to return type of the function.
-
-Function calls is an important mechanism to extend functionality of eBPF
-programs. Socket filters may let programs call one set of functions,
-whereas tracing filters may allow a completely different set.
-
-If a function is made accessible to eBPF program, it needs to be thought
-through from a safety point of view. The verifier will guarantee that the
-function is called with valid arguments.
-
-seccomp vs socket filters have different security restrictions for classic BPF.
-Seccomp solves this by two stage verifier: classic BPF verifier is followed
-by seccomp verifier. In case of eBPF one configurable verifier is shared for
-all use cases.
-
-See details of eBPF verifier in kernel/bpf/verifier.c
-
-Register value tracking
------------------------
-In order to determine the safety of an eBPF program, the verifier must track
-the range of possible values in each register and also in each stack slot.
-This is done with 'struct bpf_reg_state', defined in include/linux/
-bpf_verifier.h, which unifies tracking of scalar and pointer values. Each
-register state has a type, which is either NOT_INIT (the register has not been
-written to), SCALAR_VALUE (some value which is not usable as a pointer), or a
-pointer type. The types of pointers describe their base, as follows:
- PTR_TO_CTX Pointer to bpf_context.
- CONST_PTR_TO_MAP Pointer to struct bpf_map. "Const" because arithmetic
- on these pointers is forbidden.
- PTR_TO_MAP_VALUE Pointer to the value stored in a map element.
- PTR_TO_MAP_VALUE_OR_NULL
- Either a pointer to a map value, or NULL; map accesses
- (see section 'eBPF maps', below) return this type,
- which becomes a PTR_TO_MAP_VALUE when checked != NULL.
- Arithmetic on these pointers is forbidden.
- PTR_TO_STACK Frame pointer.
- PTR_TO_PACKET skb->data.
- PTR_TO_PACKET_END skb->data + headlen; arithmetic forbidden.
-However, a pointer may be offset from this base (as a result of pointer
-arithmetic), and this is tracked in two parts: the 'fixed offset' and 'variable
-offset'. The former is used when an exactly-known value (e.g. an immediate
-operand) is added to a pointer, while the latter is used for values which are
-not exactly known. The variable offset is also used in SCALAR_VALUEs, to track
-the range of possible values in the register.
-The verifier's knowledge about the variable offset consists of:
-* minimum and maximum values as unsigned
-* minimum and maximum values as signed
-* knowledge of the values of individual bits, in the form of a 'tnum': a u64
-'mask' and a u64 'value'. 1s in the mask represent bits whose value is unknown;
-1s in the value represent bits known to be 1. Bits known to be 0 have 0 in both
-mask and value; no bit should ever be 1 in both. For example, if a byte is read
-into a register from memory, the register's top 56 bits are known zero, while
-the low 8 are unknown - which is represented as the tnum (0x0; 0xff). If we
-then OR this with 0x40, we get (0x40; 0xbf), then if we add 1 we get (0x0;
-0x1ff), because of potential carries.
-
-Besides arithmetic, the register state can also be updated by conditional
-branches. For instance, if a SCALAR_VALUE is compared > 8, in the 'true' branch
-it will have a umin_value (unsigned minimum value) of 9, whereas in the 'false'
-branch it will have a umax_value of 8. A signed compare (with BPF_JSGT or
-BPF_JSGE) would instead update the signed minimum/maximum values. Information
-from the signed and unsigned bounds can be combined; for instance if a value is
-first tested < 8 and then tested s> 4, the verifier will conclude that the value
-is also > 4 and s< 8, since the bounds prevent crossing the sign boundary.
-
-PTR_TO_PACKETs with a variable offset part have an 'id', which is common to all
-pointers sharing that same variable offset. This is important for packet range
-checks: after adding a variable to a packet pointer register A, if you then copy
-it to another register B and then add a constant 4 to A, both registers will
-share the same 'id' but the A will have a fixed offset of +4. Then if A is
-bounds-checked and found to be less than a PTR_TO_PACKET_END, the register B is
-now known to have a safe range of at least 4 bytes. See 'Direct packet access',
-below, for more on PTR_TO_PACKET ranges.
-
-The 'id' field is also used on PTR_TO_MAP_VALUE_OR_NULL, common to all copies of
-the pointer returned from a map lookup. This means that when one copy is
-checked and found to be non-NULL, all copies can become PTR_TO_MAP_VALUEs.
-As well as range-checking, the tracked information is also used for enforcing
-alignment of pointer accesses. For instance, on most systems the packet pointer
-is 2 bytes after a 4-byte alignment. If a program adds 14 bytes to that to jump
-over the Ethernet header, then reads IHL and adds (IHL * 4), the resulting
-pointer will have a variable offset known to be 4n+2 for some n, so adding the 2
-bytes (NET_IP_ALIGN) gives a 4-byte alignment and so word-sized accesses through
-that pointer are safe.
-
-Direct packet access
---------------------
-In cls_bpf and act_bpf programs the verifier allows direct access to the packet
-data via skb->data and skb->data_end pointers.
-Ex:
-1: r4 = *(u32 *)(r1 +80) /* load skb->data_end */
-2: r3 = *(u32 *)(r1 +76) /* load skb->data */
-3: r5 = r3
-4: r5 += 14
-5: if r5 > r4 goto pc+16
-R1=ctx R3=pkt(id=0,off=0,r=14) R4=pkt_end R5=pkt(id=0,off=14,r=14) R10=fp
-6: r0 = *(u16 *)(r3 +12) /* access 12 and 13 bytes of the packet */
-
-this 2byte load from the packet is safe to do, since the program author
-did check 'if (skb->data + 14 > skb->data_end) goto err' at insn #5 which
-means that in the fall-through case the register R3 (which points to skb->data)
-has at least 14 directly accessible bytes. The verifier marks it
-as R3=pkt(id=0,off=0,r=14).
-id=0 means that no additional variables were added to the register.
-off=0 means that no additional constants were added.
-r=14 is the range of safe access which means that bytes [R3, R3 + 14) are ok.
-Note that R5 is marked as R5=pkt(id=0,off=14,r=14). It also points
-to the packet data, but constant 14 was added to the register, so
-it now points to 'skb->data + 14' and accessible range is [R5, R5 + 14 - 14)
-which is zero bytes.
-
-More complex packet access may look like:
- R0=inv1 R1=ctx R3=pkt(id=0,off=0,r=14) R4=pkt_end R5=pkt(id=0,off=14,r=14) R10=fp
- 6: r0 = *(u8 *)(r3 +7) /* load 7th byte from the packet */
- 7: r4 = *(u8 *)(r3 +12)
- 8: r4 *= 14
- 9: r3 = *(u32 *)(r1 +76) /* load skb->data */
-10: r3 += r4
-11: r2 = r1
-12: r2 <<= 48
-13: r2 >>= 48
-14: r3 += r2
-15: r2 = r3
-16: r2 += 8
-17: r1 = *(u32 *)(r1 +80) /* load skb->data_end */
-18: if r2 > r1 goto pc+2
- R0=inv(id=0,umax_value=255,var_off=(0x0; 0xff)) R1=pkt_end R2=pkt(id=2,off=8,r=8) R3=pkt(id=2,off=0,r=8) R4=inv(id=0,umax_value=3570,var_off=(0x0; 0xfffe)) R5=pkt(id=0,off=14,r=14) R10=fp
-19: r1 = *(u8 *)(r3 +4)
-The state of the register R3 is R3=pkt(id=2,off=0,r=8)
-id=2 means that two 'r3 += rX' instructions were seen, so r3 points to some
-offset within a packet and since the program author did
-'if (r3 + 8 > r1) goto err' at insn #18, the safe range is [R3, R3 + 8).
-The verifier only allows 'add'/'sub' operations on packet registers. Any other
-operation will set the register state to 'SCALAR_VALUE' and it won't be
-available for direct packet access.
-Operation 'r3 += rX' may overflow and become less than original skb->data,
-therefore the verifier has to prevent that. So when it sees 'r3 += rX'
-instruction and rX is more than 16-bit value, any subsequent bounds-check of r3
-against skb->data_end will not give us 'range' information, so attempts to read
-through the pointer will give "invalid access to packet" error.
-Ex. after insn 'r4 = *(u8 *)(r3 +12)' (insn #7 above) the state of r4 is
-R4=inv(id=0,umax_value=255,var_off=(0x0; 0xff)) which means that upper 56 bits
-of the register are guaranteed to be zero, and nothing is known about the lower
-8 bits. After insn 'r4 *= 14' the state becomes
-R4=inv(id=0,umax_value=3570,var_off=(0x0; 0xfffe)), since multiplying an 8-bit
-value by constant 14 will keep upper 52 bits as zero, also the least significant
-bit will be zero as 14 is even. Similarly 'r2 >>= 48' will make
-R2=inv(id=0,umax_value=65535,var_off=(0x0; 0xffff)), since the shift is not sign
-extending. This logic is implemented in adjust_reg_min_max_vals() function,
-which calls adjust_ptr_min_max_vals() for adding pointer to scalar (or vice
-versa) and adjust_scalar_min_max_vals() for operations on two scalars.
-
-The end result is that bpf program author can access packet directly
-using normal C code as:
- void *data = (void *)(long)skb->data;
- void *data_end = (void *)(long)skb->data_end;
- struct eth_hdr *eth = data;
- struct iphdr *iph = data + sizeof(*eth);
- struct udphdr *udp = data + sizeof(*eth) + sizeof(*iph);
-
- if (data + sizeof(*eth) + sizeof(*iph) + sizeof(*udp) > data_end)
- return 0;
- if (eth->h_proto != htons(ETH_P_IP))
- return 0;
- if (iph->protocol != IPPROTO_UDP || iph->ihl != 5)
- return 0;
- if (udp->dest == 53 || udp->source == 9)
- ...;
-which makes such programs easier to write comparing to LD_ABS insn
-and significantly faster.
-
-eBPF maps
----------
-'maps' is a generic storage of different types for sharing data between kernel
-and userspace.
-
-The maps are accessed from user space via BPF syscall, which has commands:
-- create a map with given type and attributes
- map_fd = bpf(BPF_MAP_CREATE, union bpf_attr *attr, u32 size)
- using attr->map_type, attr->key_size, attr->value_size, attr->max_entries
- returns process-local file descriptor or negative error
-
-- lookup key in a given map
- err = bpf(BPF_MAP_LOOKUP_ELEM, union bpf_attr *attr, u32 size)
- using attr->map_fd, attr->key, attr->value
- returns zero and stores found elem into value or negative error
-
-- create or update key/value pair in a given map
- err = bpf(BPF_MAP_UPDATE_ELEM, union bpf_attr *attr, u32 size)
- using attr->map_fd, attr->key, attr->value
- returns zero or negative error
-
-- find and delete element by key in a given map
- err = bpf(BPF_MAP_DELETE_ELEM, union bpf_attr *attr, u32 size)
- using attr->map_fd, attr->key
-
-- to delete map: close(fd)
- Exiting process will delete maps automatically
-
-userspace programs use this syscall to create/access maps that eBPF programs
-are concurrently updating.
-
-maps can have different types: hash, array, bloom filter, radix-tree, etc.
-
-The map is defined by:
- . type
- . max number of elements
- . key size in bytes
- . value size in bytes
-
-Pruning
--------
-The verifier does not actually walk all possible paths through the program. For
-each new branch to analyse, the verifier looks at all the states it's previously
-been in when at this instruction. If any of them contain the current state as a
-subset, the branch is 'pruned' - that is, the fact that the previous state was
-accepted implies the current state would be as well. For instance, if in the
-previous state, r1 held a packet-pointer, and in the current state, r1 holds a
-packet-pointer with a range as long or longer and at least as strict an
-alignment, then r1 is safe. Similarly, if r2 was NOT_INIT before then it can't
-have been used by any path from that point, so any value in r2 (including
-another NOT_INIT) is safe. The implementation is in the function regsafe().
-Pruning considers not only the registers but also the stack (and any spilled
-registers it may hold). They must all be safe for the branch to be pruned.
-This is implemented in states_equal().
-
-Understanding eBPF verifier messages
-------------------------------------
-
-The following are few examples of invalid eBPF programs and verifier error
-messages as seen in the log:
-
-Program with unreachable instructions:
-static struct bpf_insn prog[] = {
- BPF_EXIT_INSN(),
- BPF_EXIT_INSN(),
-};
-Error:
- unreachable insn 1
-
-Program that reads uninitialized register:
- BPF_MOV64_REG(BPF_REG_0, BPF_REG_2),
- BPF_EXIT_INSN(),
-Error:
- 0: (bf) r0 = r2
- R2 !read_ok
-
-Program that doesn't initialize R0 before exiting:
- BPF_MOV64_REG(BPF_REG_2, BPF_REG_1),
- BPF_EXIT_INSN(),
-Error:
- 0: (bf) r2 = r1
- 1: (95) exit
- R0 !read_ok
-
-Program that accesses stack out of bounds:
- BPF_ST_MEM(BPF_DW, BPF_REG_10, 8, 0),
- BPF_EXIT_INSN(),
-Error:
- 0: (7a) *(u64 *)(r10 +8) = 0
- invalid stack off=8 size=8
-
-Program that doesn't initialize stack before passing its address into function:
- BPF_MOV64_REG(BPF_REG_2, BPF_REG_10),
- BPF_ALU64_IMM(BPF_ADD, BPF_REG_2, -8),
- BPF_LD_MAP_FD(BPF_REG_1, 0),
- BPF_RAW_INSN(BPF_JMP | BPF_CALL, 0, 0, 0, BPF_FUNC_map_lookup_elem),
- BPF_EXIT_INSN(),
-Error:
- 0: (bf) r2 = r10
- 1: (07) r2 += -8
- 2: (b7) r1 = 0x0
- 3: (85) call 1
- invalid indirect read from stack off -8+0 size 8
-
-Program that uses invalid map_fd=0 while calling to map_lookup_elem() function:
- BPF_ST_MEM(BPF_DW, BPF_REG_10, -8, 0),
- BPF_MOV64_REG(BPF_REG_2, BPF_REG_10),
- BPF_ALU64_IMM(BPF_ADD, BPF_REG_2, -8),
- BPF_LD_MAP_FD(BPF_REG_1, 0),
- BPF_RAW_INSN(BPF_JMP | BPF_CALL, 0, 0, 0, BPF_FUNC_map_lookup_elem),
- BPF_EXIT_INSN(),
-Error:
- 0: (7a) *(u64 *)(r10 -8) = 0
- 1: (bf) r2 = r10
- 2: (07) r2 += -8
- 3: (b7) r1 = 0x0
- 4: (85) call 1
- fd 0 is not pointing to valid bpf_map
-
-Program that doesn't check return value of map_lookup_elem() before accessing
-map element:
- BPF_ST_MEM(BPF_DW, BPF_REG_10, -8, 0),
- BPF_MOV64_REG(BPF_REG_2, BPF_REG_10),
- BPF_ALU64_IMM(BPF_ADD, BPF_REG_2, -8),
- BPF_LD_MAP_FD(BPF_REG_1, 0),
- BPF_RAW_INSN(BPF_JMP | BPF_CALL, 0, 0, 0, BPF_FUNC_map_lookup_elem),
- BPF_ST_MEM(BPF_DW, BPF_REG_0, 0, 0),
- BPF_EXIT_INSN(),
-Error:
- 0: (7a) *(u64 *)(r10 -8) = 0
- 1: (bf) r2 = r10
- 2: (07) r2 += -8
- 3: (b7) r1 = 0x0
- 4: (85) call 1
- 5: (7a) *(u64 *)(r0 +0) = 0
- R0 invalid mem access 'map_value_or_null'
-
-Program that correctly checks map_lookup_elem() returned value for NULL, but
-accesses the memory with incorrect alignment:
- BPF_ST_MEM(BPF_DW, BPF_REG_10, -8, 0),
- BPF_MOV64_REG(BPF_REG_2, BPF_REG_10),
- BPF_ALU64_IMM(BPF_ADD, BPF_REG_2, -8),
- BPF_LD_MAP_FD(BPF_REG_1, 0),
- BPF_RAW_INSN(BPF_JMP | BPF_CALL, 0, 0, 0, BPF_FUNC_map_lookup_elem),
- BPF_JMP_IMM(BPF_JEQ, BPF_REG_0, 0, 1),
- BPF_ST_MEM(BPF_DW, BPF_REG_0, 4, 0),
- BPF_EXIT_INSN(),
-Error:
- 0: (7a) *(u64 *)(r10 -8) = 0
- 1: (bf) r2 = r10
- 2: (07) r2 += -8
- 3: (b7) r1 = 1
- 4: (85) call 1
- 5: (15) if r0 == 0x0 goto pc+1
- R0=map_ptr R10=fp
- 6: (7a) *(u64 *)(r0 +4) = 0
- misaligned access off 4 size 8
-
-Program that correctly checks map_lookup_elem() returned value for NULL and
-accesses memory with correct alignment in one side of 'if' branch, but fails
-to do so in the other side of 'if' branch:
- BPF_ST_MEM(BPF_DW, BPF_REG_10, -8, 0),
- BPF_MOV64_REG(BPF_REG_2, BPF_REG_10),
- BPF_ALU64_IMM(BPF_ADD, BPF_REG_2, -8),
- BPF_LD_MAP_FD(BPF_REG_1, 0),
- BPF_RAW_INSN(BPF_JMP | BPF_CALL, 0, 0, 0, BPF_FUNC_map_lookup_elem),
- BPF_JMP_IMM(BPF_JEQ, BPF_REG_0, 0, 2),
- BPF_ST_MEM(BPF_DW, BPF_REG_0, 0, 0),
- BPF_EXIT_INSN(),
- BPF_ST_MEM(BPF_DW, BPF_REG_0, 0, 1),
- BPF_EXIT_INSN(),
-Error:
- 0: (7a) *(u64 *)(r10 -8) = 0
- 1: (bf) r2 = r10
- 2: (07) r2 += -8
- 3: (b7) r1 = 1
- 4: (85) call 1
- 5: (15) if r0 == 0x0 goto pc+2
- R0=map_ptr R10=fp
- 6: (7a) *(u64 *)(r0 +0) = 0
- 7: (95) exit
-
- from 5 to 8: R0=imm0 R10=fp
- 8: (7a) *(u64 *)(r0 +0) = 1
- R0 invalid mem access 'imm'
-
-Testing
--------
-
-Next to the BPF toolchain, the kernel also ships a test module that contains
-various test cases for classic and internal BPF that can be executed against
-the BPF interpreter and JIT compiler. It can be found in lib/test_bpf.c and
-enabled via Kconfig:
-
- CONFIG_TEST_BPF=m
-
-After the module has been built and installed, the test suite can be executed
-via insmod or modprobe against 'test_bpf' module. Results of the test cases
-including timings in nsec can be found in the kernel log (dmesg).
-
-Misc
-----
-
-Also trinity, the Linux syscall fuzzer, has built-in support for BPF and
-SECCOMP-BPF kernel fuzzing.
-
-Written by
-----------
-
-The document was written in the hope that it is found useful and in order
-to give potential BPF hackers or security auditors a better overview of
-the underlying architecture.
-
-Jay Schulist <jschlst@...ba.org>
-Daniel Borkmann <daniel@...earbox.net>
-Alexei Starovoitov <ast@...nel.org>
--
2.17.1
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