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Date: Thu, 23 Jul 2015 11:31:13 +0200
From: Daniel Borkmann <daniel@...earbox.net>
To: "Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>,
Alexei Starovoitov <ast@...mgrid.com>
CC: linux-man <linux-man@...r.kernel.org>,
linux-kernel@...r.kernel.org, Silvan Jegen <s.jegen@...il.com>,
Walter Harms <wharms@....de>
Subject: Re: Draft 3 of bpf(2) man page for review
Hi Michael,
looks good already, a couple of comments inline, on top of Alexei's feedback:
On 07/22/2015 10:10 PM, Michael Kerrisk (man-pages) wrote:
...
> NAME
> bpf - perform a command on an extended eBPF map or program
'extended eBPF' should perhaps just say 'eBPF' or 'extended BPF' (the
'e' itself stands for 'extended')
> SYNOPSIS
> #include <linux/bpf.h>
>
> int bpf(int cmd, union bpf_attr *attr, unsigned int size);
>
> DESCRIPTION
> The bpf() system call performs a range of operations related to
> extended Berkeley Packet Filters. Extended BPF (or eBPF) is sim‐
> ilar to the original ("classic") BPF (cBPF) used to filter net‐
> work packets. For both cBPF and eBPF programs, the kernel stati‐
> cally analyzes the programs before loading them, in order to
> ensure that they cannot harm the running system.
>
> eBPF extends cBPF in multiple ways, including the ability to call
> a fixed set of in-kernel helper functions (via the BPF_CALL
> opcode extension provided by eBPF) and access shared data struc‐
> tures such as eBPF maps.
>
> Extended BPF Design/Architecture
> BPF maps are a generic data structure for storage of different
Maybe s/BPF/eBPF/ as we introduced its definition above and used 'eBPF maps'
just in the previous sentence. (I would from the onwards just use either eBPF
or cBPF, makes it probably more clear).
> data types. A user process can create multiple maps (with
> key/value-pairs being opaque bytes of data) and access them via
> file descriptors. Differnt eBPF programs can access the same
> maps in parallel. It's up to the user process and eBPF program
> to decide what they store inside maps.
>
> eBPF programs are similar to kernel modules. They are loaded by
> the user process and automatically unloaded when the process
> exits. Each program is a set of instructions that is safe to run
The 1st and 2nd sentence in that order/combination may sounds a bit weird.
Maybe I would just drop the first sentence? I would argue that there might
be a few similarities, but more differences overall. So I guess we'd either
need to elaborate on the 1st sentence or just leave it out (could perhaps
be a FIXME comment to later on introduce a new section that elaborates on
both?).
> until its completion. An in-kernel verifier statically deter‐
> mines that the eBPF program terminates and is safe to execute.
> During verification, the kernel increments reference counts for
> each of the maps that the eBPF program uses, so that the selected
> maps cannot be removed until the program is unloaded.
s/selected/attached/ ? Btw, a user obviously can close() the map fds if he
wants to, but ultimatively they're freed when the program unloads.
> eBPF programs can be attached to different events. These events
> can be the arrival of network packets, tracing events, classifi‐
> cation event by qdisc (for eBPF programs attached to a tc(8)
> classifier), and other types that may be added in the future. A
Maybe: classification events by network queuing disciplines
> new event triggers execution of the eBPF program, which may store
> information about the event in eBPF maps. Beyond storing data,
> eBPF programs may call a fixed set of in-kernel helper functions.
I think this was mentioned before, but ok.
> The same eBPF program can be attached to multiple events and dif‐
> ferent eBPF programs can access the same map:
>
> tracing tracing tracing packet packet
> event A event B event C on eth0 on eth1
> | | | | |
> | | | | |
> --> tracing <-- tracing socket socket
> prog_1 prog_2 prog_3 prog_4
> | | | |
> |--- -----| |-------| map_3
> map_1 map_2
Maybe prog_4 example could also be: s/socket/tc ingress classifier/ ;)
> Arguments
> The operation to be performed by the bpf() system call is deter‐
> mined by the cmd argument. Each operation takes an accompanying
> argument, provided via attr, which is a pointer to a union of
> type bpf_attr (see below). The size argument is the size of the
> union pointed to by attr.
>
> The value provided in cmd is one of the following:
>
> BPF_MAP_CREATE
> Create a map with and return a file descriptor that refers
> to the map.
'Create a map with and'
> BPF_MAP_LOOKUP_ELEM
> Look up an element by key in a specified map and return
> its value.
>
> BPF_MAP_UPDATE_ELEM
> Create or update an element (key/value pair) in a speci‐
> fied map.
>
> BPF_MAP_DELETE_ELEM
> Look up and delete an element by key in a specified map.
>
> BPF_MAP_GET_NEXT_KEY
> Look up an element by key in a specified map and return
> the key of the next element.
>
> BPF_PROG_LOAD
> Verify and load an eBPF program, returning a new file
> descriptor associated with the program.
>
> The bpf_attr union consists of various anonymous structures that
> are used by different bpf() commands:
>
> union bpf_attr {
> struct { /* Used by BPF_MAP_CREATE */
> __u32 map_type;
> __u32 key_size; /* size of key in bytes */
> __u32 value_size; /* size of value in bytes */
> __u32 max_entries; /* maximum number of entries
> in a map */
> };
>
> struct { /* Used by BPF_MAP_*_ELEM and BPF_MAP_GET_NEXT_KEY
> commands */
> __u32 map_fd;
> __aligned_u64 key;
> union {
> __aligned_u64 value;
> __aligned_u64 next_key;
> };
> __u64 flags;
> };
>
> struct { /* Used by BPF_PROG_LOAD */
> __u32 prog_type;
> __u32 insn_cnt;
> __aligned_u64 insns; /* 'const struct bpf_insn *' */
> __aligned_u64 license; /* 'const char *' */
> __u32 log_level; /* verbosity level of verifier */
> __u32 log_size; /* size of user buffer */
> __aligned_u64 log_buf; /* user supplied 'char *'
> buffer */
> __u32 kern_version;
> /* checked when prog_type=kprobe
> (since Linux 4.1) */
> };
> } __attribute__((aligned(8)));
>
> eBPF maps
> Maps are a generic data structure for storage of different types
> of data. They allow sharing of data between eBPF kernel pro‐
> grams, and also between kernel and user-space applications.
>
> Each map type has the following attributes:
>
> * type
> * maximum number of elements
> * key size in bytes
> * value size in bytes
>
> The following wrapper functions demonstrate how various bpf()
> commands can be used to access the maps. The functions use the
> cmd argument to invoke different operations.
>
> BPF_MAP_CREATE
> The BPF_MAP_CREATE command creates a new map, returning a
> new file descriptor that refers to the map.
>
> int
> bpf_create_map(enum bpf_map_type map_type, int key_size,
> int value_size, int max_entries)
key_size, value_size and max_entries could rather be 'unsigned int' in
this API example.
> {
> union bpf_attr attr = {
> .map_type = map_type,
> .key_size = key_size,
> .value_size = value_size,
> .max_entries = max_entries
> };
>
> return bpf(BPF_MAP_CREATE, &attr, sizeof(attr));
> }
>
> The new map has the type specified by map_type, and
> attributes as specified in key_size, value_size, and
> max_entries. On success, this operation returns a file
> descriptor. On error, -1 is returned and errno is set to
> EINVAL, EPERM, or ENOMEM.
>
> The attributes key_size and value_size will be used by the
attribute's?
> verifier during program loading to check that the program
> is calling bpf_map_*_elem() helper functions with a cor‐
> rectly initialized key and to check that the program
> doesn't access the map element value beyond the specified
> value_size. For example, when a map is created with a
> key_size of 8 and the eBPF program calls
>
> bpf_map_lookup_elem(map_fd, fp - 4)
>
> the program will be rejected, since the in-kernel helper
> function
>
> bpf_map_lookup_elem(map_fd, void *key)
>
> expects to read 8 bytes from the location pointed to by
> key, but the fp - 4 (where fp is the top of the stack)
> starting address will cause out-of-bounds stack access.
>
> Similarly, when a map is created with a value_size of 1
> and the eBPF program contains
>
> value = bpf_map_lookup_elem(...);
> *(u32 *) value = 1;
>
> the program will be rejected, since it accesses the value
> pointer beyond the specified 1 byte value_size limit.
>
> Currently, the following values are supported for
> map_type:
>
> enum bpf_map_type {
> BPF_MAP_TYPE_UNSPEC, /* Reserve 0 as invalid map type */
> BPF_MAP_TYPE_HASH,
> BPF_MAP_TYPE_ARRAY,
> BPF_MAP_TYPE_PROG_ARRAY,
> };
>
> map_type selects one of the available map implementations
> in the kernel. For all map types, eBPF programs access
> maps with the same bpf_map_lookup_elem() and
> bpf_map_update_elem() helper functions. Further details
> of the various map types are given below.
>
> BPF_MAP_LOOKUP_ELEM
> The BPF_MAP_LOOKUP_ELEM command looks up an element with a
> given key in the map referred to by the file descriptor
> fd.
>
> int
> bpf_lookup_elem(int fd, void *key, void *value)
It's just an API example implementation, and we cast the const away
in ptr_to_u64() [which is not provided here, that's ok], but it documents
the API itself better for those who implement it. I did the same in
iproute2's tc/tc_bpf.c:
const void *key
> {
> union bpf_attr attr = {
> .map_fd = fd,
> .key = ptr_to_u64(key),
> .value = ptr_to_u64(value),
> };
>
> return bpf(BPF_MAP_LOOKUP_ELEM, &attr, sizeof(attr));
> }
>
> If an element is found, the operation returns zero and
> stores the element's value into value, which must point to
> a buffer of value_size bytes.
>
> If no element is found, the operation returns -1 and sets
> errno to ENOENT.
>
> BPF_MAP_UPDATE_ELEM
> The BPF_MAP_UPDATE_ELEM command creates or updates an ele‐
> ment with a given key/value in the map referred to by the
> file descriptor fd.
>
> int
> bpf_update_elem(int fd, void *key, void *value, __u64 flags)
> {
const void *key, const void *value, uint64_t flags
The type __u64 is kernel internal, so if there's no strict reason to use it,
we should just use what's provided by stdint.h.
> union bpf_attr attr = {
> .map_fd = fd,
> .key = ptr_to_u64(key),
> .value = ptr_to_u64(value),
> .flags = flags,
> };
>
> return bpf(BPF_MAP_UPDATE_ELEM, &attr, sizeof(attr));
> }
>
> The flags argument should be specified as one of the fol‐
> lowing:
>
> BPF_ANY
> Create a new element or update an existing element.
>
> BPF_NOEXIST
> Create a new element only if it did not exist.
>
> BPF_EXIST
> Update an existing element.
>
> On success, the operation returns zero. On error, -1 is
> returned and errno is set to EINVAL, EPERM, ENOMEM, or
> E2BIG. E2BIG indicates that the number of elements in the
> map reached the max_entries limit specified at map cre‐
> ation time. EEXIST will be returned if flags specifies
> BPF_NOEXIST and the element with key already exists in the
> map. ENOENT will be returned if flags specifies BPF_EXIST
> and the element with key doesn't exist in the map.
>
> BPF_MAP_DELETE_ELEM
> The BPF_MAP_DELETE_ELEM command deleted the element whose
> key is key from the map referred to by the file descriptor
> fd.
>
> int
> bpf_delete_elem(int fd, void *key)
const void *key
> {
> union bpf_attr attr = {
> .map_fd = fd,
> .key = ptr_to_u64(key),
> };
>
> return bpf(BPF_MAP_DELETE_ELEM, &attr, sizeof(attr));
> }
>
> On success, zero is returned. If the element is not
> found, -1 is returned and errno is set to ENOENT.
>
> BPF_MAP_GET_NEXT_KEY
> The BPF_MAP_GET_NEXT_KEY command looks up an element by
> key in the map referred to by the file descriptor fd and
> sets the next_key pointer to the key of the next element.
>
> int
> bpf_get_next_key(int fd, void *key, void *next_key)
> {
const void *key
> union bpf_attr attr = {
> .map_fd = fd,
> .key = ptr_to_u64(key),
> .next_key = ptr_to_u64(next_key),
> };
>
> return bpf(BPF_MAP_GET_NEXT_KEY, &attr, sizeof(attr));
> }
>
> If key is found, the operation returns zero and sets the
> next_key pointer to the key of the next element. If key
> is not found, the operation returns zero and sets the
> next_key pointer to the key of the first element. If key
> is the last element, -1 is returned and errno is set to
> ENOENT. Other possible errno values are ENOMEM, EFAULT,
> EPERM, and EINVAL. This method can be used to iterate
> over all elements in the map.
>
> close(map_fd)
> Delete the map referred to by the file descriptor map_fd.
> When the user-space program that created a map exits, all
> maps will be deleted automatically (but see NOTES).
>
> eBPF map types
> The following map types are supported:
>
> BPF_MAP_TYPE_HASH
> Hash-table maps have the following characteristics:
>
> * Maps are created and destroyed by user-space programs.
> Both user-space and eBPF programs can perform lookup,
> update, and delete operations.
>
> * The kernel takes care of allocating and freeing
> key/value pairs.
>
> * The map_update_elem() helper with fail to insert new
> element when the max_entries limit is reached. (This
> ensures that eBPF programs cannot exhaust memory.)
>
> * map_update_elem() replaces existing elements atomi‐
> cally.
>
> Hash-table maps are optimized for speed of lookup.
>
> BPF_MAP_TYPE_ARRAY
> Array maps have the following characteristics:
>
> * Optimized for fastest possible lookup. In the future
> the verifier/JIT compiler may recognize lookup() opera‐
> tions that employ a constant key and optimize it into
> constant pointer. It is possible to optimize a non-
> constant key into direct pointer arithmetic as well,
> since pointers and value_size are constant for the life
> of the eBPF program. In other words,
> array_map_lookup_elem() may be 'inlined' by the veri‐
> fier/JIT compiler while preserving concurrent access to
> this map from user space.
>
> * All array elements pre-allocated and zero initialized
> at init time
>
> * The key is an array index, and must be exactly four
> bytes.
>
> * map_delete_elem() fails with the error EINVAL, since
> elements cannot be deleted.
>
> * map_update_elem() replaces elements in an non-atomic
> fashion; for atomic updates, a hash-table map should be
> used instead.
This point here is most important, i.e. to not have false user expecations.
Maybe it's also worth mentioning that when you have a value_size of sizeof(long),
you can however use __sync_fetch_and_add() atomic builtin from the LLVM backend.
> Among the uses for array maps are the following:
>
> * As "global" eBPF variables: an array of 1 element whose
> key is (index) 0 and where the value is a collection of
> 'global' variables which eBPF programs can use to keep
> state between events.
>
> * Aggregation of tracing events into a fixed set of buck‐
> ets.
>
> BPF_MAP_TYPE_PROG_ARRAY (since Linux 4.2)
> [To be completed]
>
> eBPF programs
> The BPF_PROG_LOAD command is used to load an eBPF program into
> the kernel. The return value for this command is a new file
> descriptor associated with this eBPF program.
>
> char bpf_log_buf[LOG_BUF_SIZE];
>
> int
> bpf_prog_load(enum bpf_prog_type prog_type,
> const struct bpf_insn *insns, int insn_cnt,
> const char *license)
Maybe:
int bpf_prog_load(enum bpf_prog_type type, const struct bpf_insn *insns,
unsigned int num_insns, const char *license)
[ The double prog_type is redundant. ]
> {
> union bpf_attr attr = {
> .prog_type = prog_type,
> .insns = ptr_to_u64(insns),
> .insn_cnt = insn_cnt,
> .license = ptr_to_u64(license),
> .log_buf = ptr_to_u64(bpf_log_buf),
> .log_size = LOG_BUF_SIZE,
> .log_level = 1,
> };
Would be nice to have this indented properly, I mean that all should
be aligned with tab before '='. That would make it much easier to read.
Also for all other code examples in this man-page (I forgot to mention
it for the above).
>
> return bpf(BPF_PROG_LOAD, &attr, sizeof(attr));
> }
>
> prog_type is one of the available program types:
>
> enum bpf_prog_type {
> BPF_PROG_TYPE_UNSPEC, /* Reserve 0 as invalid
> program type */
A pity that these *_UNSPEC types (also for the map) had to make it
into the uapi. :(
> BPF_PROG_TYPE_SOCKET_FILTER,
> BPF_PROG_TYPE_KPROBE,
> BPF_PROG_TYPE_SCHED_CLS,
> BPF_PROG_TYPE_SCHED_ACT,
> };
>
> For further details of eBPF program types, see below.
>
> The remaining fields of bpf_attr are set as follows:
>
> * insns is an array of struct bpf_insn instructions.
>
> * insn_cnt is the number of instructions in the program referred
> to by insns.
>
> * license is a license string, which must be GPL compatible to
> call helper functions marked gpl_only.
Not strictly. So here, the same rules apply as with kernel modules. I.e. what
the kernel checks for are the following license strings:
static inline int license_is_gpl_compatible(const char *license)
{
return (strcmp(license, "GPL") == 0
|| strcmp(license, "GPL v2") == 0
|| strcmp(license, "GPL and additional rights") == 0
|| strcmp(license, "Dual BSD/GPL") == 0
|| strcmp(license, "Dual MIT/GPL") == 0
|| strcmp(license, "Dual MPL/GPL") == 0);
}
With any of them, the eBPF program is declared GPL compatible. Maybe of interest
for those that want to use dual licensing of some sort.
> * log_buf is a pointer to a caller-allocated buffer in which the
> in-kernel verifier can store the verification log. This log
> is a multi-line string that can be checked by the program
> author in order to understand how the verifier came to the
> conclusion that the BPF program is unsafe. The format of the
> output can change at any time as the verifier evolves.
>
> * log_size size of the buffer pointed to by log_bug. If the
> size of the buffer is not large enough to store all verifier
> messages, -1 is returned and errno is set to ENOSPC.
>
> * log_level verbosity level of the verifier. A value of zero
> means that the verifier will not provide a log.
Note that the log buffer is optional as mentioned here log_level = 0. The
above example code of bpf_prog_load() suggests that it always needs to be
provided.
I once ran indeed into an issue where the program itself was correct, but
it got rejected by the kernel, because my log buffer size was too small, so
in tc, we now have it larger as bpf_log_buf[65536] ...
> Applying close(2) to the file descriptor returned by
> BPF_PROG_LOAD will unload the eBPF program (but see NOTES).
>
> Maps are accessible from eBPF programs and are used to exchange
> data between eBPF programs and between eBPF programs and user-
> space programs. For example, eBPF programs can process various
> events (like kprobe, packets) and store their data into a map,
> and user-space programs can then fetch data from the map. Con‐
> versely, user-space programs can use a map as a configuration
> mechanism, populating the map with values checked by the eBPF
> program, which then modifies its behavior on the fly according to
> those values.
>
> eBPF program types
> By picking prog_type, the program author selects a set of helper
> functions that can be called from the eBPF program and the corre‐
> sponding format of struct bpf_context (which is the data blob
> passed into the eBPF program as the first argument). For exam‐
I had to read this twice. ;) Maybe this needs to be reworded slightly.
It just means that depending on the program type that the author selects,
you might end up with a different subset of helper functions, and a
different program input/context. For example tracing does not have the
exact same helpers as socket filters (it might have some that can be used
by both). Also, the eBPF program input (context) for socket filters is a
network packet, wheras for tracing you operate on a set of registers.
> ple, programs loaded with a prog_type of
> BPF_PROG_TYPE_SOCKET_FILTER may call the bpf_map_lookup_elem()
> helper, whereas some other program types may not be able to
> employ this helper. The set of functions available to eBPF pro‐
> grams of a given type may increase in the future.
>
> The following program types are supported:
>
> BPF_PROG_TYPE_SOCKET_FILTER (since Linux 3.19)
> Currently, the set of functions for
> BPF_PROG_TYPE_SOCKET_FILTER is:
>
> bpf_map_lookup_elem(map_fd, void *key)
> /* look up key in a map_fd */
> bpf_map_update_elem(map_fd, void *key, void *value)
> /* update key/value */
> bpf_map_delete_elem(map_fd, void *key)
> /* delete key in a map_fd */
>
> The bpf_context argument is a pointer to a struct sk_buff.
> Programs cannot access the fields of sk_buff directly.
>
> BPF_PROG_TYPE_KPROBE (since Linux 4.1)
> [To be documented]
>
> BPF_PROG_TYPE_SCHED_CLS (since Linux 4.1)
> [To be documented]
>
> BPF_PROG_TYPE_SCHED_ACT (since Linux 4.1)
> [To be documented]
>
> Events
> Once a program is loaded, it can be attached to an event. Vari‐
> ous kernel subsystems have different ways to do so.
>
> Since Linux 3.19, the following call will attach the program
> prog_fd to the socket sockfd, which was created by an earlier
> call to socket(2):
>
> setsockopt(sockfd, SOL_SOCKET, SO_ATTACH_BPF,
> &prog_fd, sizeof(prog_fd));
>
> Since Linux 4.1, the following call may be used to attach the
> eBPF program referred to by the file descriptor prog_fd to a perf
> event file descriptor, event_fd, that was created by a previous
> call to perf_event_open(2):
>
> ioctl(event_fd, PERF_EVENT_IOC_SET_BPF, prog_fd);
>
> EXAMPLES
> /* bpf+sockets example:
> * 1. create array map of 256 elements
> * 2. load program that counts number of packets received
> * r0 = skb->data[ETH_HLEN + offsetof(struct iphdr, protocol)]
> * map[r0]++
> * 3. attach prog_fd to raw socket via setsockopt()
> * 4. print number of received TCP/UDP packets every second
> */
> int
> main(int argc, char **argv)
> {
> int sock, map_fd, prog_fd, key;
> long long value = 0, tcp_cnt, udp_cnt;
>
> map_fd = bpf_create_map(BPF_MAP_TYPE_ARRAY, sizeof(key),
> sizeof(value), 256);
> if (map_fd < 0) {
> printf("failed to create map '%s'\n", strerror(errno));
> /* likely not run as root */
> return 1;
> }
>
> struct bpf_insn prog[] = {
> BPF_MOV64_REG(BPF_REG_6, BPF_REG_1), /* r6 = r1 */
> BPF_LD_ABS(BPF_B, ETH_HLEN + offsetof(struct iphdr, protocol)),
> /* r0 = ip->proto */
> BPF_STX_MEM(BPF_W, BPF_REG_10, BPF_REG_0, -4),
> /* *(u32 *)(fp - 4) = r0 */
> BPF_MOV64_REG(BPF_REG_2, BPF_REG_10), /* r2 = fp */
> BPF_ALU64_IMM(BPF_ADD, BPF_REG_2, -4), /* r2 = r2 - 4 */
> BPF_LD_MAP_FD(BPF_REG_1, map_fd), /* r1 = map_fd */
> BPF_CALL_FUNC(BPF_FUNC_map_lookup_elem),
> /* r0 = map_lookup(r1, r2) */
> BPF_JMP_IMM(BPF_JEQ, BPF_REG_0, 0, 2),
> /* if (r0 == 0) goto pc+2 */
> BPF_MOV64_IMM(BPF_REG_1, 1), /* r1 = 1 */
> BPF_XADD(BPF_DW, BPF_REG_0, BPF_REG_1, 0, 0),
> /* lock *(u64 *) r0 += r1 */
> BPF_MOV64_IMM(BPF_REG_0, 0), /* r0 = 0 */
> BPF_EXIT_INSN(), /* return r0 */
> };
>
> prog_fd = bpf_prog_load(BPF_PROG_TYPE_SOCKET_FILTER, prog,
> sizeof(prog), "GPL");
>
> sock = open_raw_sock("lo");
>
> assert(setsockopt(sock, SOL_SOCKET, SO_ATTACH_BPF, &prog_fd,
> sizeof(prog_fd)) == 0);
>
> for (;;) {
> key = IPPROTO_TCP;
> assert(bpf_lookup_elem(map_fd, &key, &tcp_cnt) == 0);
> key = IPPROTO_UDP
> assert(bpf_lookup_elem(map_fd, &key, &udp_cnt) == 0);
> printf("TCP %lld UDP %lld packets0, tcp_cnt, udp_cnt);
> sleep(1);
> }
>
> return 0;
> }
>
> Some complete working code can be found in the samples/bpf direc‐
> tory in the kernel source tree.
>
> RETURN VALUE
> For a successful call, the return value depends on the operation:
>
> BPF_MAP_CREATE
> The new file descriptor associated with the eBPF map.
>
> BPF_PROG_LOAD
> The new file descriptor associated with the eBPF program.
>
> All other commands
> Zero.
>
> On error, -1 is returned, and errno is set appropriately.
>
> ERRORS
> EPERM The call was made without sufficient privilege (without
> the CAP_SYS_ADMIN capability).
>
> ENOMEM Cannot allocate sufficient memory.
>
> EBADF fd is not an open file descriptor
>
> EFAULT One of the pointers (key or value or log_buf or insns) is
> outside the accessible address space.
>
> EINVAL The value specified in cmd is not recognized by this ker‐
> nel.
>
> EINVAL For BPF_MAP_CREATE, either map_type or attributes are
> invalid.
>
> EINVAL For BPF_MAP_*_ELEM commands, some of the fields of union
> bpf_attr that are not used by this command are not set to
> zero.
>
> EINVAL For BPF_PROG_LOAD, indicates an attempt to load an invalid
> program. BPF programs can be deemed einvalid due to
> unrecognized instructions, the use of reserved fields,
> jumps out of range, infinite loops or calls of unknown
> functions.
>
> EACCES For BPF_PROG_LOAD, even though all program instructions
> are valid, the program has been rejected because it was
> deemed unsafe. This may be because it may have accessed a
> disallowed memory region or an uninitialized stack/regis‐
> ter or because the function constraints don't match the
> actual types or because there was a misaligned memory
> access. In this case, it is recommended to call bpf()
> again with log_level = 1 and examine log_buf for the spe‐
> cific reason provided by the verifier.
>
> ENOENT For BPF_MAP_LOOKUP_ELEM or BPF_MAP_DELETE_ELEM, indicates
> that the element with the given key was not found.
>
> E2BIG The BPF program is too large or a map reached the
> max_entries limit (maximum number of elements).
>
> VERSIONS
> The bpf() system call first appeared in Linux 3.18.
>
> CONFORMING TO
> The bpf() system call is Linux-specific.
>
> NOTES
> In the current implementation, all bpf() commands require the
> caller to have the CAP_SYS_ADMIN capability.
>
> eBPF objects (maps and programs) can be shared between processes.
> For example, after fork(2), the child inherits file descriptors
> referring to the same eBPF objects. In addition, file descrip‐
> tors referring to eBPF objects can be transferred over UNIX
> domain sockets. File descriptors referring to eBPF objects can
> be duplicated in the usual way, using dup(2) and similar calls.
> An eBPF object is deallocated only after all file descriptors
> referring to the object have been closed.
>
> eBPF programs can be written in a restricted C that is compiled
> (using the clang compiler) into eBPF bytecode and executed on the
> in-kernel virtual machine or just-in-time compiled into native
> code. (Various features are omitted from this restricted C, such
> as loops, global variables, variadic functions, floating-point
> numbers, and passing structures as function arguments.) Some
> examples can be found in the samples/bpf/*_kern.c files in the
> kernel source tree.
I would also make a note about the JIT compiler here, i.e. that it's disabled
by default, and can be enabled via:
* Normal mode: echo 1 > /proc/sys/net/core/bpf_jit_enable
* Debugging mode: echo 2 > /proc/sys/net/core/bpf_jit_enable
[opcodes dumped in hex into the kernel log, which can then be disassembled
with tools/net/bpf_jit_disasm.c from the kernel tree]
When enabled, after a eBPF program gets loaded, it's transparently compiled /
translated inside the kernel into machine opcodes for better performance,
currently on x86_64, arm64 and s390.
> SEE ALSO
> seccomp(2), socket(7), tc(8), tc-bpf(8)
>
> Both classic and extended BPF are explained in the kernel source
> file Documentation/networking/filter.txt.
>
Thanks for all the work!
Cheers,
Daniel
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