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Date: Wed, 22 Jul 2015 22:10:39 +0200
From: "Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>
To: Alexei Starovoitov <ast@...mgrid.com>,
Daniel Borkmann <daniel@...earbox.net>
CC: mtk.manpages@...il.com, 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
On 07/22/2015 09:22 PM, Alexei Starovoitov wrote:
> On 7/22/15 11:43 AM, Michael Kerrisk (man-pages) wrote:
>> .TH BPF 2 2015-03-10 "Linux" "Linux Programmer's Manual"
>
> should the date be updated ?
It'll get updated later, by scripts.
>> BPF maps are a generic data structure for storage of different data types.
>> A user process can create multiple maps (with key/value-pairs being
>> opaque bytes of data) and access them via file descriptors.
>> eBPF programs can access maps from inside the kernel in parallel.
>> .\"
>> .\" FIXME!! What does the previous sentence mean?
>> .\"
>> .\" Isn't "from inside the kernel" redundant? (I mean: all eBPF programs
>> .\" are running inside the kernel, right?)
>
> 99.9% of the time. yes. all eBPF programs are running inside the kernel,
> though recently I've seen two versions of 'user space eBPF' where
> kernel interpreter/x64_jit were ported to user space.
> If you think 'from kernel' is redundant, just drop it.
Okay. Done.
>> .\" And what does "in parallel" mean?
>> .\" Would a simpler version of this sentence be correct? As in:
>> .\" "Different eBPF programs can access the same maps in parallel."
>
> yes. different eBPF programs and user space processes can access the
> same maps in parallel.
Okay.
>> The new map has the type specified by
>> .IR map_type ,
>> and attributes as specified in
>> .IR key_size ,
>> .IR value_size ,
>> and
>> .IR max_entries .
>> .\" FIXME!! In the next sentence, what does "process-local" mean?
>> On success, this operation returns a process-local file descriptor.
>
> Just drop this unnecessary qualifier. Just 'returns a file descriptor'
Done.
>> .in +4n
>> .nf
>> bpf_map_lookup_elem(map_fd, fp - 4)
>> .fi
>> .in
>>
>> 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
>> .I key
>> pointer, but
>> .IR "fp\ -\ 4"
>> .\" FIXME!! I'm lost! What is 'fp' in this context?
>
> it refers to 2nd argument of 'bpf_map_lookup_elem(map_fd, fp - 4)'
> fp = top of the stack.
> fp - 4 = pointer to 4 bytes below top of the stack.
> So 8 byte access from there will be out of bounds.
Okay. I added some words mentioning that 'fp' is top of stack.
>> The following map types are supported:
>> .TP
>> .B BPF_MAP_TYPE_HASH
>> .\" commit 0f8e4bd8a1fc8c4185f1630061d0a1f2d197a475
>> .\" FIXME!! Please review the following list of points, which draws
>> .\" heavily from the commit message, but reworks the text significantly
>> .\" and so may have introduced errors.
>> Hash-table maps have the following characteristics:
>> .RS
>> .IP * 3
>> Maps are created and destroyed by user-space programs.
>> Both user-space and eBPF programs
>> can perform lookuo, update, and delete operations.
>
> typo 'lookup'
Thanks, fixed.
>> .IP *
>> The kernel takes care of allocating and freeing key/value pairs.
>> .IP *
>> The
>> .BR map_update_elem ()
>> helper with fail to insert new element when the
>> .I max_entries
>> limit is reached.
>> (This ensures that eBPF programs cannot exhaust memory.)
>> .IP *
>> .BR map_update_elem ()
>> replaces existing elements atomically.
>> .RE
>> .IP
>> Hash-table maps are
>> optimized for speed of lookup.
>> .TP
>> .B BPF_MAP_TYPE_ARRAY
>> .\" commit 28fbcfa08d8ed7c5a50d41a0433aad222835e8e3
>> .\" FIXME!! Please review the following list of points, which draws
>> .\" heavily from the commit message, but reworks the text significantly
>> .\" and so may have introduced errors.
>> Array maps have the following characteristics:
>> .RS
>> .IP * 3
>> Optimized for fastest possible lookup.
>> In the future ithe verifier/JIT compiler
>
> typo 'the'
Fixed.
>> may recognize lookup() operations 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
>> .I value_size
>> are constant for the life of the eBPF program.
>> In other words,
>> .BR array_map_lookup_elem ()
>> may be 'inlined' by the verifier/JIT compiler
>> while preserving concurrent access to this map from user space.
>> .IP *
>> All array elements pre-allocated and zero initialized at init time
>> .IP *
>> The key is an array index, and must be exactly four bytes.
>> .IP *
>> .BR map_delete_elem ()
>> fails with the error
>> .BR EINVAL ,
>> since elements cannot be deleted.
>> .IP *
>> .BR map_update_elem ()
>> replaces elements in an non-atomic fashion;
>> for atomic updates, a hash-table map should be used instead.
>
> the description of hash and array maps looks good.
Okay. Thanks for checking.
>> .\" FIXME The following paragraph needs amending. Alexei commented:
>> .\"
>> .\" Actually now in case of SOCKET_FILTER, SCHED_CLS, SCHED_ACT
>> .\" the program can now access skb fields.
>> .\" See 'struct __sk_buff' and commit 9bac3d6d548e5
>> .\"
>> .\" Do we want some text here to explain how the program access __sk_buff?
>
> I think commit 9bac3d6d548e5 tried to explain it, but translating
> that to english would be nice :)
Yes, but my C-to-English translator failed.
>> .\" FIXME!! Alexei, is the following correct?
>> eBPF objects (maps and programs) can be shared between processes.
>> For example, after
>> .BR fork (2),
>> the child inherits file descriptors referring to the same eBPF objects.
>> In addition, file descriptors 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
>> .BR dup (2)
>> and similar calls.
>> An eBPF object is deallocated only after all file descriptors
>> referring to the object have been closed.
>
> yes. all correct.
Thanks.
>> eBPF programs can be written in a restricted C that is compiled (using the
>> .B 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
>> .I samples/bpf/*_kern.c
>> files in the kernel source tree.
>
> thanks. whole thing looks good.
Thanks.
Below is the current rendered version of the man page.
Cheers,
Michael
NAME
bpf - perform a command on an extended eBPF map or program
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
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
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.
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
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.
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
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.
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)
{
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
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)
{
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)
{
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)
{
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)
{
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.
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)
{
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,
};
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 */
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.
* 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.
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‐
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.
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.
--
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
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