bpf.2

.\" Copyright (C) 2015 Alexei Starovoitov <ast@kernel.org>
.\" and Copyright (C) 2015 Michael Kerrisk <mtk.manpages@gmail.com>
.\"
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.\"
.TH BPF 2 2021-08-27 "Linux" "Linux Programmer's Manual"
.SH NAME
bpf \- perform a command on an extended BPF map or program
.SH SYNOPSIS
.nf
.B #include <linux/bpf.h>
.PP
.BI "int bpf(int " cmd ", union bpf_attr *" attr ", unsigned int " size );
.fi
.SH DESCRIPTION
The
.BR bpf ()
system call performs a range of operations related to extended
Berkeley Packet Filters.
Extended BPF (or eBPF) is similar to
the original ("classic") BPF (cBPF) used to filter network packets.
For both cBPF and eBPF programs,
the kernel statically analyzes the programs before loading them,
in order to ensure that they cannot harm the running system.
.PP
eBPF extends cBPF in multiple ways, including the ability to call
a fixed set of in-kernel helper functions
.\" See 'enum bpf_func_id' in include/uapi/linux/bpf.h
(via the
.B BPF_CALL
opcode extension provided by eBPF)
and access shared data structures such as eBPF maps.
.\"
.SS Extended BPF Design/Architecture
eBPF maps are a generic data structure for storage of different data types.
Data types are generally treated as binary blobs, so a user just specifies
the size of the key and the size of the value at map-creation time.
In other words, a key/value for a given map can have an arbitrary structure.
.PP
A user process can create multiple maps (with key/value-pairs being
opaque bytes of data) and access them via file descriptors.
Different 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.
.PP
There's one special map type, called a program array.
This type of map stores file descriptors referring to other eBPF programs.
When a lookup in the map is performed, the program flow is
redirected in-place to the beginning of another eBPF program and does not
return back to the calling program.
The level of nesting has a fixed limit of 32,
.\" Defined by the kernel constant MAX_TAIL_CALL_CNT in include/linux/bpf.h
so that infinite loops cannot be crafted.
At run time, the program file descriptors stored in the map can be modified,
so program functionality can be altered based on specific requirements.
All programs referred to in a program-array map must
have been previously loaded into the kernel via
.BR bpf ().
If a map lookup fails, the current program continues its execution.
See
.B BPF_MAP_TYPE_PROG_ARRAY
below for further details.
.PP
Generally, eBPF programs are loaded by the user process and automatically
unloaded when the process exits.
In some cases, for example,
.BR tc\-bpf (8),
the program will continue to stay alive inside the kernel even after the
process that loaded the program exits.
In that case,
the tc subsystem holds a reference to the eBPF program after the
file descriptor has been closed by the user-space program.
Thus, whether a specific program continues to live inside the kernel
depends on how it is further attached to a given kernel subsystem
after it was loaded via
.BR bpf ().
.PP
Each eBPF program is a set of instructions that is safe to run until
its completion.
An in-kernel verifier statically determines 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 attached maps can't be removed until the program is unloaded.
.PP
eBPF programs can be attached to different events.
These events can be the arrival of network packets, tracing
events, classification events by network queueing  disciplines
(for eBPF programs attached to a
.BR 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.
.PP
The same eBPF program can be attached to multiple events and different
eBPF programs can access the same map:
.PP
.in +4n
.EX
tracing     tracing    tracing    packet      packet     packet
event A     event B    event C    on eth0     on eth1    on eth2
 |             |         |          |           |          \(ha
 |             |         |          |           v          |
 \-\-> tracing <\-\-     tracing      socket    tc ingress   tc egress
      prog_1          prog_2      prog_3    classifier    action
      |  |              |           |         prog_4      prog_5
   |\-\-\-  \-\-\-\-\-|  |\-\-\-\-\-\-|          map_3        |           |
 map_1       map_2                              \-\-| map_4 |\-\-
.EE
.in
.\"
.SS Arguments
The operation to be performed by the
.BR bpf ()
system call is determined by the
.I cmd
argument.
Each operation takes an accompanying argument,
provided via
.IR attr ,
which is a pointer to a union of type
.I bpf_attr
(see below).
The
.I size
argument is the size of the union pointed to by
.IR attr .
.PP
The value provided in
.I cmd
is one of the following:
.TP
.B BPF_MAP_CREATE
Create a map and return a file descriptor that refers to the map.
The close-on-exec file descriptor flag (see
.BR fcntl (2))
is automatically enabled for the new file descriptor.
.TP
.B BPF_MAP_LOOKUP_ELEM
Look up an element by key in a specified map and return its value.
.TP
.B BPF_MAP_UPDATE_ELEM
Create or update an element (key/value pair) in a specified map.
.TP
.B BPF_MAP_DELETE_ELEM
Look up and delete an element by key in a specified map.
.TP
.B BPF_MAP_GET_NEXT_KEY
Look up an element by key in a specified map and return the key
of the next element.
.TP
.B BPF_PROG_LOAD
Verify and load an eBPF program,
returning a new file descriptor associated with the program.
The close-on-exec file descriptor flag (see
.BR fcntl (2))
is automatically enabled for the new file descriptor.
.IP
The
.I bpf_attr
union consists of various anonymous structures that are used by different
.BR bpf ()
commands:
.PP
.in +4n
.EX
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;      /* \(aqconst struct bpf_insn *\(aq */
        __aligned_u64 license;    /* \(aqconst char *\(aq */
        __u32         log_level;  /* verbosity level of verifier */
        __u32         log_size;   /* size of user buffer */
        __aligned_u64 log_buf;    /* user supplied \(aqchar *\(aq
                                     buffer */
        __u32         kern_version;
                                  /* checked when prog_type=kprobe
                                     (since Linux 4.1) */
.\"                 commit 2541517c32be2531e0da59dfd7efc1ce844644f5
    };
} __attribute__((aligned(8)));
.EE
.in
.\"
.SS eBPF maps
Maps are a generic data structure for storage of different types of data.
They allow sharing of data between eBPF kernel programs,
and also between kernel and user-space applications.
.PP
Each map type has the following attributes:
.IP * 3
type
.IP *
maximum number of elements
.IP *
key size in bytes
.IP *
value size in bytes
.PP
The following wrapper functions demonstrate how various
.BR bpf ()
commands can be used to access the maps.
The functions use the
.I cmd
argument to invoke different operations.
.TP
.B BPF_MAP_CREATE
The
.B BPF_MAP_CREATE
command creates a new map,
returning a new file descriptor that refers to the map.
.IP
.in +4n
.EX
int
bpf_create_map(enum bpf_map_type map_type,
               unsigned int key_size,
               unsigned int value_size,
               unsigned 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));
}
.EE
.in
.IP
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 .
On success, this operation returns a file descriptor.
On error, \-1 is returned and
.I errno
is set to
.BR EINVAL ,
.BR EPERM ,
or
.BR ENOMEM .
.IP
The
.I key_size
and
.I value_size
attributes will be used by the verifier during program loading
to check that the program is calling
.BR bpf_map_*_elem ()
helper functions with a correctly initialized
.I key
and to check that the program doesn't access the map element
.I value
beyond the specified
.IR value_size .
For example, when a map is created with a
.I key_size
of 8 and the eBPF program calls
.IP
.in +4n
.EX
bpf_map_lookup_elem(map_fd, fp \- 4)
.EE
.in
.IP
the program will be rejected,
since the in-kernel helper function
.IP
.in +4n
.EX
bpf_map_lookup_elem(map_fd, void *key)
.EE
.in
.IP
expects to read 8 bytes from the location pointed to by
.IR key ,
but the
.I fp\ \-\ 4
(where
.I fp
is the top of the stack)
starting address will cause out-of-bounds stack access.
.IP
Similarly, when a map is created with a
.I value_size
of 1 and the eBPF program contains
.IP
.in +4n
.EX
value = bpf_map_lookup_elem(...);
*(u32 *) value = 1;
.EE
.in
.IP
the program will be rejected, since it accesses the
.I value
pointer beyond the specified 1 byte
.I value_size
limit.
.IP
Currently, the following values are supported for
.IR map_type :
.IP
.in +4n
.EX
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,
    BPF_MAP_TYPE_PERF_EVENT_ARRAY,
    BPF_MAP_TYPE_PERCPU_HASH,
    BPF_MAP_TYPE_PERCPU_ARRAY,
    BPF_MAP_TYPE_STACK_TRACE,
    BPF_MAP_TYPE_CGROUP_ARRAY,
    BPF_MAP_TYPE_LRU_HASH,
    BPF_MAP_TYPE_LRU_PERCPU_HASH,
    BPF_MAP_TYPE_LPM_TRIE,
    BPF_MAP_TYPE_ARRAY_OF_MAPS,
    BPF_MAP_TYPE_HASH_OF_MAPS,
    BPF_MAP_TYPE_DEVMAP,
    BPF_MAP_TYPE_SOCKMAP,
    BPF_MAP_TYPE_CPUMAP,
    BPF_MAP_TYPE_XSKMAP,
    BPF_MAP_TYPE_SOCKHASH,
    BPF_MAP_TYPE_CGROUP_STORAGE,
    BPF_MAP_TYPE_REUSEPORT_SOCKARRAY,
    BPF_MAP_TYPE_PERCPU_CGROUP_STORAGE,
    BPF_MAP_TYPE_QUEUE,
    BPF_MAP_TYPE_STACK,
    /* See /usr/include/linux/bpf.h for the full list. */
};
.EE
.in
.IP
.I map_type
selects one of the available map implementations in the kernel.
.\" FIXME We need an explanation of why one might choose each of
.\" these map implementations
For all map types,
eBPF programs access maps with the same
.BR bpf_map_lookup_elem ()
and
.BR bpf_map_update_elem ()
helper functions.
Further details of the various map types are given below.
.TP
.B BPF_MAP_LOOKUP_ELEM
The
.B BPF_MAP_LOOKUP_ELEM
command looks up an element with a given
.I key
in the map referred to by the file descriptor
.IR fd .
.IP
.in +4n
.EX
int
bpf_lookup_elem(int fd, const 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));
}
.EE
.in
.IP
If an element is found,
the operation returns zero and stores the element's value into
.IR value ,
which must point to a buffer of
.I value_size
bytes.
.IP
If no element is found, the operation returns \-1 and sets
.I errno
to
.BR ENOENT .
.TP
.B BPF_MAP_UPDATE_ELEM
The
.B BPF_MAP_UPDATE_ELEM
command
creates or updates an element with a given
.I key/value
in the map referred to by the file descriptor
.IR fd .
.IP
.in +4n
.EX
int
bpf_update_elem(int fd, const void *key, const void *value,
                uint64_t 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));
}
.EE
.in
.IP
The
.I flags
argument should be specified as one of the following:
.RS
.TP
.B BPF_ANY
Create a new element or update an existing element.
.TP
.B BPF_NOEXIST
Create a new element only if it did not exist.
.TP
.B BPF_EXIST
Update an existing element.
.RE
.IP
On success, the operation returns zero.
On error, \-1 is returned and
.I errno
is set to
.BR EINVAL ,
.BR EPERM ,
.BR ENOMEM ,
or
.BR E2BIG .
.B E2BIG
indicates that the number of elements in the map reached the
.I max_entries
limit specified at map creation time.
.B EEXIST
will be returned if
.I flags
specifies
.B BPF_NOEXIST
and the element with
.I key
already exists in the map.
.B ENOENT
will be returned if
.I flags
specifies
.B BPF_EXIST
and the element with
.I key
doesn't exist in the map.
.TP
.B BPF_MAP_DELETE_ELEM
The
.B BPF_MAP_DELETE_ELEM
command
deletes the element whose key is
.I key
from the map referred to by the file descriptor
.IR fd .
.IP
.in +4n
.EX
int
bpf_delete_elem(int fd, const void *key)
{
    union bpf_attr attr = {
        .map_fd = fd,
        .key    = ptr_to_u64(key),
    };

    return bpf(BPF_MAP_DELETE_ELEM, &attr, sizeof(attr));
}
.EE
.in
.IP
On success, zero is returned.
If the element is not found, \-1 is returned and
.I errno
is set to
.BR ENOENT .
.TP
.B BPF_MAP_GET_NEXT_KEY
The
.B BPF_MAP_GET_NEXT_KEY
command looks up an element by
.I key
in the map referred to by the file descriptor
.I fd
and sets the
.I next_key
pointer to the key of the next element.
.IP
.in +4n
.EX
int
bpf_get_next_key(int fd, const 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));
}
.EE
.in
.IP
If
.I key
is found, the operation returns zero and sets the
.I next_key
pointer to the key of the next element.
If
.I key
is not found, the operation returns zero and sets the
.I next_key
pointer to the key of the first element.
If
.I key
is the last element, \-1 is returned and
.I errno
is set to
.BR ENOENT .
Other possible
.I errno
values are
.BR ENOMEM ,
.BR EFAULT ,
.BR EPERM ,
and
.BR EINVAL .
This method can be used to iterate over all elements in the map.
.TP
.B close(map_fd)
Delete the map referred to by the file descriptor
.IR map_fd .
When the user-space program that created a map exits, all maps will
be deleted automatically (but see NOTES).
.\"
.SS eBPF map types
The following map types are supported:
.TP
.B BPF_MAP_TYPE_HASH
.\" commit 0f8e4bd8a1fc8c4185f1630061d0a1f2d197a475
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 lookup, update, and delete operations.
.IP *
The kernel takes care of allocating and freeing key/value pairs.
.IP *
The
.BR map_update_elem ()
helper will 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
Array maps have the following characteristics:
.RS
.IP * 3
Optimized for fastest possible lookup.
In the future the verifier/JIT compiler
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 a
.B nonatomic
fashion;
for atomic updates, a hash-table map should be used instead.
There is however one special case that can also be used with arrays:
the atomic built-in
.B __sync_fetch_and_add()
can be used on 32 and 64 bit atomic counters.
For example, it can be
applied on the whole value itself if it represents a single counter,
or in case of a structure containing multiple counters, it could be
used on individual counters.
This is quite often useful for aggregation and accounting of events.
.RE
.IP
Among the uses for array maps are the following:
.RS
.IP * 3
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.
.IP *
Aggregation of tracing events into a fixed set of buckets.
.IP *
Accounting of networking events, for example, number of packets and packet
sizes.
.RE
.TP
.BR BPF_MAP_TYPE_PROG_ARRAY " (since Linux 4.2)"
A program array map is a special kind of array map whose map values
contain only file descriptors referring to other eBPF programs.
Thus, both the
.I key_size
and
.I value_size
must be exactly four bytes.
This map is used in conjunction with the
.BR bpf_tail_call ()
helper.
.IP
This means that an eBPF program with a program array map attached to it
can call from kernel side into
.IP
.in +4n
.EX
void bpf_tail_call(void *context, void *prog_map,
                   unsigned int index);
.EE
.in
.IP
and therefore replace its own program flow with the one from the program
at the given program array slot, if present.
This can be regarded as kind of a jump table to a different eBPF program.
The invoked program will then reuse the same stack.
When a jump into the new program has been performed,
it won't return to the old program anymore.
.IP
If no eBPF program is found at the given index of the program array
(because the map slot doesn't contain a valid program file descriptor,
the specified lookup index/key is out of bounds,
or the limit of 32
.\" MAX_TAIL_CALL_CNT
nested calls has been exceed),
execution continues with the current eBPF program.
This can be used as a fall-through for default cases.
.IP
A program array map is useful, for example, in tracing or networking, to
handle individual system calls or protocols in their own subprograms and
use their identifiers as an individual map index.
This approach may result in performance benefits,
and also makes it possible to overcome the maximum
instruction limit of a single eBPF program.
In dynamic environments,
a user-space daemon might atomically replace individual subprograms
at run-time with newer versions to alter overall program behavior,
for instance, if global policies change.
.\"
.SS eBPF programs
The
.B 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.
.PP
.in +4n
.EX
char bpf_log_buf[LOG_BUF_SIZE];

int
bpf_prog_load(enum bpf_prog_type type,
              const struct bpf_insn *insns, int insn_cnt,
              const char *license)
{
    union bpf_attr attr = {
        .prog_type = 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));
}
.EE
.in
.PP
.I prog_type
is one of the available program types:
.IP
.in +4n
.EX
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,
    BPF_PROG_TYPE_TRACEPOINT,
    BPF_PROG_TYPE_XDP,
    BPF_PROG_TYPE_PERF_EVENT,
    BPF_PROG_TYPE_CGROUP_SKB,
    BPF_PROG_TYPE_CGROUP_SOCK,
    BPF_PROG_TYPE_LWT_IN,
    BPF_PROG_TYPE_LWT_OUT,
    BPF_PROG_TYPE_LWT_XMIT,
    BPF_PROG_TYPE_SOCK_OPS,
    BPF_PROG_TYPE_SK_SKB,
    BPF_PROG_TYPE_CGROUP_DEVICE,
    BPF_PROG_TYPE_SK_MSG,
    BPF_PROG_TYPE_RAW_TRACEPOINT,
    BPF_PROG_TYPE_CGROUP_SOCK_ADDR,
    BPF_PROG_TYPE_LWT_SEG6LOCAL,
    BPF_PROG_TYPE_LIRC_MODE2,
    BPF_PROG_TYPE_SK_REUSEPORT,
    BPF_PROG_TYPE_FLOW_DISSECTOR,
    /* See /usr/include/linux/bpf.h for the full list. */
};
.EE
.in
.PP
For further details of eBPF program types, see below.
.PP
The remaining fields of
.I bpf_attr
are set as follows:
.IP * 3
.I insns
is an array of
.I "struct bpf_insn"
instructions.
.IP *
.I insn_cnt
is the number of instructions in the program referred to by
.IR insns .
.IP *
.I license
is a license string, which must be GPL compatible to call helper functions
marked
.IR gpl_only .
(The licensing rules are the same as for kernel modules,
so that also dual licenses, such as "Dual BSD/GPL", may be used.)
.IP *
.I 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 eBPF program is unsafe.
The format of the output can change at any time as the verifier evolves.
.IP *
.I log_size
size of the buffer pointed to by
.IR log_buf .
If the size of the buffer is not large enough to store all
verifier messages, \-1 is returned and
.I errno
is set to
.BR ENOSPC .
.IP *
.I log_level
verbosity level of the verifier.
A value of zero means that the verifier will not provide a log;
in this case,
.I log_buf
must be a NULL pointer, and
.I log_size
must be zero.
.PP
Applying
.BR close (2)
to the file descriptor returned by
.B BPF_PROG_LOAD
will unload the eBPF program (but see NOTES).
.PP
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.
Conversely, 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.
.\"
.\"
.SS eBPF program types
The eBPF program type
.RI ( prog_type )
determines the subset of kernel helper functions that the program
may call.
The program type also determines the program input (context)\(emthe
format of
.I "struct bpf_context"
(which is the data blob passed into the eBPF program as the first argument).
.\"
.\" FIXME
.\" Somewhere in this page we need a general introduction to the
.\" bpf_context. For example, how does a BPF program access the
.\" context?
.PP
For example, a tracing program does not have the exact same
subset of helper functions as a socket filter program
(though they may have some helpers in common).
Similarly,
the input (context) for a tracing program is a set of register values,
while for a socket filter it is a network packet.
.PP
The set of functions available to eBPF programs of a given type may increase
in the future.
.PP
The following program types are supported:
.TP
.BR BPF_PROG_TYPE_SOCKET_FILTER " (since Linux 3.19)"
Currently, the set of functions for
.B BPF_PROG_TYPE_SOCKET_FILTER
is:
.IP
.in +4n
.EX
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 */
.EE
.in
.IP
The
.I bpf_context
argument is a pointer to a
.IR "struct __sk_buff" .
.\" FIXME: We need some text here to explain how the program
.\" accesses __sk_buff.
.\" See 'struct __sk_buff' and commit 9bac3d6d548e5
.\"
.\" Alexei commented:
.\" Actually now in case of SOCKET_FILTER, SCHED_CLS, SCHED_ACT
.\" the program can now access skb fields.
.\"
.TP
.BR BPF_PROG_TYPE_KPROBE " (since Linux 4.1)"
.\" commit 2541517c32be2531e0da59dfd7efc1ce844644f5
[To be documented]
.\" FIXME Document this program type
.\"	  Describe allowed helper functions for this program type
.\"	  Describe bpf_context for this program type
.\"
.\" FIXME We need text here to describe 'kern_version'
.TP
.BR BPF_PROG_TYPE_SCHED_CLS " (since Linux 4.1)"
.\" commit 96be4325f443dbbfeb37d2a157675ac0736531a1
.\" commit e2e9b6541dd4b31848079da80fe2253daaafb549
[To be documented]
.\" FIXME Document this program type
.\"	  Describe allowed helper functions for this program type
.\"	  Describe bpf_context for this program type
.TP
.BR BPF_PROG_TYPE_SCHED_ACT " (since Linux 4.1)"
.\" commit 94caee8c312d96522bcdae88791aaa9ebcd5f22c
.\" commit a8cb5f556b567974d75ea29c15181c445c541b1f
[To be documented]
.\" FIXME Document this program type
.\"	  Describe allowed helper functions for this program type
.\"	  Describe bpf_context for this program type
.SS Events
Once a program is loaded, it can be attached to an event.
Various kernel subsystems have different ways to do so.
.PP
Since Linux 3.19,
.\" commit 89aa075832b0da4402acebd698d0411dcc82d03e
the following call will attach the program
.I prog_fd
to the socket
.IR sockfd ,
which was created by an earlier call to
.BR socket (2):
.PP
.in +4n
.EX
setsockopt(sockfd, SOL_SOCKET, SO_ATTACH_BPF,
           &prog_fd, sizeof(prog_fd));
.EE
.in
.PP
Since Linux 4.1,
.\" commit 2541517c32be2531e0da59dfd7efc1ce844644f5
the following call may be used to attach
the eBPF program referred to by the file descriptor
.I prog_fd
to a perf event file descriptor,
.IR event_fd ,
that was created by a previous call to
.BR perf_event_open (2):
.PP
.in +4n
.EX
ioctl(event_fd, PERF_EVENT_IOC_SET_BPF, prog_fd);
.EE
.in
.\"
.\"
.SH RETURN VALUE
For a successful call, the return value depends on the operation:
.TP
.B BPF_MAP_CREATE
The new file descriptor associated with the eBPF map.
.TP
.B BPF_PROG_LOAD
The new file descriptor associated with the eBPF program.
.TP
All other commands
Zero.
.PP
On error, \-1 is returned, and
.I errno
is set to indicate the error.
.SH ERRORS
.TP
.B E2BIG
The eBPF program is too large or a map reached the
.I max_entries
limit (maximum number of elements).
.TP
.B EACCES
For
.BR 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/register 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
.BR bpf ()
again with
.I log_level = 1
and examine
.I log_buf
for the specific reason provided by the verifier.
.TP
.B EBADF
.I fd
is not an open file descriptor.
.TP
.B EFAULT
One of the pointers
.RI ( key
or
.I value
or
.I log_buf
or
.IR insns )
is outside the accessible address space.
.TP
.B EINVAL
The value specified in
.I cmd
is not recognized by this kernel.
.TP
.B EINVAL
For
.BR BPF_MAP_CREATE ,
either
.I map_type
or attributes are invalid.
.TP
.B EINVAL
For
.B BPF_MAP_*_ELEM
commands,
some of the fields of
.I "union bpf_attr"
that are not used by this command
are not set to zero.
.TP
.B EINVAL
For
.BR BPF_PROG_LOAD ,
indicates an attempt to load an invalid program.
eBPF programs can be deemed
invalid due to unrecognized instructions, the use of reserved fields, jumps
out of range, infinite loops or calls of unknown functions.
.TP
.B ENOENT
For
.B BPF_MAP_LOOKUP_ELEM
or
.BR BPF_MAP_DELETE_ELEM ,
indicates that the element with the given
.I key
was not found.
.TP
.B ENOMEM
Cannot allocate sufficient memory.
.TP
.B EPERM
The call was made without sufficient privilege
(without the
.B CAP_SYS_ADMIN
capability).
.SH VERSIONS
The
.BR bpf ()
system call first appeared in Linux 3.18.
.SH CONFORMING TO
The
.BR bpf ()
system call is Linux-specific.
.SH NOTES
Prior to Linux 4.4, all
.BR bpf ()
commands require the caller to have the
.B CAP_SYS_ADMIN
capability.
From Linux 4.4 onwards,
.\" commit 1be7f75d1668d6296b80bf35dcf6762393530afc
an unprivileged user may create limited programs of type
.BR BPF_PROG_TYPE_SOCKET_FILTER
and associated maps.
However they may not store kernel pointers within
the maps and are presently limited to the following helper functions:
.\" [Linux 5.6] mtk: The list of available functions is, I think, governed
.\" by the check in net/core/filter.c::bpf_base_func_proto().
.IP * 3
get_random
.PD 0
.IP *
get_smp_processor_id
.IP *
tail_call
.IP *
ktime_get_ns
.PD 1
.PP
Unprivileged access may be blocked by writing the value 1 to the file
.IR /proc/sys/kernel/unprivileged_bpf_disabled .
.PP
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.
.PP
eBPF programs can be written in a restricted C that is compiled (using the
.B clang
compiler) into eBPF bytecode.
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.
.\" There are also examples for the tc classifier, in the iproute2
.\" project, in examples/bpf
.PP
The kernel contains a just-in-time (JIT) compiler that translates
eBPF bytecode into native machine code for better performance.
In kernels before Linux 4.15,
the JIT compiler is disabled by default,
but its operation can be controlled by writing one of the
following integer strings to the file
.IR /proc/sys/net/core/bpf_jit_enable :
.IP 0 3
Disable JIT compilation (default).
.IP 1
Normal compilation.
.IP 2
Debugging mode.
The generated opcodes are dumped in hexadecimal into the kernel log.
These opcodes can then be disassembled using the program
.I tools/net/bpf_jit_disasm.c
provided in the kernel source tree.
.PP
Since Linux 4.15,
.\" commit 290af86629b25ffd1ed6232c4e9107da031705cb
the kernel may configured with the
.B CONFIG_BPF_JIT_ALWAYS_ON
option.
In this case, the JIT compiler is always enabled, and the
.I bpf_jit_enable
is initialized to 1 and is immutable.
(This kernel configuration option was provided as a mitigation for
one of the Spectre attacks against the BPF interpreter.)
.PP
The JIT compiler for eBPF is currently
.\" Last reviewed in Linux 4.18-rc by grepping for BPF_ALU64 in arch/
.\" and by checking the documentation for bpf_jit_enable in
.\" Documentation/sysctl/net.txt
available for the following architectures:
.IP * 3
x86-64 (since Linux 3.18; cBPF since Linux 3.0);
.\" commit 0a14842f5a3c0e88a1e59fac5c3025db39721f74
.PD 0
.IP *
ARM32 (since Linux 3.18; cBPF since Linux 3.4);
.\" commit ddecdfcea0ae891f782ae853771c867ab51024c2
.IP *
SPARC 32 (since Linux 3.18; cBPF since Linux 3.5);
.\" commit 2809a2087cc44b55e4377d7b9be3f7f5d2569091
.IP *
ARM-64 (since Linux 3.18);
.\" commit e54bcde3d69d40023ae77727213d14f920eb264a
.IP *
s390 (since Linux 4.1; cBPF since Linux 3.7);
.\" commit c10302efe569bfd646b4c22df29577a4595b4580
.IP *
PowerPC 64 (since Linux 4.8; cBPF since Linux 3.1);
.\" commit 0ca87f05ba8bdc6791c14878464efc901ad71e99
.\" commit 156d0e290e969caba25f1851c52417c14d141b24
.IP *
SPARC 64 (since Linux 4.12);
.\" commit 7a12b5031c6b947cc13918237ae652b536243b76
.IP *
x86-32 (since Linux 4.18);
.\" commit 03f5781be2c7b7e728d724ac70ba10799cc710d7
.IP *
MIPS 64 (since Linux 4.18; cBPF since Linux 3.16);
.\" commit c6610de353da5ca6eee5b8960e838a87a90ead0c
.\" commit f381bf6d82f032b7410185b35d000ea370ac706b
.IP *
riscv (since Linux 5.1).
.\" commit 2353ecc6f91fd15b893fa01bf85a1c7a823ee4f2
.PD
.SH EXAMPLES
.EX
/* 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 \(aq%s\(aq\en", 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 */
.\"                                == atomic64_add
        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) / sizeof(prog[0]), "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 packets\en", tcp_cnt, udp_cnt);
        sleep(1);
    }

    return 0;
}
.EE
.PP
Some complete working code can be found in the
.I samples/bpf
directory in the kernel source tree.
.SH SEE ALSO
.BR seccomp (2),
.BR bpf\-helpers (7),
.BR socket (7),
.BR tc (8),
.BR tc\-bpf (8)
.PP
Both classic and extended BPF are explained in the kernel source file
.IR Documentation/networking/filter.txt .