bpf(2) - Linux manual page (original) (raw)
bpf(2) System Calls Manual bpf(2)
NAME top
bpf - perform a command on an extended BPF map or programSYNOPSIS top
**#include <linux/bpf.h>**
**int bpf(int** _cmd_**, union bpf_attr ***_attr_**, unsigned int** _size_**);**DESCRIPTION top
The **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.
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 structures such
as eBPF maps.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.
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.
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, 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
**bpf**(). If a map lookup fails, the current program continues its
execution. See **BPF_MAP_TYPE_PROG_ARRAY** below for further details.
Generally, eBPF programs are loaded by the user process and
automatically unloaded when the process exits. In some cases, for
example, [tc-bpf(8)](../man8/tc-bpf.8.html), 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 **bpf**().
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.
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 [tc(8)](../man8/tc.8.html) 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
different eBPF programs can access the same map:
tracing tracing tracing packet packet packet
event A event B event C on eth0 on eth1 on eth2
| | | | | ^
| | | | 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 |--Arguments The operation to be performed by the bpf() system call is determined by the cmd argument. Each operation takes an accompanying argument, provided via attr, which is a pointer to a union of type bpfattr (see below). The unused fields and padding must be zeroed out before the call. 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 and return a file descriptor that refers to
the map. The close-on-exec file descriptor flag (see
[fcntl(2)](../man2/fcntl.2.html)) is automatically enabled for the new file
descriptor.
**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 specified
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 close-on-exec
file descriptor flag (see [fcntl(2)](../man2/fcntl.2.html)) is automatically
enabled for the new file descriptor.
The _bpfattr_ 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 programs, 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,
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));
}
The new map has the type specified by _maptype_, and
attributes as specified in _keysize_, _valuesize_, and
_maxentries_. On success, this operation returns a file
descriptor. On error, -1 is returned and _[errno](../man3/errno.3.html)_ is set to
**EINVAL**, **EPERM**, or **ENOMEM**.
The _keysize_ and _valuesize_ attributes will be used by the
verifier during program loading to check that the program
is calling **bpf_map_*_elem**() helper functions with a
correctly initialized _key_ and to check that the program
doesn't access the map element _value_ beyond the specified
_valuesize_. For example, when a map is created with a
_keysize_ 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 _valuesize_ 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 _valuesize_ limit.
Currently, the following values are supported for _maptype_:
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. */
};
_maptype_ 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, 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));
}
If an element is found, the operation returns zero and
stores the element's value into _value_, which must point to
a buffer of _valuesize_ bytes.
If no element is found, the operation returns -1 and sets
_[errno](../man3/errno.3.html)_ to **ENOENT**.
**BPF_MAP_UPDATE_ELEM**
The **BPF_MAP_UPDATE_ELEM** command creates or updates an
element with a given _key/value_ in the map referred to by
the file descriptor _fd_.
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));
}
The _flags_ argument should be specified as one of the
following:
**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](../man3/errno.3.html)_ is set to **EINVAL**, **EPERM**, **ENOMEM**, or
**E2BIG**. **E2BIG** indicates that the number of elements in the
map reached the _maxentries_ limit specified at map creation
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 deletes the element whose
key is _key_ from the map referred to by the file descriptor
_fd_.
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));
}
On success, zero is returned. If the element is not found,
-1 is returned and _[errno](../man3/errno.3.html)_ 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 _nextkey_ pointer to the key of the next element.
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));
}
If _key_ is found, the operation returns zero and sets the
_nextkey_ pointer to the key of the next element. If _key_ is
not found, the operation returns zero and sets the _nextkey_
pointer to the key of the first element. If _key_ is the
last element, -1 is returned and _[errno](../man3/errno.3.html)_ is set to **ENOENT**.
Other possible _[errno](../man3/errno.3.html)_ 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 _mapfd_.
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 will fail to insert new
element when the _maxentries_ limit is reached. (This
ensures that eBPF programs cannot exhaust memory.)
• **map_update_elem**() replaces existing elements atomically.
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()
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 _valuesize_ are constant for the life
of the eBPF program. In other words,
**array_map_lookup_elem**() may be 'inlined' by the
verifier/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 a **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
**__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.
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
buckets.
• Accounting of networking events, for example, number of
packets and packet sizes.
**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 _keysize_ and _valuesize_ must
be exactly four bytes. This map is used in conjunction
with the **bpf_tail_call**() helper.
This means that an eBPF program with a program array map
attached to it can call from kernel side into
void bpf_tail_call(void *context, void *prog_map,
unsigned int index);
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.
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 nested calls has been
exceed), execution continues with the current eBPF program.
This can be used as a fall-through for default cases.
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.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 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));
}
_progtype_ 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,
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. */
};
For further details of eBPF program types, see below.
The remaining fields of _bpfattr_ are set as follows:
• _insns_ is an array of _struct bpfinsn_ instructions.
• _insncnt_ 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 _gplonly_. (The licensing rules
are the same as for kernel modules, so that also dual licenses,
such as "Dual BSD/GPL", may be used.)
• _logbuf_ 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.
• _logsize_ size of the buffer pointed to by _logbuf_. If the size
of the buffer is not large enough to store all verifier
messages, -1 is returned and _[errno](../man3/errno.3.html)_ is set to **ENOSPC**.
• _loglevel_ verbosity level of the verifier. A value of zero
means that the verifier will not provide a log; in this case,
_logbuf_ must be a null pointer, and _logsize_ must be zero.
Applying [close(2)](../man2/close.2.html) 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. 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.eBPF program types The eBPF program type (progtype) determines the subset of kernel helper functions that the program may call. The program type also determines the program input (context)—the format of struct bpfcontext (which is the data blob passed into the eBPF program as the first argument).
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.
The set of functions available to eBPF programs 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 _bpfcontext_ argument is a pointer to a _struct_
__skbuff_.
**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. Various kernel subsystems have different ways to do so.
Since Linux 3.19, the following call will attach the program
_progfd_ to the socket _sockfd_, which was created by an earlier call
to [socket(2)](../man2/socket.2.html):
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 _progfd_ to a perf event
file descriptor, _eventfd_, that was created by a previous call to
[perf_event_open(2)](../man2/perf%5Fevent%5Fopen.2.html):
ioctl(event_fd, PERF_EVENT_IOC_SET_BPF, prog_fd);RETURN VALUE top
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](../man3/errno.3.html)_ is set to indicate the error.ERRORS top
**E2BIG** The eBPF program is too large or a map reached the
_maxentries_ limit (maximum number of elements).
**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/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 **bpf**() again with
_loglevel = 1_ and examine _logbuf_ for the specific reason
provided by the verifier.
**EAGAIN** For **BPF_PROG_LOAD**, indicates that needed resources are
blocked. This happens when the verifier detects pending
signals while it is checking the validity of the bpf
program. In this case, just call **bpf**() again with the same
parameters.
**EBADF** _fd_ is not an open file descriptor.
**EFAULT** One of the pointers (_key_ or _value_ or _logbuf_ or _insns_) is
outside the accessible address space.
**EINVAL** The value specified in _cmd_ is not recognized by this
kernel.
**EINVAL** For **BPF_MAP_CREATE**, either _maptype_ or attributes are
invalid.
**EINVAL** For **BPF_MAP_*_ELEM** commands, some of the fields of _union_
_bpfattr_ 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. 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.
**ENOENT** For **BPF_MAP_LOOKUP_ELEM** or **BPF_MAP_DELETE_ELEM**, indicates
that the element with the given _key_ was not found.
**ENOMEM** Cannot allocate sufficient memory.
**EPERM** The call was made without sufficient privilege (without the
**CAP_SYS_ADMIN** capability).STANDARDS top
Linux.HISTORY top
Linux 3.18.NOTES top
Prior to Linux 4.4, all **bpf**() commands require the caller to have
the **CAP_SYS_ADMIN** capability. From Linux 4.4 onwards, an
unprivileged user may create limited programs of type
**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:
• get_random
• get_smp_processor_id
• tail_call
• ktime_get_ns
Unprivileged access may be blocked by writing the value 1 to the
file _/proc/sys/kernel/unprivilegedbpfdisabled_.
eBPF objects (maps and programs) can be shared between processes.
For example, after [fork(2)](../man2/fork.2.html), 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 [dup(2)](../man2/dup.2.html) 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. 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.
The kernel contains a just-in-time (JIT) compiler that translates
eBPF bytecode into native machine code for better performance.
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 _/proc/sys/net/core/bpfjitenable_:
**0** Disable JIT compilation (default).
**1** Normal compilation.
**2** Debugging mode. The generated opcodes are dumped in
hexadecimal into the kernel log. These opcodes can then be
disassembled using the program _tools/net/bpfjitdisasm.c_
provided in the kernel source tree.
Since Linux 4.15, the kernel may be configured with the
**CONFIG_BPF_JIT_ALWAYS_ON** option. In this case, the JIT compiler
is always enabled, and the _bpfjitenable_ 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.)
The JIT compiler for eBPF is currently available for the following
architectures:
• x86-64 (since Linux 3.18; cBPF since Linux 3.0);
• ARM32 (since Linux 3.18; cBPF since Linux 3.4);
• SPARC 32 (since Linux 3.18; cBPF since Linux 3.5);
• ARM-64 (since Linux 3.18);
• s390 (since Linux 4.1; cBPF since Linux 3.7);
• PowerPC 64 (since Linux 4.8; cBPF since Linux 3.1);
• SPARC 64 (since Linux 4.12);
• x86-32 (since Linux 4.18);
• MIPS 64 (since Linux 4.18; cBPF since Linux 3.16);
• riscv (since Linux 5.1).EXAMPLES top
/* 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) / 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\n", tcp_cnt, udp_cnt);
sleep(1);
}
return 0;
}
Some complete working code can be found in the _samples/bpf_
directory in the kernel source tree.SEE ALSO top
[seccomp(2)](../man2/seccomp.2.html), [bpf-helpers(7)](../man7/bpf-helpers.7.html), [socket(7)](../man7/socket.7.html), [tc(8)](../man8/tc.8.html), [tc-bpf(8)](../man8/tc-bpf.8.html)
Both classic and extended BPF are explained in the kernel source
file _Documentation/networking/filter.txt_.COLOPHON top
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man-pages@man7.orgLinux man-pages 6.10 2025-01-05 bpf(2)
Pages that refer to this page:perf_event_open(2), seccomp(2), syscalls(2), lirc(4), proc_pid_fd(5), proc_sys_net(5), bpf-helpers(7), capabilities(7), socket(7), tc-bpf(8)