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       bpf - perform a command on an extended BPF map or program


       #include <linux/bpf.h>

       int bpf(int cmd, union bpf_attr *attr, unsigned int size);


       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

       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

       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 runtime, 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), 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) 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 |--

       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 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:

              Create a map and return a file descriptor that refers to the map.

              Look up an element by key in a specified map and return its value.

              Create or update an element (key/value pair) in a specified map.

              Look up and delete an element by key in a specified map.

              Look up an element by key in a specified  map  and  return  the  key  of  the  next

              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 programs,  and  also  between  kernel  and  user-space

       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.

              The BPF_MAP_CREATE command creates a new map, returning a new file descriptor  that
              refers to the map.

                  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 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  key_size and value_size 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 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

                  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 */

              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.

              The  BPF_MAP_LOOKUP_ELEM  command  looks  up an element with a given key in the map
              referred to by the file descriptor fd.

                  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 value_size bytes.

              If no element is found, the operation returns -1 and sets errno to ENOENT.

              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.

                  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:

                     Create a new element or update an existing element.

                     Create a new element only if it did not 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 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.

              The  BPF_MAP_DELETE_ELEM  command deleted the element whose key is key from the map
              referred to by the file descriptor fd.

                  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 is set to ENOENT.

              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

                  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 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.

              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

   eBPF map types
       The following map types are supported:

              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

              *  map_update_elem() replaces existing elements atomically.

              Hash-table maps are optimized for speed of lookup.

              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  value_size  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

       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 key_size and
              value_size 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];

           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));

       prog_type is one of the available program types:

           enum bpf_prog_type {
               BPF_PROG_TYPE_UNSPEC,        /* Reserve 0 as invalid
                                               program type */

       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.  (The licensing rules are the same as for kernel modules, so that also
          dual licenses, such as "Dual BSD/GPL", may be used.)

       *  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.

       *  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; in this case, log_buf must be a NULL pointer, and log_size must
          be zero.

       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.  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 (prog_type) 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 bpf_context (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

       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.

       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]

       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 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);


       /* 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
       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 */
                                       /* 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);

           return 0;

       Some  complete working code can be found in the samples/bpf directory in the kernel source


       For a successful call, the return value depends on the operation:

              The new file descriptor associated with the eBPF map.

              The new file descriptor associated with the eBPF program.

       All other commands

       On error, -1 is returned, and errno is set appropriately.


       EPERM  The  call  was  made  without  sufficient  privilege  (without  the   CAP_SYS_ADMIN

       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 kernel.

       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.  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.

       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
              log_level = 1 and examine log_buf for the specific 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  eBPF  program  is  too  large  or a map reached the max_entries limit (maximum
              number of elements).


       The bpf() system call first appeared in Linux 3.18.


       The bpf() system call is Linux-specific.


       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  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)  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.  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/bpf_jit_enable:

       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/bpf_jit_disasm.c
          provided in the kernel source tree.

       JIT   compiler   for  eBPF  is  currently  available  for  the  x86-64,  arm64,  and  s390


       seccomp(2), socket(7), tc(8), tc-bpf(8)

       Both  classic   and   extended   BPF   are   explained   in   the   kernel   source   file


       This  page  is  part of release 4.04 of the Linux man-pages project.  A description of the
       project, information about reporting bugs, and the latest version of  this  page,  can  be
       found at