bpf

BPF(2)                     Linux Programmer's Manual                    BPF(2)

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

SYNOPSIS
       #include <linux/bpf.h>

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

DESCRIPTION
       The  bpf()  system  call  performs  a  range  of  operations related to
       extended Berkeley Packet Filters.  Extended BPF (or eBPF) is 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 exten-
       sion 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 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  automati-
       cally 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 descrip-
       tor  has  been  closed by the user-space program.  Thus, whether a spe-
       cific 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 informa-
       tion 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, pro-
       vided via attr, which is a pointer to a union  of  type  bpf_attr  (see
       below).  The size argument is the size of the union pointed to by attr.

       The value provided in cmd is one of the following:

       BPF_MAP_CREATE
              Create  a  map  and  return a file descriptor that refers to the
              map.  The close-on-exec file descriptor flag (see  fcntl(2))  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))  is automatically enabled for the new file
              descriptor.

              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 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 map_type,  and  attributes
              as  specified in key_size, value_size, and max_entries.  On suc-
              cess, 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 veri-
              fier 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  func-
              tion

                  bpf_map_lookup_elem(map_fd, void *key)

              expects to read 8 bytes from the location pointed to by key, but
              the fp - 4 (where fp is the top of the stack)  starting  address
              will cause out-of-bounds stack access.

              Similarly,  when a map is created with a value_size of 1 and the
              eBPF program contains

                  value = bpf_map_lookup_elem(...);
                  *(u32 *) value = 1;

              the program will  be  rejected,  since  it  accesses  the  value
              pointer beyond the specified 1 byte value_size limit.

              Currently, the following values are supported for map_type:

                  enum bpf_map_type {
                      BPF_MAP_TYPE_UNSPEC,  /* Reserve 0 as invalid map type */
                      BPF_MAP_TYPE_HASH,
                      BPF_MAP_TYPE_ARRAY,
                      BPF_MAP_TYPE_PROG_ARRAY,
                      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,
                  };

              map_type selects one of the available map implementations in the
              kernel.  For all map types, eBPF programs access maps  with  the
              same   bpf_map_lookup_elem()  and  bpf_map_update_elem()  helper
              functions.  Further details of the various map types  are  given
              below.

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

                  int
                  bpf_lookup_elem(int fd, 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.

       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 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.

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

                  int
                  bpf_delete_elem(int fd, const void *key)
                  {
                      union bpf_attr attr = {
                          .map_fd = fd,
                          .key    = ptr_to_u64(key),
                      };

                      return bpf(BPF_MAP_DELETE_ELEM, &attr, sizeof(attr));

                  }

              On  success,  zero is returned.  If the element is not found, -1
              is returned and errno is set to ENOENT.

       BPF_MAP_GET_NEXT_KEY
              The BPF_MAP_GET_NEXT_KEY command looks up an element by  key  in
              the  map  referred  to  by  the  file descriptor fd and sets the
              next_key pointer to the key of the next element.

                  int
                  bpf_get_next_key(int fd, 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.

       close(map_fd)
              Delete the map referred to by the file descriptor map_fd.   When
              the  user-space  program that created a map exits, all maps will
              be deleted automatically (but see NOTES).

   eBPF map types
       The following map types are supported:

       BPF_MAP_TYPE_HASH
              Hash-table maps have the following characteristics:

              *  Maps are created and destroyed by user-space programs.   Both
                 user-space  and eBPF programs can perform lookup, update, and
                 delete operations.

              *  The kernel takes care of  allocating  and  freeing  key/value
                 pairs.

              *  The  map_update_elem() helper will fail to insert new element
                 when the max_entries limit is reached.   (This  ensures  that
                 eBPF programs cannot exhaust memory.)

              *  map_update_elem() replaces existing elements 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 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  sin-
                 gle  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 pack-
                 ets 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  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 contin-
              ues  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  net-
              working, 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 ker-
       nel.   The return value for this command is a new file descriptor asso-
       ciated 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));
           }

       prog_type is one of the available program types:

                  enum bpf_prog_type {
                      BPF_PROG_TYPE_UNSPEC,        /* Reserve 0 as invalid
                                                      program type */
                      BPF_PROG_TYPE_SOCKET_FILTER,
                      BPF_PROG_TYPE_KPROBE,
                      BPF_PROG_TYPE_SCHED_CLS,
                      BPF_PROG_TYPE_SCHED_ACT,
                  };

       For further details of eBPF program types, see below.

       The remaining fields of bpf_attr are set as follows:

       *  insns is an array of struct bpf_insn instructions.

       *  insn_cnt is the number of instructions in the program referred to by
          insns.

       *  license  is  a  license string, which must be GPL compatible to call
          helper functions marked gpl_only.  (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_buf.  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 pro-
       grams.  For example, eBPF programs can  process  various  events  (like
       kprobe,  packets)  and store their data into a map, and user-space pro-
       grams can then fetch data from the map.   Conversely,  user-space  pro-
       grams  can  use  a map as a configuration mechanism, populating the map
       with values checked by the eBPF program, which then modifies its behav-
       ior 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_con-
       text (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 pro-
       gram 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 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]

   Events
       Once a program is loaded, it can be attached to an event.  Various ker-
       nel 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 pro-
       gram referred to by the file descriptor prog_fd to a  perf  event  file
       descriptor,   event_fd,   that  was  created  by  a  previous  call  to
       perf_event_open(2):

           ioctl(event_fd, PERF_EVENT_IOC_SET_BPF, prog_fd);

EXAMPLES
       /* bpf+sockets example:
        * 1. create array map of 256 elements
        * 2. load program that counts number of packets received
        *    r0 = skb->data[ETH_HLEN + offsetof(struct iphdr, protocol)]
        *    map[r0]++
        * 3. attach prog_fd to raw socket via setsockopt()
        * 4. print number of received TCP/UDP packets every second
        */
       int
       main(int argc, char **argv)
       {
           int sock, map_fd, prog_fd, key;
           long long value = 0, tcp_cnt, udp_cnt;

           map_fd = bpf_create_map(BPF_MAP_TYPE_ARRAY, sizeof(key),
                                   sizeof(value), 256);
           if (map_fd < 0) {
               printf("failed to create map '%s'\n", strerror(errno));
               /* likely not run as root */
               return 1;
           }

           struct bpf_insn prog[] = {
               BPF_MOV64_REG(BPF_REG_6, BPF_REG_1),        /* r6 = r1 */
               BPF_LD_ABS(BPF_B, ETH_HLEN + offsetof(struct iphdr, protocol)),
                                       /* r0 = ip->proto */
               BPF_STX_MEM(BPF_W, BPF_REG_10, BPF_REG_0, -4),
                                       /* *(u32 *)(fp - 4) = r0 */
               BPF_MOV64_REG(BPF_REG_2, BPF_REG_10),       /* r2 = fp */
               BPF_ALU64_IMM(BPF_ADD, BPF_REG_2, -4),      /* r2 = r2 - 4 */
               BPF_LD_MAP_FD(BPF_REG_1, map_fd),           /* r1 = map_fd */
               BPF_CALL_FUNC(BPF_FUNC_map_lookup_elem),
                                       /* r0 = map_lookup(r1, r2) */
               BPF_JMP_IMM(BPF_JEQ, BPF_REG_0, 0, 2),
                                       /* if (r0 == 0) goto pc+2 */
               BPF_MOV64_IMM(BPF_REG_1, 1),                /* r1 = 1 */
               BPF_XADD(BPF_DW, BPF_REG_0, BPF_REG_1, 0, 0),
                                       /* lock *(u64 *) r0 += r1 */
               BPF_MOV64_IMM(BPF_REG_0, 0),                /* r0 = 0 */
               BPF_EXIT_INSN(),                            /* return r0 */
           };

           prog_fd = bpf_prog_load(BPF_PROG_TYPE_SOCKET_FILTER, prog,
                                   sizeof(prog), "GPL");

           sock = open_raw_sock("lo");

           assert(setsockopt(sock, SOL_SOCKET, SO_ATTACH_BPF, &prog_fd,
                             sizeof(prog_fd)) == 0);

           for (;;) {
               key = IPPROTO_TCP;
               assert(bpf_lookup_elem(map_fd, &key, &tcp_cnt) == 0);
               key = IPPROTO_UDP;
               assert(bpf_lookup_elem(map_fd, &key, &udp_cnt) == 0);
               printf("TCP %lld UDP %lld 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.

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

       BPF_MAP_CREATE
              The new file descriptor associated with the eBPF map.

       BPF_PROG_LOAD
              The new file descriptor associated with the eBPF program.

       All other commands
              Zero.

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

ERRORS
       E2BIG  The  eBPF  program is too large or a map reached the max_entries
              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 rec-
              ommended to call bpf() again with  log_level  =  1  and  examine
              log_buf for the specific reason provided by the verifier.

       EBADF  fd is not an open file descriptor.

       EFAULT One  of  the pointers (key or value or log_buf or insns) is out-
              side 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 pro-
              gram.  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).

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

CONFORMING TO
       The bpf() system call is Linux-specific.

NOTES
       In the current implementation, all bpf() commands require the caller to
       have the CAP_SYS_ADMIN capability.

       eBPF  objects (maps and programs) can be shared between processes.  For
       example, after fork(2), the child inherits file  descriptors  referring
       to  the  same eBPF objects.  In addition, file 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 func-
       tions, floating-point numbers, and passing structures as function argu-
       ments.  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 com-
       piler  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 architectures.

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

       Both  classic  and extended BPF are explained in the kernel source file
       Documentation/networking/filter.txt.

COLOPHON
       This page is part of release 4.15 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
       https://www.kernel.org/doc/man-pages/.

Linux                             2018-02-02                            BPF(2)
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