bpf

       #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  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.

       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  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_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
              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,
                  };

              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.
                          .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)
                  {

              to the key of the first element.  If key is the last element, -1
              is  returned  and  errno is set to ENOENT.  Other possible errno
              values are ENOMEM, EFAULT, EPERM, and EINVAL.  This  method  can
              be used to iterate over all elements in the map.

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

   eBPF map types
       The following map types are supported:

       BPF_MAP_TYPE_HASH
              Hash-table maps have the following characteristics:

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

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

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

              *  map_update_elem() replaces existing elements 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.
                 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,

               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_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  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.
       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
               return 1;
           }

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

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

           sock = open_raw_sock("lo");

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

           for (;;) {
               key = IPPROTO_TCP;
               assert(bpf_lookup_elem(map_fd, &key, &tcp_cnt) == 0);
               key = IPPROTO_UDP
               assert(bpf_lookup_elem(map_fd, &key, &udp_cnt) == 0);
               printf("TCP %lld UDP %lld packets0, tcp_cnt, udp_cnt);
               sleep(1);
           }

           return 0;
       }

       Some complete working code can be found in the samples/bpf 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.

       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.

       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.

       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).

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-
       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 4.04 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
       http://www.kernel.org/doc/man-pages/.

Linux                             2015-07-23                            BPF(2)
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