bpf(2)                                                  System Calls Manual                                                  bpf(2)

       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 system.

       eBPF  extends  cBPF  in  multiple  ways,  including  the  ability to call a fixed set of in-kernel helper functions (via the
       BPF_CALL opcode extension provided by eBPF) and access shared data structures such as eBPF maps.

   Extended BPF Design/Architecture
       eBPF maps are a generic data structure for storage of different data types.  Data types  are  generally  treated  as  binary
       blobs,  so  a  user  just  specifies  the size of the key and the size of the value at map-creation time.  In other words, a
       key/value for a given map can have an arbitrary structure.

       A user process can create multiple maps (with key/value-pairs being opaque bytes of data) and access them via file  descrip‐
       tors.  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 pro‐
       grams.  When a lookup in the map is performed, the program flow is redirected in-place to the beginning of another eBPF pro‐
       gram and does not return back to the calling program.  The level of nesting has a fixed limit of 32, so that infinite  loops
       cannot  be  crafted.   At run time, the program file descriptors stored in the map can be modified, so program functionality
       can be altered based on specific requirements.  All programs referred to in a program-array map must  have  been  previously
       loaded  into  the  kernel  via  bpf().   If  a  map  lookup  fails,  the  current  program  continues  its  execution.   See
       BPF_MAP_TYPE_PROG_ARRAY below for further details.

       Generally, eBPF programs are loaded by the user process and automatically unloaded when the process exits.  In  some  cases,
       for  example,  tc-bpf(8),  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  de‐
       termines  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  un‐

       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  accompa‐
       nying  argument,  provided via attr, which is a pointer to a union of type bpf_attr (see below).  The unused fields and pad‐
       ding must be zeroed out before the call.  The size argument is the size of the union pointed to by attr.

       The value provided in cmd is one of the following:

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

              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 element.

              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.

              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

              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) start‐
              ing 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 */
                      /* See /usr/include/linux/bpf.h for the full list. */

              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

                  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  cre‐
              ation  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 deletes the element whose key is key from the map referred to by the file  descriptor

                  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 element.

                  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 NOTES).

   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,  up‐
                 date, 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.

              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  pro‐
                 gram.  In other words, array_map_lookup_elem() may be 'inlined' by the verifier/JIT compiler while preserving con‐
                 current access to this map from user space.

              •  All array elements pre-allocated and zero initialized at init time

              •  The key is an array index, and must be exactly four bytes.

              •  map_delete_elem() fails with the error EINVAL, since elements cannot be deleted.

              •  map_update_elem() replaces elements in a nonatomic fashion; for atomic updates, a hash-table map  should  be  used
                 instead.   There  is  however  one  special  case  that  can  also  be  used  with  arrays:  the  atomic  built-in
                 __sync_fetch_and_add() can be used on 32 and 64 bit atomic counters.  For example, it can be applied on the  whole
                 value  itself  if it represents a single counter, or in case of a structure containing multiple counters, it could
                 be used on individual counters.  This is quite often useful for aggregation and accounting of events.

              Among the uses for array maps are the following:

              •  As "global" eBPF variables: an array of 1 element whose key is (index) 0 and where the value is  a  collection  of
                 'global' variables which eBPF programs can use to keep state between events.

              •  Aggregation of tracing events into a fixed set of buckets.

              •  Accounting of networking events, for example, number of packets and packet sizes.

       BPF_MAP_TYPE_PROG_ARRAY (since Linux 4.2)
              A  program array map is a special kind of array map whose map values contain only file descriptors referring to other
              eBPF programs.  Thus, both the 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  perfor‐
              mance  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 ver‐
              sions 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 */
                      /* See /usr/include/linux/bpf.h for the full list. */

       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 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 config‐
       uration 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 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]

       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);

       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 to indicate the error.

       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/regis‐
              ter 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.

       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

       EINVAL For BPF_PROG_LOAD, indicates an attempt to load an invalid program.  eBPF programs can be deemed invalid due  to  un‐
              recognized  instructions,  the  use  of reserved fields, jumps out of range, infinite loops or calls of unknown func‐

       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).

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

       The bpf() system call is Linux-specific.

       Prior to Linux 4.4, all bpf() commands require the caller to have the CAP_SYS_ADMIN capability.  From Linux 4.4 onwards,  an
       unprivileged user may create limited programs of type BPF_PROG_TYPE_SOCKET_FILTER and associated maps.  However they may not
       store kernel pointers within the maps and are presently limited to the following helper functions:

       •  get_random
       •  get_smp_processor_id
       •  tail_call
       •  ktime_get_ns

       Unprivileged access may be blocked by writing the value 1 to the file /proc/sys/kernel/unprivileged_bpf_disabled.

       eBPF objects (maps and programs) can be shared between processes.  For example, after fork(2), the child inherits  file  de‐
       scriptors  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 fea‐
       tures 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

       The kernel contains a just-in-time (JIT) compiler that translates eBPF bytecode into native machine code for better  perfor‐
       mance.   Before  Linux  4.15, the JIT compiler is disabled by default, but its operation can be controlled by writing one of
       the following integer strings to the file /proc/sys/net/core/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 dis‐
              assembled using the program tools/net/bpf_jit_disasm.c provided in the kernel source tree.

       Since Linux 4.15, the kernel may configured with the CONFIG_BPF_JIT_ALWAYS_ON option.  In this case, the JIT compiler is al‐
       ways enabled, and the bpf_jit_enable is initialized to 1 and is immutable.  (This kernel configuration option  was  provided
       as a mitigation for one of the Spectre attacks against the BPF interpreter.)

       The JIT compiler for eBPF is currently available for the following architectures:

       •  x86-64 (since Linux 3.18; cBPF since Linux 3.0);
       •  ARM32 (since Linux 3.18; cBPF since Linux 3.4);
       •  SPARC 32 (since Linux 3.18; cBPF since Linux 3.5);
       •  ARM-64 (since Linux 3.18);
       •  s390 (since Linux 4.1; cBPF since Linux 3.7);
       •  PowerPC 64 (since Linux 4.8; cBPF since Linux 3.1);
       •  SPARC 64 (since Linux 4.12);
       •  x86-32 (since Linux 4.18);
       •  MIPS 64 (since Linux 4.18; cBPF since Linux 3.16);
       •  riscv (since Linux 5.1).

       /* 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) / sizeof(prog[0]), "GPL");

           sock = open_raw_sock("lo");

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

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

           return 0;

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

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

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

Linux man-pages 6.03                                         2023-02-05                                                      bpf(2)