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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 extension provided by eBPF)
       and access shared data structures such as eBPF maps.

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

       A  user process can create multiple maps (with key/value-pairs being opaque bytes of data) and access them via file descriptors.  Different eBPF programs can acā€
       cess 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 nestā€
       ing 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 determines that the eBPF program termiā€
       nates 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 proā€
       gram, which may store information about the event in eBPF maps.  Beyond storing data, eBPF programs may call a fixed set of in-kernel helper functions.

       The same eBPF program can be attached to multiple events and different eBPF programs can access the same map:

           tracing     tracing    tracing    packet      packet     packet
           event A     event B    event C    on eth0     on eth1    on eth2
            |             |         |          |           |          ^
            |             |         |          |           v          |
            --> tracing <--     tracing      socket    tc ingress   tc egress
                 prog_1          prog_2      prog_3    classifier    action
                 |  |              |           |         prog_4      prog_5
              |---  -----|  |------|          map_3        |           |
            map_1       map_2                              --| map_4 |--

   Arguments
       The  operation  to  be  performed  by the bpf() system call is determined by the cmd argument.  Each operation takes an accompanying argument, provided via attr,
       which is a pointer to a union of type 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 success, this operation  returns
              a file descriptor.  On error, -1 is returned and errno is set to EINVAL, EPERM, or ENOMEM.

              The  key_size  and  value_size attributes will be used by the verifier during program loading to check that the program is calling bpf_map_*_elem() helper
              functions with a correctly initialized key and to check that the program doesn't access the map element value beyond the specified value_size.  For  examā€
              ple, when a map is created with a key_size of 8 and the eBPF program calls

                  bpf_map_lookup_elem(map_fd, fp - 4)

              the program will be rejected, since the in-kernel helper function

                  bpf_map_lookup_elem(map_fd, void *key)

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

              Similarly, when a map is created with a value_size of 1 and the eBPF program 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,
                      BPF_MAP_TYPE_XSKMAP,
                      BPF_MAP_TYPE_SOCKHASH,
                      BPF_MAP_TYPE_CGROUP_STORAGE,
                      BPF_MAP_TYPE_REUSEPORT_SOCKARRAY,
                      BPF_MAP_TYPE_PERCPU_CGROUP_STORAGE,
                      BPF_MAP_TYPE_QUEUE,
                      BPF_MAP_TYPE_STACK,
                      /* See /usr/include/linux/bpf.h for the full list. */
                  };

              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 deletes the element whose key is key from the map referred to by the file descriptor fd.

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

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

              On success, zero is returned.  If the element is not found, -1 is returned and errno 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 erā€
              rno 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  optiā€
                 mize 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 conā€
                 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 inā€
                 dividual 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 performance benefits, and also makes it possible to overcome the maximum inā€
              struction limit of a single eBPF program.  In dynamic environments, a user-space daemon might atomically replace individual subprograms at  run-time  with
              newer versions to alter overall program behavior, for instance, if global policies change.

   eBPF programs
       The  BPF_PROG_LOAD command is used to load an eBPF program into the kernel.  The return value for this command is a new file descriptor associated with this eBPF
       program.

           char bpf_log_buf[LOG_BUF_SIZE];

           int
           bpf_prog_load(enum bpf_prog_type type,
                         const struct bpf_insn *insns, int insn_cnt,
                         const char *license)
           {
               union bpf_attr attr = {
                   .prog_type = type,
                   .insns     = ptr_to_u64(insns),
                   .insn_cnt  = insn_cnt,
                   .license   = ptr_to_u64(license),
                   .log_buf   = ptr_to_u64(bpf_log_buf),
                   .log_size  = LOG_BUF_SIZE,
                   .log_level = 1,
               };

               return bpf(BPF_PROG_LOAD, &attr, sizeof(attr));
           }

       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,
                      BPF_PROG_TYPE_TRACEPOINT,
                      BPF_PROG_TYPE_XDP,
                      BPF_PROG_TYPE_PERF_EVENT,
                      BPF_PROG_TYPE_CGROUP_SKB,
                      BPF_PROG_TYPE_CGROUP_SOCK,
                      BPF_PROG_TYPE_LWT_IN,
                      BPF_PROG_TYPE_LWT_OUT,
                      BPF_PROG_TYPE_LWT_XMIT,
                      BPF_PROG_TYPE_SOCK_OPS,
                      BPF_PROG_TYPE_SK_SKB,
                      BPF_PROG_TYPE_CGROUP_DEVICE,
                      BPF_PROG_TYPE_SK_MSG,
                      BPF_PROG_TYPE_RAW_TRACEPOINT,
                      BPF_PROG_TYPE_CGROUP_SOCK_ADDR,
                      BPF_PROG_TYPE_LWT_SEG6LOCAL,
                      BPF_PROG_TYPE_LIRC_MODE2,
                      BPF_PROG_TYPE_SK_REUSEPORT,
                      BPF_PROG_TYPE_FLOW_DISSECTOR,
                      /* See /usr/include/linux/bpf.h for the full list. */
                  };

       For further details of eBPF program types, see below.

       The remaining fields of 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.   Conā€
       versely,  user-space programs can use a map as a configuration mechanism, populating the map with values checked by the eBPF program, which then modifies its beā€
       havior 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)ā€”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]

   Events
       Once a program is loaded, it can be attached to an event.  Various kernel subsystems have different ways to do so.

       Since Linux 3.19, the following call will attach the program 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);

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 to indicate the error.

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 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 zero.

       EINVAL For BPF_PROG_LOAD, indicates an attempt to load an invalid program.  eBPF programs can be deemed invalid due to unrecognized instructions, the use of  reā€
              served 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
       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 descriptors referring to the same eBPF obā€
       jects.  In addition, file descriptors referring to eBPF objects can be transferred over UNIX domain sockets.  File descriptors referring to eBPF objects  can  be
       duplicated  in  the  usual  way, using dup(2) and similar calls.  An eBPF object is deallocated only after all file descriptors referring to the object have been
       closed.

       eBPF programs can be written in a restricted C that is compiled (using the clang compiler) into eBPF bytecode.  Various features are omitted from this restricted
       C,  such as loops, global variables, variadic functions, floating-point numbers, and passing structures as function arguments.  Some examples can be found in the
       samples/bpf/*_kern.c files in the kernel source tree.

       The kernel contains a just-in-time (JIT) compiler that translates eBPF bytecode into native machine code for better performance.  In kernels 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  disassembled  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 always 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  interā€
       preter.)

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

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

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

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

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

           prog_fd = bpf_prog_load(BPF_PROG_TYPE_SOCKET_FILTER, prog,
                                   sizeof(prog) / sizeof(prog[0]), "GPL");

           sock = open_raw_sock("lo");

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

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

           return 0;
       }

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

SEE ALSO
       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                                                                          2021-08-27                                                                         BPF(2)