Re: Draft 3 of bpf(2) man page for review

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On 07/22/2015 09:22 PM, Alexei Starovoitov wrote:
> On 7/22/15 11:43 AM, Michael Kerrisk (man-pages) wrote:
>> .TH BPF 2 2015-03-10 "Linux" "Linux Programmer's Manual"
> 
> should the date be updated ?

It'll get updated later, by scripts.

>> BPF maps are a generic data structure for storage of different data types.
>> A user process can create multiple maps (with key/value-pairs being
>> opaque bytes of data) and access them via file descriptors.
>> eBPF programs can access maps from inside the kernel in parallel.
>> .\"
>> .\" FIXME!! What does the previous sentence mean?
>> .\"
>> .\" Isn't "from inside the kernel" redundant? (I mean: all eBPF programs
>> .\" are running inside the kernel, right?)
> 
> 99.9% of the time. yes. all eBPF programs are running inside the kernel,
> though recently I've seen two versions of 'user space eBPF' where
> kernel interpreter/x64_jit were ported to user space.
> If you think 'from kernel' is redundant, just drop it.

Okay. Done.

>> .\" And what does "in parallel" mean?
>> .\" Would a simpler version of this sentence be correct? As in:
>> .\"     "Different eBPF programs can access the same maps in parallel."
> 
> yes. different eBPF programs and user space processes can access the
> same maps in parallel.

Okay.

>> The new map has the type specified by
>> .IR map_type ,
>> and attributes as specified in
>> .IR key_size ,
>> .IR value_size ,
>> and
>> .IR max_entries .
>> .\" FIXME!! In the next sentence, what does "process-local" mean?
>> On success, this operation returns a process-local file descriptor.
> 
> Just drop this unnecessary qualifier. Just 'returns a file descriptor'

Done.

>> .in +4n
>> .nf
>> bpf_map_lookup_elem(map_fd, fp - 4)
>> .fi
>> .in
>>
>> 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
>> .I key
>> pointer, but
>> .IR "fp\ -\ 4"
>> .\" FIXME!! I'm lost! What is 'fp' in this context?
> 
> it refers to 2nd argument of 'bpf_map_lookup_elem(map_fd, fp - 4)'
> fp = top of the stack.
> fp - 4 = pointer to 4 bytes below top of the stack.
> So 8 byte access from there will be out of bounds.

Okay. I added some words mentioning that 'fp' is top of stack.

>> The following map types are supported:
>> .TP
>> .B BPF_MAP_TYPE_HASH
>> .\" commit 0f8e4bd8a1fc8c4185f1630061d0a1f2d197a475
>> .\" FIXME!! Please review the following list of points, which draws
>> .\" heavily from the commit message, but reworks the text significantly
>> .\" and so may have introduced errors.
>> Hash-table maps have the following characteristics:
>> .RS
>> .IP * 3
>> Maps are created and destroyed by user-space programs.
>> Both user-space and eBPF programs
>> can perform lookuo, update, and delete operations.
> 
> typo 'lookup'

Thanks, fixed.

>> .IP *
>> The kernel takes care of allocating and freeing key/value pairs.
>> .IP *
>> The
>> .BR map_update_elem ()
>> helper with fail to insert new element when the
>> .I max_entries
>> limit is reached.
>> (This ensures that eBPF programs cannot exhaust memory.)
>> .IP *
>> .BR map_update_elem ()
>> replaces existing elements atomically.
>> .RE
>> .IP
>> Hash-table maps are
>> optimized for speed of lookup.
>> .TP
>> .B BPF_MAP_TYPE_ARRAY
>> .\" commit 28fbcfa08d8ed7c5a50d41a0433aad222835e8e3
>> .\" FIXME!! Please review the following list of points, which draws
>> .\" heavily from the commit message, but reworks the text significantly
>> .\" and so may have introduced errors.
>> Array maps have the following characteristics:
>> .RS
>> .IP * 3
>> Optimized for fastest possible lookup.
>> In the future ithe verifier/JIT compiler
> 
> typo 'the'

Fixed.

>> 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
>> .I value_size
>> are constant for the life of the eBPF program.
>> In other words,
>> .BR array_map_lookup_elem ()
>> may be 'inlined' by the verifier/JIT compiler
>> while preserving concurrent access to this map from user space.
>> .IP *
>> All array elements pre-allocated and zero initialized at init time
>> .IP *
>> The key is an array index, and must be exactly four bytes.
>> .IP *
>> .BR map_delete_elem ()
>> fails with the error
>> .BR EINVAL ,
>> since elements cannot be deleted.
>> .IP *
>> .BR map_update_elem ()
>> replaces elements in an non-atomic fashion;
>> for atomic updates, a hash-table map should be used instead.
> 
> the description of hash and array maps looks good.

Okay. Thanks for checking.

>> .\" FIXME The following paragraph needs amending. Alexei commented:
>> .\"
>> .\"     Actually now in case of SOCKET_FILTER, SCHED_CLS, SCHED_ACT
>> .\"     the program can now access skb fields.
>> .\"     See 'struct __sk_buff' and commit 9bac3d6d548e5
>> .\"
>> .\" Do we want some text here to explain how the program access __sk_buff?
> 
> I think commit 9bac3d6d548e5 tried to explain it, but translating
> that to english would be nice :)

Yes, but my C-to-English translator failed.

>> .\" FIXME!! Alexei, is the following correct?
>> eBPF objects (maps and programs) can be shared between processes.
>> For example, after
>> .BR 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
>> .BR dup (2)
>> and similar calls.
>> An eBPF object is deallocated only after all file descriptors
>> referring to the object have been closed.
> 
> yes. all correct.


Thanks.

>> eBPF programs can be written in a restricted C that is compiled (using the
>> .B clang
>> compiler) into eBPF bytecode and executed on the in-kernel virtual machine or
>> just-in-time compiled into native code.
>> (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
>> .I samples/bpf/*_kern.c
>> files in the kernel source tree.
> 
> thanks. whole thing looks good.

Thanks.

Below is the current rendered version of the man page.

Cheers,

Michael


NAME
       bpf - perform a command on an extended eBPF 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 sim‐
       ilar  to  the original ("classic") BPF (cBPF) used to filter net‐
       work packets.  For both cBPF and eBPF programs, the kernel stati‐
       cally  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  struc‐
       tures such as eBPF maps.

   Extended BPF Design/Architecture
       BPF  maps  are  a generic data structure for storage of different
       data types.  A  user  process  can  create  multiple  maps  (with
       key/value-pairs  being  opaque bytes of data) and access them via
       file descriptors.  Differnt 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.

       eBPF programs are similar to kernel modules.  They are loaded  by
       the  user  process  and  automatically  unloaded when the process
       exits.  Each program is a set of instructions that is safe to run
       until  its  completion.   An in-kernel verifier statically deter‐
       mines 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 selected
       maps cannot 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,  classifi‐
       cation  event  by  qdisc  (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 dif‐
       ferent eBPF programs can access the same map:

           tracing     tracing     tracing     packet     packet
           event A     event B     event C     on eth0    on eth1
            |             |          |           |          |
            |             |          |           |          |
            --> tracing <--      tracing       socket     socket
                 prog_1           prog_2       prog_3     prog_4
                 |  |               |            |
              |---  -----|  |-------|           map_3
            map_1       map_2

   Arguments
       The  operation to be performed by the bpf() system call is deter‐
       mined 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 with 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 speci‐
              fied 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_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  pro‐
       grams, 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, int key_size,
                                 int value_size, 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 attributes key_size and value_size will be used by the
              verifier  during program loading to check that the program
              is calling bpf_map_*_elem() helper functions with  a  cor‐
              rectly  initialized  key  and  to  check  that the program
              doesn't access the map element value beyond the  specified
              value_size.   For  example,  when  a map is created with a
              key_size of 8 and the eBPF program calls

                  bpf_map_lookup_elem(map_fd, fp - 4)

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

                  bpf_map_lookup_elem(map_fd, void *key)

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

              Similarly,  when  a  map is created with a value_size of 1
              and the eBPF program 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, 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 ele‐
              ment with a given key/value in the map referred to by  the
              file descriptor fd.

                  int
                  bpf_update_elem(int fd, void *key, void *value, __u64 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 fol‐
              lowing:

              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 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.

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

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

              If key is found, the operation returns zero and  sets  the
              next_key  pointer  to the key of the next element.  If key
              is not found, the operation  returns  zero  and  sets  the
              next_key  pointer to the key of the first element.  If key
              is the last element, -1 is returned and errno  is  set  to
              ENOENT.   Other  possible errno values are ENOMEM, EFAULT,
              EPERM, and EINVAL.  This method can  be  used  to  iterate
              over all elements in the map.

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

   eBPF map types
       The following map types are supported:

       BPF_MAP_TYPE_HASH
              Hash-table maps have the following characteristics:

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

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

              *  The map_update_elem() helper 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  atomi‐
                 cally.

              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() opera‐
                 tions  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  veri‐
                 fier/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 an non-atomic
                 fashion; for atomic updates, a hash-table map should be
                 used instead.

              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 buck‐
                 ets.

       BPF_MAP_TYPE_PROG_ARRAY (since Linux 4.2)
              [To be completed]

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

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

       prog_type is one of the available program types:

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

       For further details of eBPF program types, see below.

       The remaining fields of bpf_attr are set as follows:

       *  insns is an array of struct bpf_insn instructions.

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

       *  license is a license string, which must be GPL  compatible  to
          call helper functions marked gpl_only.

       *  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 BPF 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.

       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.   Con‐
       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 behavior on the fly according to
       those values.

   eBPF program types
       By picking prog_type, the program author selects a set of  helper
       functions that can be called from the eBPF program and the corre‐
       sponding format of struct bpf_context (which  is  the  data  blob
       passed  into  the eBPF program as the first argument).  For exam‐
       ple,     programs     loaded     with     a     prog_type      of
       BPF_PROG_TYPE_SOCKET_FILTER  may  call  the bpf_map_lookup_elem()
       helper, whereas some other program  types  may  not  be  able  to
       employ  this helper.  The set of functions available to eBPF pro‐
       grams 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.
              Programs cannot access the fields of sk_buff directly.

       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.   Vari‐
       ous 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);

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

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

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

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

           sock = open_raw_sock("lo");

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

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

           return 0;
       }

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

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

       BPF_MAP_CREATE
              The new file descriptor associated with the eBPF map.

       BPF_PROG_LOAD
              The new file descriptor associated with the eBPF program.

       All other commands
              Zero.

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

ERRORS
       EPERM  The  call  was  made without sufficient privilege (without
              the CAP_SYS_ADMIN capability).

       ENOMEM Cannot allocate sufficient memory.

       EBADF  fd is not an open file descriptor

       EFAULT One of the pointers (key or value or log_buf or insns)  is
              outside the accessible address space.

       EINVAL The  value specified in cmd is not recognized by this ker‐
              nel.

       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.  BPF programs  can  be  deemed  einvalid  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/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  spe‐
              cific 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 BPF  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  descrip‐
       tors  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 and executed on the
       in-kernel  virtual  machine  or just-in-time compiled into native
       code.  (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.

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.


-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
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