Message-ID: <556583B4.4000607@gmail.com>
Date:	Wed, 27 May 2015 10:43:32 +0200
From:	"Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>
To:	Alexei Starovoitov <ast@...mgrid.com>
CC:	mtk.manpages@...il.com, Silvan Jegen <s.jegen@...il.com>,
	linux-man@...r.kernel.org, linux-kernel@...r.kernel.org,
	Daniel Borkmann <daniel@...earbox.net>,
	Walter Harms <wharms@....de>
Subject: Edited draft of bpf(2) man page for review/enhancement

Hello Alexei,

I took the draft 3 of the bpf(2) man page that you sent back in March 
and did some substantial editing to clarify the language and add a 
few technical details. Could you please check the revised  version
below, to ensure I did not inject any errors.

I also added a number of FIXMEs for pieces of the page that need
further work. Could you take a look at these and let me know your
thoughts, please.

Cheers,

Michael

.\" Copyright (C) 2015 Alexei Starovoitov <ast@...nel.org>
.\"
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.\"
.TH BPF 2 2015-03-10 "Linux" "Linux Programmer's Manual"
.SH NAME
bpf - perform a command on an extended BPF map or program
.SH SYNOPSIS
.nf
.B #include <linux/bpf.h>
.sp
.BI "int bpf(int cmd, union bpf_attr *attr, unsigned int size);

.SH DESCRIPTION
The
.BR bpf ()
system call performs a range of operations related to extended
Berkeley Packet Filters.
Extended BPF (or eBPF) is similar to
the original BPF (or classic BPF) used to filter network packets.
For both BPF and eBPF programs,
the kernel statically analyzes the programs before loading them,
in order to ensure that they cannot harm the running system.
.P
eBPF extends classic BPF in multiple ways including the ability to call
in-kernel helper functions (via the
.B BPF_CALL
opcode extension provided by eBPF)
and access shared data structures such as BPF maps.
The programs can be written in a restricted C that is compiled into
.\" FIXME In the next line, what is "a restricted C"? Where does
.\"       one get further information about it?
eBPF bytecode and executed on the in-kernel virtual machine or
just-in-time compiled into native code.
.SS Extended BPF Design/Architecture
.P
.\" FIXME In the following line, what does "different data types" mean?
.\"       Are the values in a map not just blobs?
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.
BPF programs can access maps from inside the kernel in parallel.
It's up to the user process and BPF program to decide what they store
inside maps.
.P
BPF programs are similar to kernel modules.
They are loaded by the user
process and automatically unloaded when the process exits.
Each BPF program is a set of instructions that is safe to run until
its completion.
The in-kernel BPF verifier statically determines that the program
terminates and is safe to execute.
.\" FIXME In the following sentence, what does "takes hold" mean?
During verification, the program takes hold of maps that it intends to use,
so selected maps cannot be removed until the program is unloaded.

BPF programs can be attached to different events.
.\" FIXME: In the next sentence , "packets" are not "events". What
.\" do you really mean to say here? ("the arrival of a network packet"?)
These events can be packets, tracing
events, and other types that may be added in the future.
A new event triggers execution of the BPF program, which
may store information about the event in the maps.
Beyond storing data, BPF programs may call into in-kernel helper functions.
The same program can be attached to multiple events and different programs can
access the same map:
.\" FIXME Can maps be shared between processes? (E.g., what happens
.\"       when fork() is called?)

.in +4n
.nf
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
.fi
.in
.SS Arguments
The operation to be performed by the
.BR bpf ()
system call is determined by the
.IR cmd
argument, which can be one of the following:
.TP
.B BPF_MAP_CREATE
Create a map with the specified type and attributes and return 
a file descriptor that refers to the map.
.TP
.B BPF_MAP_LOOKUP_ELEM
Look up an element by key in a specified map and return its value.
.TP
.B BPF_MAP_UPDATE_ELEM
Create or update an element (key/value pair) in a specified map.
.TP
.B BPF_MAP_DELETE_ELEM
Look up and delete an element by key in a specified map.
.TP
.B BPF_MAP_GET_NEXT_KEY
Look up an element by key in a specified map and return the key
of the next element.
.TP
.B BPF_PROG_LOAD
Verify and load a BPF program.
.PP
The
.I attr
argument is a pointer to a union of type
.IR bpf_attr
(see below);
.I size
is the size of the union pointed to by
.IR attr .
.P
The
.I bpf_attr
union consists of various anonymous structures that are used by different
.BR bpf ()
commands:

.in +4n
.nf
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 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 */
    };
} __attribute__((aligned(8)));
.fi
.in
.SS BPF maps
Maps are a generic data structure for storage of different types
and sharing data between the kernel and user-space programs.

Each map type has the following attributes:

.PD 0
.IP * 3
type
.IP *
maximum number of elements
.IP *
key size in bytes
.IP *
value size in bytes
.PD
.PP
The following wrapper functions demonstrate how various
.BR bpf ()
commands can be used to access the maps.
The functions use the
.IR cmd
argument to invoke different operations.
.TP 4
.B BPF_MAP_CREATE
The
.B BPF_MAP_CREATE
command creates a new map.

.in +4n
.nf
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));
}
.fi
.in

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.
On error, \-1 is returned and
.I errno
is set to
.BR EINVAL ,
.BR EPERM ,
or
.BR ENOMEM .

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

.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?
starting address will cause out-of-bounds stack access.

Similarly, when a map is created with a
.I value_size
of 1 and the program calls

.in +4n
.nf
value = bpf_map_lookup_elem(...);
*(u32 *) value = 1;
.fi
.in

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

Currently, two
.I map_type
are supported:

.in +4n
.nf
enum bpf_map_type {
    BPF_MAP_TYPE_UNSPEC,
    BPF_MAP_TYPE_HASH,
    BPF_MAP_TYPE_ARRAY,
};
.fi
.in
.\" FIXME Explain the purpose of BPF_MAP_TYPE_UNSPEC

.I map_type
selects one of the available map implementations in the kernel.
.\" FIXME We need an explanation of BPF_MAP_TYPE_HASH here
.\" FIXME We need an explanation of BPF_MAP_TYPE_ARRAY here
.\" FIXME We need an explanation of why one might choose HASH versus ARRAY
For all map types,
programs access maps with the same
.BR bpf_map_lookup_elem ()/
.BR bpf_map_update_elem ()
helper functions.
.TP
.B BPF_MAP_LOOKUP_ELEM
The
.B BPF_MAP_LOOKUP_ELEM
command looks up an element with a given
.I key
in the map referred to by the file descriptor
.IR fd .

.in +4n
.nf
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));
}
.fi
.in

If an element is found,
the operation returns zero and stores the element's value into
.I value.
.\" FIXME Here, I think we need some statement about what 'value' must
.\"       point to. Presumable, it must be a buffer at least as large as
.\"       the map's 'value_size' attribute?

If no element is found, the operation returns \-1 and sets
.I errno
to
.BR ENOENT .
.TP
.B BPF_MAP_UPDATE_ELEM
The
.B BPF_MAP_UPDATE_ELEM
command
creates or updates an element with a given
.I key/value
in the map referred to by the file descriptor
.IR fd .

.in +4n
.nf
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));
}
.fi
.in

The
.I flags
argument should be specified as one of the following:
.RS
.TP
.B BPF_ANY
Create a new element or update an existing element.
.TP
.B BPF_NOEXIST
Create a new element only if it did not exist.
.TP
.B BPF_EXIST
Update an existing element.
.RE
.IP
On success, the operation returns zero.
On error, \-1 is returned and
.I errno
is set to
.BR EINVAL ,
.BR EPERM ,
.BR ENOMEM ,
or
.BR E2BIG .
.B E2BIG
indicates that the number of elements in the map reached the
.I max_entries
limit specified at map creation time.
.B EEXIST
will be returned if
.I flags
specifies
.B BPF_NOEXIST
and the element with
.I key
already exists in the map.
.B ENOENT
will be returned if 
.I flags
specifies
.B BPF_EXIST
and the element with
.I key
doesn't exist in the map.
.TP
.B BPF_MAP_DELETE_ELEM
The
.B BPF_MAP_DELETE_ELEM
command
deleted the element whose key is
.I key
from the map referred to by the file descriptor
.IR fd .

.in +4n
.nf
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));
}
.fi
.in

On success, zero is returned.
If the element is not found, \-1 is returned and
.I errno
is set to
.BR ENOENT .
.TP
.B BPF_MAP_GET_NEXT_KEY
The
.B BPF_MAP_GET_NEXT_KEY
command looks up an element by
.I key
in the map referred to by the file descriptor
.IR fd 
and sets the
.I next_key
pointer to the key of the next element.

.nf
.in +4n
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));
}
.fi
.in

.\" FIXME Need to explain the return value on success here.
If
.I key
is not found, the operation returns zero and sets the
.I next_key
pointer to the key of the first element.
If
.I key
is the last element, \-1 is returned and
.I errno
is set to
.BR ENOENT .
Other possible
.I errno
values are
.BR ENOMEM ,
.BR EFAULT ,
.BR EPERM ,
and
.BR EINVAL .
This method can be used to iterate over all elements in the map.
.TP
.B close(map_fd)
Delete the map referred to by the file descriptor
.IR map_fd .
When the user-space program that created a map exits, all maps will
be deleted automatically.
.\" FIXME What are the semantics when a file descriptor is duplicated
.\"       (dup() etc.)? (I.e., when is a map deallocated automatically?)
.\"
.SS BPF programs
.TP 4
.B BPF_PROG_LOAD
The
.B BPF_PROG_LOAD
command is used to load an extended BPF program into the kernel.

.in +4n
.nf
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));
}
.fi
.in

.I prog_type
is one of the available program types:

.in +4n
.nf
enum bpf_prog_type {
    BPF_PROG_TYPE_UNSPEC,
.\" FIXME Explain the purpose of BPF_PROG_TYPE_UNSPEC
    BPF_PROG_TYPE_SOCKET_FILTER,
    BPF_PROG_TYPE_SCHED_CLS,
.\" FIXME BPF_PROG_TYPE_SCHED_CLS appears not to exist?
};
.fi
.in

By picking
.IR prog_type ,
the program author selects a set of helper functions that can be called from
the BPF program and the corresponding format of
.I struct bpf_context
(which is the data blob passed into the program as the first argument).
For example, programs loaded with

    prog_type = BPF_PROG_TYPE_SOCKET_FILTER

may call the
.BR bpf_map_lookup_elem ()
helper,
whereas some future program types may not.
The set of functions available to BPF programs of a given type may increase
in the future.

Currently, the set of functions for
.B BPF_PROG_TYPE_SOCKET_FILTER
is:

.in +4n
.nf
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 */
.fi
.in

.\" FIXME The next sentence fragment is incomplete
and
.I bpf_context
is a pointer to a
.IR "struct sk_buff" .
Programs cannot access fields of
.I sk_buff
directly.

More program types may be added in the future.
.\" FIXME The following sentence is grammatically broken.
.\"       What should it say?
Like
.B BPF_PROG_TYPE_KPROBE
and
.I bpf_context
for it may be defined as a pointer to a
.IR "struct pt_regs" .

The fields of
.I bpf_attr
are set as follows:
.RS
.IP * 3
.I insns
is an array of
.I "struct bpf_insn"
instructions.
.IP *
.I insn_cnt
is the number of instructions in the program referred to by
.IR insns .
.IP *
.I license
is a license string, which must be GPL compatible to call helper functions
.\" FIXME Maybe we should list the GPL compatible strings that can be
.\"       specified?
marked
.IR gpl_only .
.IP *
.I 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.
.IP *
.I log_size
size of the buffer pointed to by
.IR log_bug .
If the size of the buffer is not large enough to store all
verifier messages, \-1 is returned and
.I errno
is set to
.BR ENOSPC .
.IP *
.I log_level
verbosity level of the verifier.
A value of zero means that the verifier will
not provide a log.
.RE
.TP
.B close(prog_fd)
will unload the BPF program.
.P
Maps are accessible from BPF programs and are used to exchange data between
BPF programs and between BPF programs and user-space programs.
Programs process various events (like kprobe, packets) and
store their data into maps.
User-space programs fetch data from the maps.
.\" FIXME We need some elaboration here... What does the next sentence mean?
Either the same or a different map may be used by user space
as a configuration space to alter program behavior on the fly.
.SS Events
Once a program is loaded, it can be attached to an event.
Various kernel
subsystems have different ways to do so.
For example:

.in +4n
.nf
setsockopt(sockfd, SOL_SOCKET, SO_ATTACH_BPF,
           &prog_fd, sizeof(prog_fd));
.fi
.in

will attach the program
.I prog_fd
to the socket
.IR sockfd ,
which was received from a prior call to
.BR socket (2).

In the future,

.in +4n
.nf
ioctl(event_fd, PERF_EVENT_IOC_SET_BPF, prog_fd);
.fi
.in

may attach the program
.I prog_fd
to perf event
.I event_fd
which was received by prior call to
.BR perf_event_open (2).

.SH EXAMPLES
.\" FIXME It would be nice if this was a complete working example
.nf
/* 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),
.\" FIXME What does 'lock' in the line below mean?
                                /* 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,
.\" FIXME The next line looks wrong. Should it not be
.\"
.\"           sizeof(prog) / sizeof(struct bpf_insn) ?
                            sizeof(prog), "GPL");

    sock = open_raw_sock("lo");

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

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

    return 0;
}
.fi
.SH RETURN VALUE
For a successful call, the return value depends on the operation:
.TP
.B BPF_MAP_CREATE
The new file descriptor associated with the BPF map.
.TP
.B BPF_PROG_LOAD
The new file descriptor associated with the BPF program.
.TP
All other commands
Zero.
.PP
On error, \-1 is returned, and
.I errno
is set appropriately.
.SH ERRORS
.TP
.B EPERM
The call was made without sufficient privilege
(without the
.B CAP_SYS_ADMIN
capability).
.TP
.B ENOMEM
Cannot allocate sufficient memory.
.TP
.B EBADF
.I fd
is not an open file descriptor
.TP
.B EFAULT
One of the pointers
.RI ( key
or
.I value
or
.I log_buf
or
.IR insns )
is outside the accessible address space.
.TP
.B EINVAL
The value specified in
.I cmd
is not recognized by this kernel.
.TP
.B EINVAL
For
.BR BPF_MAP_CREATE ,
either
.I map_type
or attributes are invalid.
.TP
.B EINVAL
For
.BR BPF_MAP_*_ELEM
commands,
some of the fields of
.I "union bpf_attr"
that are not used by this command
are not set to zero.
.TP
.B EINVAL
For
.BR BPF_PROG_LOAD,
indicates an attempt to load an invalid program.
BPF programs can be deemed
invalid due to unrecognized instructions, the use of reserved fields, jumps
out of range, infinite loops or calls of unknown functions.
.TP
.BR EACCES
For
.BR 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
.BR bpf ()
again with
.I log_level = 1
and examine
.I log_buf
for the specific reason provided by the verifier.
.TP
.BR ENOENT
For
.B BPF_MAP_LOOKUP_ELEM
or
.BR BPF_MAP_DELETE_ELEM ,
indicates that the element with the given
.I key
was not found.
.TP
.BR E2BIG
The BPF program is too large or a map reached the
.I max_entries
limit (maximum number of elements).
.SH VERSIONS
The
.BR bpf ()
system call first appeared in Linux 3.18.
.SH CONFORMING TO
The
.BR bpf ()
system call is Linux-specific.
.SH NOTES
In the current implementation, all
.BR bpf ()
commands require the caller to have the
.B CAP_SYS_ADMIN
capability.
.SH SEE ALSO
.BR seccomp (2),
.BR socket (7)

Both classic and extended BPF are explained in the kernel source file
.IR 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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