[PATCH v12 net-next 00/11] eBPF syscall, verifier, testsuite

From: Alexei Starovoitov
Date: Mon Sep 15 2014 - 15:24:05 EST

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Hi All,

the list of changes v11 -> v12:
- dropped patch 11 and copied few macros to libbpf.h (suggested by Daniel)
- replaced 'enum bpf_prog_type' with u32 to be safe in compat (.. Andy)
- implemented and tested compat support (not part of this set) (.. Daniel)
- changed 'void *log_buf' to 'char *' (.. Daniel)
- combined struct bpf_work_struct and bpf_prog_info (.. Daniel)
- added better return value explanation to manpage (.. Andy)
- added log_buf/log_size explanation to manpage (.. Andy & Daniel)
- added a lot more info about prog_type and map_type to manpage (.. Andy)
- rebased, tweaked test_stubs

Patches 1-4 establish BPF syscall shell for maps and programs.
Patches 5-10 add verifier step by step
Patch 11 adds test stubs for 'unspec' program type and verifier testsuite
from user space

Note that patches 1,3,4,7 add commands and attributes to the syscall
while being backwards compatible from each other, which should demonstrate
how other commands can be added in the future.

After this set the programs can be loaded for testing only. They cannot
be attached to any events. Though manpage talks about tracing and sockets,
it will be a subject of future patches.

Please take a look at manpage:

BPF(2) Linux Programmer's Manual BPF(2)

NAME
bpf - perform a command on eBPF map or program

SYNOPSIS
#include <linux/bpf.h>

int bpf(int cmd, union bpf_attr *attr, unsigned int size);

DESCRIPTION
bpf() syscall is a multiplexor for a range of different operations on
eBPF which can be characterized as "universal in-kernel virtual
machine". eBPF is similar to original Berkeley Packet Filter (or
"classic BPF") used to filter network packets. Both statically analyze
the programs before loading them into the kernel to ensure that
programs cannot harm the running system.

eBPF extends classic BPF in multiple ways including ability to call in-
kernel helper functions and access shared data structures like eBPF
maps. The programs can be written in a restricted C that is compiled
into eBPF bytecode and executed on the eBPF virtual machine or JITed
into native instruction set.

eBPF Design/Architecture
eBPF maps is a generic storage of different types. User process can
create multiple maps (with key/value being opaque bytes of data) and
access them via file descriptor. In parallel eBPF programs can access
maps from inside the kernel. It's up to 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 process exits. Each eBPF
program is a safe run-to-completion set of instructions. eBPF verifier
statically determines that the program terminates and is safe to
execute. During verification the program takes a hold of maps that it
intends to use, so selected maps cannot be removed until the program is
unloaded. The program can be attached to different events. These events
can be packets, tracepoint events and other types in the future. A new
event triggers execution of the program which may store information
about the event in the maps. Beyond storing data the programs may call
into in-kernel helper functions which may, for example, dump stack, do
trace_printk or other forms of live kernel debugging. The same program
can be attached to multiple events. Different programs can access the
same map:
tracepoint tracepoint tracepoint sk_buff sk_buff
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

Syscall Arguments
bpf() syscall operation is determined by cmd which can be one of the
following:

BPF_MAP_CREATE
Create a map with given type and attributes and return map FD

BPF_MAP_LOOKUP_ELEM
Lookup element by key in a given map and return its value

BPF_MAP_UPDATE_ELEM
Create or update element (key/value pair) in a given map

BPF_MAP_DELETE_ELEM
Lookup and delete element by key in a given map

BPF_MAP_GET_NEXT_KEY
Lookup element by key in a given map and return key of next
element

BPF_PROG_LOAD
Verify and load eBPF program

attr is a pointer to a union of type bpf_attr as defined below.

size is the size of the union.

union bpf_attr {
struct { /* anonymous struct used by BPF_MAP_CREATE command */
__u32 map_type;
__u32 key_size; /* size of key in bytes */
__u32 value_size; /* size of value in bytes */
__u32 max_entries; /* max number of entries in a map */
};

struct { /* anonymous struct used by BPF_MAP_*_ELEM commands */
int map_fd;
void *key;
union {
void *value;
void *next_key;
};
};

struct { /* anonymous struct used by BPF_PROG_LOAD command */
__u32 prog_type;
__u32 insn_cnt;
const struct bpf_insn *insns;
const char *license;
__u32 log_level; /* verbosity level of eBPF verifier */
__u32 log_size; /* size of user buffer */
char *log_buf; /* user supplied buffer */
};
};

eBPF maps
maps is a generic storage of different types for sharing data between
kernel and userspace.

Any map type has the following attributes:
. type
. max number of elements
. key size in bytes
. value size in bytes

The following wrapper functions demonstrate how this syscall can be
used to access the maps. The functions use the cmd argument to invoke
different operations.

BPF_MAP_CREATE
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));
}
bpf() syscall creates a map of map_type type and given
attributes key_size, value_size, max_entries. On success it
returns process-local file descriptor. On error, -1 is returned
and errno is set to EINVAL or EPERM or ENOMEM.

The attributes key_size and value_size will be used by verifier
during program loading to check that program is calling
bpf_map_*_elem() helper functions with correctly initialized key
and that program doesn't access map element value beyond
specified value_size. For example, when map is created with
key_size = 8 and program does:
bpf_map_lookup_elem(map_fd, fp - 4)
such program will be rejected, since in-kernel helper function
bpf_map_lookup_elem(map_fd, void *key) expects to read 8 bytes
from 'key' pointer, but 'fp - 4' starting address will cause out
of bounds stack access.

Similarly, when map is created with value_size = 1 and program
does:
value = bpf_map_lookup_elem(...);
*(u32 *)value = 1;
such program will be rejected, since it accesses value pointer
beyond specified 1 byte value_size limit.

Currently only hash table map_type is supported:
enum bpf_map_type {
BPF_MAP_TYPE_UNSPEC,
BPF_MAP_TYPE_HASH,
};
map_type selects one of the available map implementations in
kernel. For all map_types eBPF programs access maps with the
same bpf_map_lookup_elem()/bpf_map_update_elem() helper
functions.

BPF_MAP_LOOKUP_ELEM
int bpf_lookup_elem(int fd, void *key, void *value)
{
union bpf_attr attr = {
.map_fd = fd,
.key = key,
.value = value,
};

return bpf(BPF_MAP_LOOKUP_ELEM, &attr, sizeof(attr));
}
bpf() syscall looks up an element with given key in a map fd.
If element is found it returns zero and stores element's value
into value. If element is not found it returns -1 and sets
errno to ENOENT.

BPF_MAP_UPDATE_ELEM
int bpf_update_elem(int fd, void *key, void *value)
{
union bpf_attr attr = {
.map_fd = fd,
.key = key,
.value = value,
};

return bpf(BPF_MAP_UPDATE_ELEM, &attr, sizeof(attr));
}
The call creates or updates element with given key/value in a
map fd. On success it returns zero. On error, -1 is returned
and errno is set to EINVAL or EPERM or ENOMEM or E2BIG. E2BIG
indicates that number of elements in the map reached max_entries
limit specified at map creation time.

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

return bpf(BPF_MAP_DELETE_ELEM, &attr, sizeof(attr));
}
The call deletes an element in a map fd with given key. Returns
zero on success. If element is not found it returns -1 and sets
errno to ENOENT.

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

return bpf(BPF_MAP_GET_NEXT_KEY, &attr, sizeof(attr));
}
The call looks up an element by key in a given map fd and
returns key of the next element into next_key pointer. If key is
not found, it return zero and returns key of the first element
into next_key. If key is the last element, it returns -1 and
sets errno to ENOENT. Other possible errno values are ENOMEM,
EFAULT, EPERM, EINVAL. This method can be used to iterate over
all elements of the map.

close(map_fd)
will delete the map map_fd. Exiting process will delete all
maps automatically.

eBPF programs
BPF_PROG_LOAD
This cmd is used to load eBPF program into the kernel.

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 = insns,
.insn_cnt = insn_cnt,
.license = license,
.log_buf = 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,
BPF_PROG_TYPE_SOCKET,
BPF_PROG_TYPE_TRACING,
};
By picking prog_type program author selects a set of helper
functions callable from eBPF program and corresponding format of
struct bpf_context (which is the data blob passed into the
program as the first argument). For example, the programs
loaded with prog_type = TYPE_TRACING may call bpf_printk()
helper, whereas TYPE_SOCKET programs may not. The set of
functions available to the programs under given type may
increase in the future.

Currently the set of functions for TYPE_TRACING is:
bpf_map_lookup_elem(map_fd, void *key) // lookup 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
bpf_ktime_get_ns(void) // returns current ktime
bpf_printk(char *fmt, int fmt_size, ...) // prints into trace buffer
bpf_memcmp(void *ptr1, void *ptr2, int size) // non-faulting memcmp
bpf_fetch_ptr(void *ptr) // non-faulting load pointer from any address
bpf_fetch_u8(void *ptr) // non-faulting 1 byte load
bpf_fetch_u16(void *ptr) // other non-faulting loads
bpf_fetch_u32(void *ptr)
bpf_fetch_u64(void *ptr)

and bpf_context is defined as:
struct bpf_context {
/* argN fields match one to one to arguments passed to trace events */
u64 arg1, arg2, arg3, arg4, arg5, arg6;
/* return value from kretprobe event or from syscall_exit event */
u64 ret;
};

The set of helper functions for TYPE_SOCKET is TBD.

More program types may be added in the future. Like
BPF_PROG_TYPE_USER_TRACING for unprivileged programs.

BPF_PROG_TYPE_UNSPEC is used for testing only. Such programs
cannot be attached to events.

insns array of "struct bpf_insn" instructions

insn_cnt number of instructions in the program

license license string, which must be GPL compatible to call
helper functions marked gpl_only

log_buf user supplied buffer that in-kernel verifier is using to
store verification log. Log is a multi-line string that should
be used by program author to understand how verifier came to
conclusion that program is unsafe. The format of the output can
change at any time as verifier evolves.

log_size size of user buffer. If 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 eBPF verifier, where zero means no
logs provided

close(prog_fd)
will unload eBPF program

The maps are accesible from programs and generally tie the two
together. Programs process various events (like tracepoint, kprobe,
packets) and store the data into maps. User space fetches data from
maps. Either the same or a different map may be used by user space as
configuration space to alter program behavior on the fly.

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

setsockopt(sock, SOL_SOCKET, SO_ATTACH_BPF, &prog_fd, sizeof(prog_fd));
will attach the program prog_fd to socket sock which was received by
prior call to socket().

ioctl(event_fd, PERF_EVENT_IOC_SET_BPF, prog_fd);
will attach the program prog_fd to perf event event_fd which was
received by prior call to perf_event_open().

Another way to attach the program to a tracing event is:
event_fd = open("/sys/kernel/debug/tracing/events/skb/kfree_skb/filter");
write(event_fd, "bpf-123"); /* where 123 is eBPF program FD */
/* here program is attached and will be triggered by events */
close(event_fd); /* to detach from event */

EXAMPLES
/* eBPF+sockets example:
* 1. create map with maximum of 2 elements
* 2. set map[6] = 0 and map[17] = 0
* 3. load eBPF program that counts number of TCP and UDP packets received
* via map[skb->ip->proto]++
* 4. attach prog_fd to raw socket via setsockopt()
* 5. print number of received TCP/UDP packets every second
*/
int main(int ac, char **av)
{
int sock, map_fd, prog_fd, key;
long long value = 0, tcp_cnt, udp_cnt;

map_fd = bpf_create_map(BPF_MAP_TYPE_HASH, sizeof(key), sizeof(value), 2);
if (map_fd < 0) {
printf("failed to create map '%s'\n", strerror(errno));
/* likely not run as root */
return 1;
}

key = 6; /* ip->proto == tcp */
assert(bpf_update_elem(map_fd, &key, &value) == 0);

key = 17; /* ip->proto == udp */
assert(bpf_update_elem(map_fd, &key, &value) == 0);

struct bpf_insn prog[] = {
BPF_MOV64_REG(BPF_REG_6, BPF_REG_1), /* r6 = r1 */
BPF_LD_ABS(BPF_B, 14 + 9), /* 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, prog, sizeof(prog), "GPL");
assert(prog_fd >= 0);

sock = open_raw_sock("lo");

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

for (;;) {
key = 6;
assert(bpf_lookup_elem(map_fd, &key, &tcp_cnt) == 0);
key = 17;
assert(bpf_lookup_elem(map_fd, &key, &udp_cnt) == 0);
printf("TCP %lld UDP %lld packets0, tcp_cnt, udp_cnt);
sleep(1);
}

return 0;
}

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

BPF_MAP_CREATE
The new file descriptor associated with eBPF map.

BPF_PROG_LOAD
The new file descriptor associated with eBPF program.

All other commands
Zero.

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

ERRORS
EPERM bpf() syscall 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 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" unused by this command are not set to zero.

EINVAL For BPF_PROG_LOAD, attempt to load invalid program (unrecognized
instruction or uses reserved fields or jumps out of range or
loop detected or calls unknown function).

EACCES For BPF_PROG_LOAD, though program has valid instructions, it was
rejected, since it was deemed unsafe (may access disallowed
memory region or uninitialized stack/register or function
constraints don't match actual types or misaligned access). In
such case it is recommended to call bpf() again with log_level =
1 and examine log_buf for specific reason provided by verifier.

ENOENT For BPF_MAP_LOOKUP_ELEM or BPF_MAP_DELETE_ELEM, indicates that
element with given key was not found.

E2BIG program is too large or a map reached max_entries limit (max
number of elements).

NOTES
These commands may be used only by a privileged process (one having the
CAP_SYS_ADMIN capability).

SEE ALSO
eBPF architecture and instruction set is explained in
Documentation/networking/filter.txt

Linux 2014-09-15 BPF(2)
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Next message: Alexei Starovoitov: "[PATCH v12 net-next 01/11] bpf: introduce BPF syscall and maps"
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