| bpf(2) | System Calls Manual | bpf(2) |
bpf - perform a command on an extended BPF map or program
#include <linux/bpf.h>
int bpf(int cmd, union bpf_attr *attr, unsigned int size);
The bpf() system call performs a range of operations related to extended Berkeley Packet Filters. Extended BPF (or eBPF) is similar to the original ("classic") BPF (cBPF) used to filter network packets. For both cBPF and eBPF programs, the kernel statically analyzes the programs before loading them, in order to ensure that they cannot harm the running system.
eBPF extends cBPF in multiple ways, including the ability to call a fixed set of in-kernel helper functions (via the BPF_CALL opcode extension provided by eBPF) and access shared data structures such as eBPF maps.
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 access the same maps in parallel. It's up to the user process and eBPF program to decide what they store inside maps.
There's one special map type, called a program array. This type of map stores file descriptors referring to other eBPF 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 nesting has a fixed limit of 32, so that infinite loops cannot be crafted. At run time, the program file descriptors stored in the map can be modified, so program functionality can be altered based on specific requirements. All programs referred to in a program-array map must have been previously loaded into the kernel via bpf(). If a map lookup fails, the current program continues its execution. See BPF_MAP_TYPE_PROG_ARRAY below for further details.
Generally, eBPF programs are loaded by the user process and automatically unloaded when the process exits. In some cases, for example, tc-bpf(8), the program will continue to stay alive inside the kernel even after the process that loaded the program exits. In that case, the tc subsystem holds a reference to the eBPF program after the file descriptor has been closed by the user-space program. Thus, whether a specific program continues to live inside the kernel depends on how it is further attached to a given kernel subsystem after it was loaded via bpf().
Each eBPF program is a set of instructions that is safe to run until its completion. An in-kernel verifier statically determines that the eBPF program terminates and is safe to execute. During verification, the kernel increments reference counts for each of the maps that the eBPF program uses, so that the attached maps can't be removed until the program is 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 program, which may store information about the event in eBPF maps. Beyond storing data, eBPF programs may call a fixed set of in-kernel helper functions.
The same eBPF program can be attached to multiple events and different eBPF programs can access the same map:
tracing tracing tracing packet packet packet
event A event B event C on eth0 on eth1 on eth2
| | | | | ^
| | | | v |
--> tracing <-- tracing socket tc ingress tc egress
prog_1 prog_2 prog_3 classifier action
| | | | prog_4 prog_5
|--- -----| |------| map_3 | |
map_1 map_2 --| map_4 |--
The operation to be performed by the bpf() system call is determined by the cmd argument. Each operation takes an accompanying argument, provided via attr, which is a pointer to a union of type bpf_attr (see below). The unused fields and padding must be zeroed out before the call. The size argument is the size of the union pointed to by attr.
The value provided in cmd is one of the following:
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)));
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:
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.
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));
}
bpf_map_lookup_elem(map_fd, fp - 4)
bpf_map_lookup_elem(map_fd, void *key)
value = bpf_map_lookup_elem(...);
*(u32 *) value = 1;
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. */
};
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));
}
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));
}
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));
}
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));
}
The following map types are supported:
void bpf_tail_call(void *context, void *prog_map,
unsigned int index);
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:
Applying close(2) to the file descriptor returned by BPF_PROG_LOAD will unload the eBPF program (but see NOTES).
Maps are accessible from eBPF programs and are used to exchange data between eBPF programs and between eBPF programs and user-space programs. For example, eBPF programs can process various events (like kprobe, packets) and store their data into a map, and user-space programs can then fetch data from the map. Conversely, user-space programs can use a map as a configuration mechanism, populating the map with values checked by the eBPF program, which then modifies its behavior on the fly according to those values.
The eBPF program type (prog_type) determines the subset of kernel helper functions that the program may call. The program type also determines the program input (context)—the format of struct bpf_context (which is the data blob passed into the eBPF program as the first argument).
For example, a tracing program does not have the exact same subset of helper functions as a socket filter program (though they may have some helpers in common). Similarly, the input (context) for a tracing program is a set of register values, while for a socket filter it is a network packet.
The set of functions available to eBPF programs of a given type may increase in the future.
The following program types are supported:
bpf_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 */
Once a program is loaded, it can be attached to an event. Various kernel subsystems have different ways to do so.
Since Linux 3.19, the following call will attach the program prog_fd to the socket sockfd, which was created by an earlier call to socket(2):
setsockopt(sockfd, SOL_SOCKET, SO_ATTACH_BPF,
&prog_fd, sizeof(prog_fd));
Since Linux 4.1, the following call may be used to attach the eBPF program referred to by the file descriptor prog_fd to a perf event file descriptor, event_fd, that was created by a previous call to perf_event_open(2):
ioctl(event_fd, PERF_EVENT_IOC_SET_BPF, prog_fd);
For a successful call, the return value depends on the operation:
On error, -1 is returned, and errno is set to indicate the error.
Linux.
Linux 3.18.
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:
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 objects. In addition, file descriptors referring to eBPF objects can be transferred over UNIX domain sockets. File descriptors referring to eBPF objects can be duplicated in the usual way, using dup(2) and similar calls. An eBPF object is deallocated only after all file descriptors referring to the object have been closed.
eBPF programs can be written in a restricted C that is compiled (using the clang compiler) into eBPF bytecode. Various 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. 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:
Since Linux 4.15, the kernel may be 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 interpreter.)
The JIT compiler for eBPF is currently available for the following architectures:
/* 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.
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.
| 2024-03-18 | Linux man-pages 6.7 |