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bpf: Add bpf_loop helper 

This patch adds the kernel-side and API changes for a new helper
function, bpf_loop:

long bpf_loop(u32 nr_loops, void *callback_fn, void *callback_ctx,
u64 flags);

where long (*callback_fn)(u32 index, void *ctx);

bpf_loop invokes the "callback_fn" **nr_loops** times or until the
callback_fn returns 1. The callback_fn can only return 0 or 1, and
this is enforced by the verifier. The callback_fn index is zero-indexed.

A few things to please note:
~ The "u64 flags" parameter is currently unused but is included in
case a future use case for it arises.
~ In the kernel-side implementation of bpf_loop (kernel/bpf/bpf_iter.c),
bpf_callback_t is used as the callback function cast.
~ A program can have nested bpf_loop calls but the program must
still adhere to the verifier constraint of its stack depth (the stack depth
cannot exceed MAX_BPF_STACK))
~ Recursive callback_fns do not pass the verifier, due to the call stack
for these being too deep.
~ The next patch will include the tests and benchmark

Signed-off-by: Joanne Koong <joannekoong@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20211130030622.4131246-2-joannekoong@fb.com
This commit is contained in:
Joanne Koong 2021-11-29 19:06:19 -08:00 committed by Alexei Starovoitov
parent 88691e9e1e
commit e6f2dd0f80
6 changed files with 142 additions and 34 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

1
include/linux/bpf.h
View File

@ -2164,6 +2164,7 @@ extern const struct bpf_func_proto bpf_sk_setsockopt_proto;
extern const struct bpf_func_proto bpf_sk_getsockopt_proto;
extern const struct bpf_func_proto bpf_kallsyms_lookup_name_proto;
extern const struct bpf_func_proto bpf_find_vma_proto;
extern const struct bpf_func_proto bpf_loop_proto;
const struct bpf_func_proto *tracing_prog_func_proto(
enum bpf_func_id func_id, const struct bpf_prog *prog);

25
include/uapi/linux/bpf.h
View File

@ -4957,6 +4957,30 @@ union bpf_attr {
* **-ENOENT** if *task->mm* is NULL, or no vma contains *addr*.
* **-EBUSY** if failed to try lock mmap_lock.
* **-EINVAL** for invalid **flags**.
*
* long bpf_loop(u32 nr_loops, void *callback_fn, void *callback_ctx, u64 flags)
* Description
* For **nr_loops**, call **callback_fn** function
* with **callback_ctx** as the context parameter.
* The **callback_fn** should be a static function and
* the **callback_ctx** should be a pointer to the stack.
* The **flags** is used to control certain aspects of the helper.
* Currently, the **flags** must be 0. Currently, nr_loops is
* limited to 1 << 23 (~8 million) loops.
*
* long (\*callback_fn)(u32 index, void \*ctx);
*
* where **index** is the current index in the loop. The index
* is zero-indexed.
*
* If **callback_fn** returns 0, the helper will continue to the next
* loop. If return value is 1, the helper will skip the rest of
* the loops and return. Other return values are not used now,
* and will be rejected by the verifier.
*
* Return
* The number of loops performed, **-EINVAL** for invalid **flags**,
* **-E2BIG** if **nr_loops** exceeds the maximum number of loops.
*/
#define __BPF_FUNC_MAPPER(FN) \
FN(unspec), \
@ -5140,6 +5164,7 @@ union bpf_attr {
FN(skc_to_unix_sock), \
FN(kallsyms_lookup_name), \
FN(find_vma), \
FN(loop), \
/* */
/* integer value in 'imm' field of BPF_CALL instruction selects which helper

35
kernel/bpf/bpf_iter.c
View File

@ -714,3 +714,38 @@ const struct bpf_func_proto bpf_for_each_map_elem_proto = {
.arg3_type = ARG_PTR_TO_STACK_OR_NULL,
.arg4_type = ARG_ANYTHING,
};
/* maximum number of loops */
#define MAX_LOOPS BIT(23)
BPF_CALL_4(bpf_loop, u32, nr_loops, void *, callback_fn, void *, callback_ctx,
u64, flags)
{
bpf_callback_t callback = (bpf_callback_t)callback_fn;
u64 ret;
u32 i;
if (flags)
return -EINVAL;
if (nr_loops > MAX_LOOPS)
return -E2BIG;
for (i = 0; i < nr_loops; i++) {
ret = callback((u64)i, (u64)(long)callback_ctx, 0, 0, 0);
/* return value: 0 - continue, 1 - stop and return */
if (ret)
return i + 1;
}
return i;
}
const struct bpf_func_proto bpf_loop_proto = {
.func = bpf_loop,
.gpl_only = false,
.ret_type = RET_INTEGER,
.arg1_type = ARG_ANYTHING,
.arg2_type = ARG_PTR_TO_FUNC,
.arg3_type = ARG_PTR_TO_STACK_OR_NULL,
.arg4_type = ARG_ANYTHING,
};

2
kernel/bpf/helpers.c
View File

@ -1378,6 +1378,8 @@ bpf_base_func_proto(enum bpf_func_id func_id)
return &bpf_ringbuf_query_proto;
case BPF_FUNC_for_each_map_elem:
return &bpf_for_each_map_elem_proto;
case BPF_FUNC_loop:
return &bpf_loop_proto;
default:
break;
}

88
kernel/bpf/verifier.c
View File

@ -6085,6 +6085,27 @@ static int set_map_elem_callback_state(struct bpf_verifier_env *env,
return 0;
}
static int set_loop_callback_state(struct bpf_verifier_env *env,
struct bpf_func_state *caller,
struct bpf_func_state *callee,
int insn_idx)
{
/* bpf_loop(u32 nr_loops, void *callback_fn, void *callback_ctx,
* u64 flags);
* callback_fn(u32 index, void *callback_ctx);
*/
callee->regs[BPF_REG_1].type = SCALAR_VALUE;
callee->regs[BPF_REG_2] = caller->regs[BPF_REG_3];
/* unused */
__mark_reg_not_init(env, &callee->regs[BPF_REG_3]);
__mark_reg_not_init(env, &callee->regs[BPF_REG_4]);
__mark_reg_not_init(env, &callee->regs[BPF_REG_5]);
callee->in_callback_fn = true;
return 0;
}
static int set_timer_callback_state(struct bpf_verifier_env *env,
struct bpf_func_state *caller,
struct bpf_func_state *callee,
@ -6458,13 +6479,7 @@ static int check_helper_call(struct bpf_verifier_env *env, struct bpf_insn *insn
return err;
}
if (func_id == BPF_FUNC_tail_call) {
err = check_reference_leak(env);
if (err) {
verbose(env, "tail_call would lead to reference leak\n");
return err;
}
} else if (is_release_function(func_id)) {
if (is_release_function(func_id)) {
err = release_reference(env, meta.ref_obj_id);
if (err) {
verbose(env, "func %s#%d reference has not been acquired before\n",
@ -6475,41 +6490,46 @@ static int check_helper_call(struct bpf_verifier_env *env, struct bpf_insn *insn
regs = cur_regs(env);
/* check that flags argument in get_local_storage(map, flags) is 0,
* this is required because get_local_storage() can't return an error.
*/
if (func_id == BPF_FUNC_get_local_storage &&
!register_is_null(&regs[BPF_REG_2])) {
verbose(env, "get_local_storage() doesn't support non-zero flags\n");
return -EINVAL;
}
if (func_id == BPF_FUNC_for_each_map_elem) {
switch (func_id) {
case BPF_FUNC_tail_call:
err = check_reference_leak(env);
if (err) {
verbose(env, "tail_call would lead to reference leak\n");
return err;
}
break;
case BPF_FUNC_get_local_storage:
/* check that flags argument in get_local_storage(map, flags) is 0,
* this is required because get_local_storage() can't return an error.
*/
if (!register_is_null(&regs[BPF_REG_2])) {
verbose(env, "get_local_storage() doesn't support non-zero flags\n");
return -EINVAL;
}
break;
case BPF_FUNC_for_each_map_elem:
err = __check_func_call(env, insn, insn_idx_p, meta.subprogno,
set_map_elem_callback_state);
if (err < 0)
return -EINVAL;
}
if (func_id == BPF_FUNC_timer_set_callback) {
break;
case BPF_FUNC_timer_set_callback:
err = __check_func_call(env, insn, insn_idx_p, meta.subprogno,
set_timer_callback_state);
if (err < 0)
return -EINVAL;
}
if (func_id == BPF_FUNC_find_vma) {
break;
case BPF_FUNC_find_vma:
err = __check_func_call(env, insn, insn_idx_p, meta.subprogno,
set_find_vma_callback_state);
if (err < 0)
return -EINVAL;
break;
case BPF_FUNC_snprintf:
err = check_bpf_snprintf_call(env, regs);
break;
case BPF_FUNC_loop:
err = __check_func_call(env, insn, insn_idx_p, meta.subprogno,
set_loop_callback_state);
break;
}
if (func_id == BPF_FUNC_snprintf) {
err = check_bpf_snprintf_call(env, regs);
if (err < 0)
return err;
}
if (err)
return err;
/* reset caller saved regs */
for (i = 0; i < CALLER_SAVED_REGS; i++) {

25
tools/include/uapi/linux/bpf.h
View File

@ -4957,6 +4957,30 @@ union bpf_attr {
* **-ENOENT** if *task->mm* is NULL, or no vma contains *addr*.
* **-EBUSY** if failed to try lock mmap_lock.
* **-EINVAL** for invalid **flags**.
*
* long bpf_loop(u32 nr_loops, void *callback_fn, void *callback_ctx, u64 flags)
* Description
* For **nr_loops**, call **callback_fn** function
* with **callback_ctx** as the context parameter.
* The **callback_fn** should be a static function and
* the **callback_ctx** should be a pointer to the stack.
* The **flags** is used to control certain aspects of the helper.
* Currently, the **flags** must be 0. Currently, nr_loops is
* limited to 1 << 23 (~8 million) loops.
*
* long (\*callback_fn)(u32 index, void \*ctx);
*
* where **index** is the current index in the loop. The index
* is zero-indexed.
*
* If **callback_fn** returns 0, the helper will continue to the next
* loop. If return value is 1, the helper will skip the rest of
* the loops and return. Other return values are not used now,
* and will be rejected by the verifier.
*
* Return
* The number of loops performed, **-EINVAL** for invalid **flags**,
* **-E2BIG** if **nr_loops** exceeds the maximum number of loops.
*/
#define __BPF_FUNC_MAPPER(FN) \
FN(unspec), \
@ -5140,6 +5164,7 @@ union bpf_attr {
FN(skc_to_unix_sock), \
FN(kallsyms_lookup_name), \
FN(find_vma), \
FN(loop), \
/* */
/* integer value in 'imm' field of BPF_CALL instruction selects which helper