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bpf: Add bpf_loop helper

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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 Andrii Nakryiko
parent bafda72319
commit 73c8768db7
1 changed files with 25 additions and 0 deletions
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25
include/uapi/linux/bpf.h
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@@ -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