[bpf-next,1/2] libbpf: handle nulled-out program in struct_ops correctly

Commit Message

Andrii Nakryiko April 28, 2024, 3:09 a.m. UTC

If struct_ops has one of program callbacks set declaratively and host
kernel is old and doesn't support this callback, libbpf will allow to
load such struct_ops as long as that callback was explicitly nulled-out
(presumably through skeleton). This is all working correctly, except we
won't reset corresponding program slot to NULL before bailing out, which
will lead to libbpf not detecting that BPF program has to be not
auto-loaded. Fix this by unconditionally resetting corresponding program
slot to NULL.

Fixes: c911fc61a7ce ("libbpf: Skip zeroed or null fields if not found in the kernel type.")
Signed-off-by: Andrii Nakryiko <[e-mail]>
---
 tools/lib/bpf/libbpf.c | 1 +
 1 file changed, 1 insertion(+)

Comments

[e-mail] April 30, 2024, midnight UTC | #1

Hello:

This series was applied to bpf/bpf-next.git (master)
by Martin KaFai Lau <[e-mail]>:

On Sat, 27 Apr 2024 20:09:53 -0700 you wrote:
> If struct_ops has one of program callbacks set declaratively and host
> kernel is old and doesn't support this callback, libbpf will allow to
> load such struct_ops as long as that callback was explicitly nulled-out
> (presumably through skeleton). This is all working correctly, except we
> won't reset corresponding program slot to NULL before bailing out, which
> will lead to libbpf not detecting that BPF program has to be not
> auto-loaded. Fix this by unconditionally resetting corresponding program
> slot to NULL.
> 
> [...]

Here is the summary with links:
  - [bpf-next,1/2] libbpf: handle nulled-out program in struct_ops correctly
    https://git.kernel.org/bpf/bpf-next/c/f973fccd43d3
  - [bpf-next,2/2] selftests/bpf: validate nulled-out struct_ops program is handled properly
    https://git.kernel.org/bpf/bpf-next/c/1bba3b3d373d

You are awesome, thank you!

Patch

diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index 97eb6e5dd7c8..898d5d34ecea 100644
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c
@@ -1148,6 +1148,7 @@  static int bpf_map__init_kern_struct_ops(struct bpf_map *map)
 			 * presented in the kernel BTF.
 			 */
 			if (libbpf_is_mem_zeroed(mdata, msize)) {
+				st_ops->progs[i] = NULL;
 				pr_info("struct_ops %s: member %s not found in kernel, skipping it as it's set to zero\n",
 					map->name, mname);
 				continue;