> bpf: Add log support for tracing link_create

TL;DR: Logging is incomplete: the freplace max_ctx_offset check in
bpf_tracing_prog_attach() still returns -EINVAL with an empty log, and the
saved_dst type mismatch may too. No functional regression.

> diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
> --- a/kernel/bpf/syscall.c
> +++ b/kernel/bpf/syscall.c

[ ... ]

> @@ -3843,6 +3849,7 @@ static int bpf_tracing_prog_attach(struct bpf_prog *prog,
>  	 */
>  	if (prog->type == BPF_PROG_TYPE_EXT &&
>  	    prog->aux->kprobe_write_ctx != tgt_prog->aux->kprobe_write_ctx) {
> +		bpf_log(log, "Invalid kprobe_write_ctx.\n");
>  		err = -EINVAL;
>  		goto out_unlock;
>  	}

The check right after this one still returns -EINVAL without writing
anything to the log:

	if (prog->type == BPF_PROG_TYPE_EXT &&
	    (tgt_prog->type == BPF_PROG_TYPE_RAW_TRACEPOINT ||
	     tgt_prog->type == BPF_PROG_TYPE_TRACEPOINT ||
	     tgt_prog->type == BPF_PROG_TYPE_SYSCALL) &&
	    prog->aux->max_ctx_offset > tgt_prog->aux->max_ctx_offset) {
		err = -EINVAL;
		goto out_unlock;
	}

An freplace program whose max_ctx_offset is larger than that of a
RAW_TRACEPOINT, TRACEPOINT or SYSCALL target will get -EINVAL from
link_create with an empty log and log_true_size of 0.  That is the same
kind of invalid freplace target as the kprobe_write_ctx mismatch above,
which now logs.  This isn't a functional regression, but was this check
missed?

Would something like this cover it?

	bpf_log(log, "Extension accesses more ctx than its target.\n");

Also, bpf_check_attach_target() still returns a bare -EINVAL for the
saved_dst_prog_type / saved_dst_attach_type mismatch, which is the
"freplace attachment to a new target" case the commit message mentions.
That code is in verifier.c, so it is outside this patch, but is it intended
to be left without a message?


---
AI reviewed your patch. Please fix the bug or email reply why it's not a bug.
See: https://github.com/kernel-patches/vmtest/blob/master/ci/claude/README.md

CI run summary: https://github.com/kernel-patches/bpf/actions/runs/42633880300