Re: [PATCH bpf-next 06/13] bpf: Add tracing_multi link fdinfo support for bpf progs

> diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
> index a805a5b73303f..80928401830c7 100644
> --- a/kernel/trace/bpf_trace.c
> +++ b/kernel/trace/bpf_trace.c
> @@ -3759,26 +3759,46 @@ static int bpf_tracing_multi_link_fill_link_info(const struct bpf_link *link,
>  }
>
>  #ifdef CONFIG_PROC_FS
> +static const char *bpf_prog_func_name(struct bpf_prog *prog, u32 btf_id)
> +{
> +	const struct btf *btf = prog->aux->btf;
> +	const struct btf_type *t;
> +
> +	t = btf_type_by_id(btf, btf_id);
> +	if (!t || !btf_type_is_func(t))
> +		return "";
> +
> +	return btf_name_by_offset(btf, t->name_off);
> +}
> +
>  static void bpf_tracing_multi_show_fdinfo(const struct bpf_link *link,
>  					  struct seq_file *seq)
>  {
>  	struct bpf_tracing_multi_link *tr_link =
>  		container_of(link, struct bpf_tracing_multi_link, link);
>  	bool has_cookies = !!tr_link->cookies;
> +	bool has_progs = !!tr_link->progs;
>
>  	seq_printf(seq, "attach_type:\t%u\n", tr_link->link.attach_type);
>  	seq_printf(seq, "cnt:\t%u\n", tr_link->nodes_cnt);
>
> -	seq_printf(seq, "%s\t %s\t %s\t %s\n", "obj-id", "btf-id", "cookie", "func");
> +	seq_printf(seq, "%s\t %s\t %s\t %s\n", has_progs ? "prog-id" : "obj-id", "btf-id", "cookie",
> +		   "func");
>  	for (int i = 0; i < tr_link->nodes_cnt; i++) {
>  		struct bpf_tracing_multi_node *mnode = &tr_link->nodes[i];
> +		u64 cookie = has_cookies ? tr_link->cookies[i] : 0;
>  		u32 btf_id, obj_id;
>
>  		bpf_trampoline_unpack_key(mnode->trampoline->key, &obj_id, &btf_id);
> +		btf_id = has_progs ? (u32) mnode->trampoline->key : btf_id;
                        ^^^^

This isn't a bug, but since bpf_trampoline_unpack_key() only masks off
bit 31, which is never set for prog targets, does this re-assignment
change anything, or could the unpacked value be used directly?

bpf_trampoline_unpack_key() already sets btf_id = key & 0x7FFFFFFF, and
bit 31 is only set by the !tgt_prog branch of bpf_trampoline_compute_key()
(the kernel-BTF case).

In the has_progs case the key was built as:

    ((u64)tgt_prog->aux->id << 32) | ids[i]

and ids[i] must be a valid BTF type id (<= BTF_MAX_TYPE, 0xfffff) or
bpf_check_attach_target() would have rejected the attach, so (u32)key
and the unpacked btf_id are always equal here.

> +		if (has_progs)
> +			seq_printf(seq, "%u\t %u\t %llu\t %s\n",
> +				   obj_id, btf_id, cookie,
> +				   bpf_prog_func_name(tr_link->progs[i], btf_id));
> +		else
> +			seq_printf(seq, "%u\t %u\t %llu\t %pS\n",
> +				   obj_id, btf_id, cookie,
> +				   (void *) mnode->trampoline->ip);
>
>  		cond_resched();
>  	}

The commit message shows the sample fdinfo output but doesn't say much
about the implementation or why this change is needed. In particular:

> Show the target bpf prog info in the fdinfo.
>
> $ cat /proc/self/fdinfo/13
> pos:    0
> flags:  02000000
> mnt_id: 18
> ino:    1045
> link_type:      tracing_multi
> link_id:        9
> prog_tag:       9a7f9e3ab6f138ab
> prog_id:        25
> attach_type:    61
> cnt:    2
> prog-id  btf-id  cookie  func
> 21       4       16706819        target_1
> 22       5       16706819        target_2

This isn't a bug, but could the changelog say why the trampoline ip is
not usable as the func column for prog targets (tr->ip is 0 for JIT'd
prog addresses), rather than mostly showing the resulting output? It
also doesn't mention that the first column header changes from "obj-id"
to "prog-id" for prog-target links.


---
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/31320427087