linux-kernel - Re: [RFC bpf-next v2 0/1] libbpf: add compile-time OOB warning to bpf_tail_call

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Message-ID: <3pevfugpcd2j44b2wkrjhspn2a2ta627nhnqxc6ty7dxy3nt3v@qhytbn7lmqum>
Date: Fri, 5 Sep 2025 16:42:43 +0800
From: Shung-Hsi Yu <shung-hsi.yu@...e.com>
To: bpf@...r.kernel.org
Cc: Hoyeon Lee <hoyeon.lee@...e.com>, netdev@...r.kernel.org, 
	Andrii Nakryiko <andrii@...nel.org>, Eduard Zingerman <eddyz87@...il.com>, 
	Alexei Starovoitov <ast@...nel.org>, Daniel Borkmann <daniel@...earbox.net>, 
	Martin KaFai Lau <martin.lau@...ux.dev>, Song Liu <song@...nel.org>, 
	Yonghong Song <yonghong.song@...ux.dev>, John Fastabend <john.fastabend@...il.com>, 
	KP Singh <kpsingh@...nel.org>, Stanislav Fomichev <sdf@...ichev.me>, 
	Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>, 
	Nathan Chancellor <nathan@...nel.org>, Nick Desaulniers <nick.desaulniers+lkml@...il.com>, 
	Bill Wendling <morbo@...gle.com>, Justin Stitt <justinstitt@...gle.com>, 
	open list <linux-kernel@...r.kernel.org>, 
	"open list:CLANG/LLVM BUILD SUPPORT:Keyword:b(?i:clang|llvm)b" <llvm@...ts.linux.dev>
Subject: Re: [RFC bpf-next v2 0/1] libbpf: add compile-time OOB warning to
 bpf_tail_call_static

Adding some context that I think was miss per off-list discussion with
Hoyeon.

On Fri, Sep 05, 2025 at 02:18:11PM +0900, Hoyeon Lee wrote:
> This RFC adds a compile-time check to bpf_tail_call_static() to warn
> when a constant slot(index) is >= map->max_entries. This uses a small
> BPF_MAP_ENTRIES() macro together with Clang's diagnose_if attribute.

This is an attempt to see if it is possible to warn user of out-of-bound
tail calls, with the assumption being that with bpf_tail_call_static()
users would not be intentionally calling with an index that is superior
to the number of entries.

However, there concerns with the current implementation, so this is
being sent as RFC to gather feedback, and to see if it can be better
done. Currently the concerns are:
- use macro to override bpf_tail_call_static()
- only works for Clang and not GCC
- uncertain whether this fit into libbpf conventions

> Clang front-end keeps the map type with a '(*max_entries)[N]' field,
> so the expression
> 
>     sizeof(*(m)->max_entries) / sizeof(**(m)->max_entries)
> 
> is resolved to N entirely at compile time. This allows diagnose_if()
> to emit a warning when a constant slot index is out of range.
> 
> Example:
> 
>     struct { /* BPF_MAP_TYPE_PROG_ARRAY = 3 */
>         __uint(type, 3);             // int (*type)[3];
>         __uint(max_entries, 100);    // int (*max_entries)[100];
>         __type(key, __u32);          // typeof(__u32) *key;
>         __type(value, __u32);        // typeof(__u32) *value;
>     } progs SEC(".maps");
> 
>     bpf_tail_call_static(ctx, &progs, 111);
> 
> produces:
> 
>     bound.bpf.c:26:9: warning: bpf_tail_call: slot >= max_entries [-Wuser-defined-warnings]
>        26 |         bpf_tail_call_static(ctx, &progs, 111);
>           |         ^
>     /usr/local/include/bpf/bpf_helpers.h:190:54: note: expanded from macro 'bpf_tail_call_static'
>       190 |          __bpf_tail_call_warn(__slot >= BPF_MAP_ENTRIES(map));                  \
>           |                                                             ^
>     /usr/local/include/bpf/bpf_helpers.h:183:20: note: from 'diagnose_if' attribute on '__bpf_tail_call_warn':
>       183 |     __attribute__((diagnose_if(oob, "bpf_tail_call: slot >= max_entries", "warning")));
>           |                    ^           ~~~
> 
> Out-of-bounds tail call checkup is no-ops at runtime. Emitting a
> compile-time warning can help developers detect mistakes earlier. The
> check is currently limited to Clang (due to diagnose_if) and constant
> indices, but should catch common errors.
...