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Message-ID: <6b52caf8-51f0-45ed-8f59-11f24dd2861e@linux.dev>
Date: Thu, 4 Sep 2025 21:00:41 -0700
From: Yonghong Song <yonghong.song@...ux.dev>
To: Hoyeon Lee <hoyeon.lee@...e.com>
Cc: 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>,
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:BPF [LIBRARY] (libbpf)" <bpf@...r.kernel.org>,
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 1/1] libbpf: add compile-time OOB warning to
bpf_tail_call_static
On 9/4/25 7:53 PM, Hoyeon Lee wrote:
> Add 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.
>
> 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.
>
> 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.
>
> Signed-off-by: Hoyeon Lee <hoyeon.lee@...e.com>
> ---
> tools/lib/bpf/bpf_helpers.h | 20 ++++++++++++++++++++
> 1 file changed, 20 insertions(+)
>
> diff --git a/tools/lib/bpf/bpf_helpers.h b/tools/lib/bpf/bpf_helpers.h
> index 80c028540656..0d9551bb90c0 100644
> --- a/tools/lib/bpf/bpf_helpers.h
> +++ b/tools/lib/bpf/bpf_helpers.h
> @@ -173,6 +173,26 @@ bpf_tail_call_static(void *ctx, const void *map, const __u32 slot)
> :: [ctx]"r"(ctx), [map]"r"(map), [slot]"i"(slot)
> : "r0", "r1", "r2", "r3", "r4", "r5");
> }
> +
> +#if __has_attribute(diagnose_if)
> +static __always_inline void __bpf_tail_call_warn(int oob)
> + __attribute__((diagnose_if(oob, "bpf_tail_call: slot >= max_entries", "warning")));
> +
> +#define BPF_MAP_ENTRIES(m) \
> + ((__u32)(sizeof(*(m)->max_entries) / sizeof(**(m)->max_entries)))
> +
> +#ifndef bpf_tail_call_static
> +#define bpf_tail_call_static(ctx, map, slot) \
> +({ \
> + /* wrapped to avoid double evaluation. */ \
> + const __u32 __slot = (slot); \
> + __bpf_tail_call_warn(__slot >= BPF_MAP_ENTRIES(map)); \
> + /* Avoid re-expand & invoke original as (bpf_tail_call_static)(..) */ \
> + (bpf_tail_call_static)(ctx, map, __slot); \
> +})
> +#endif /* bpf_tail_call_static */
> +#endif
> +
> #endif
> #endif
I got compilation failures with llvm21.
In file included from progs/bpf_flow.c:19:
/home/yhs/work/bpf-next/tools/testing/selftests/bpf/tools/include/bpf/bpf_helpers.h:178:29: error: function '__bpf_tail_call_warn' has internal
linkage but is not defined [-Werror,-Wundefined-internal]
178 | static __always_inline void __bpf_tail_call_warn(int oob)
| CLNG-BPF [test_progs] btf_type_tag_percpu.bpf.o
^
progs/bpf_flow.c:122:3: note: used here
122 | bpf_tail_call_static(skb, &jmp_table, IP);
| ^
/home/yhs/work/bpf-next/tools/testing/selftests/bpf/tools/include/bpf/bpf_helpers.h:189:2: note: expanded from macro 'bpf_tail_call_static'
189 | __bpf_tail_c CLNG-BPF [test_progs] btf_type_tag_user.bpf.o
all_warn(__slot >= BPF_MAP_ENTRIES(map)); CLNG-BPF [test_progs] cb_refs.bpf.o
\
| ^
>
> --
> 2.51.0
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