Re: [RFC bpf-next 1/1] libbpf: add compile-time OOB warning to bpf_tail_call_static

[Yonghong Song <yonghong.song@linux.dev>]

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@suse.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