linux-kernel - Re: [PATCH] libbpf: increase probe_name buffer size to avoid format-truncation

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Message-ID: <CAEf4BzbM+mG5EYQKdp1r4mntP2bXTb-KgJWg4ir6zzrSRrQURA@mail.gmail.com>
Date: Wed, 17 Sep 2025 13:39:27 -0700
From: Andrii Nakryiko <andrii.nakryiko@...il.com>
To: Sanjay Chitroda <quic_ckantibh@...cinc.com>
Cc: Alexei Starovoitov <ast@...nel.org>, Sanjay Chitroda <sanjayembeddese@...il.com>, 
	Daniel Borkmann <daniel@...earbox.net>, Andrii Nakryiko <andrii@...nel.org>, 
	Martin KaFai Lau <martin.lau@...ux.dev>, Eduard Zingerman <eddyz87@...il.com>, 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>, bpf@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] libbpf: increase probe_name buffer size to avoid format-truncation

On Wed, Sep 17, 2025 at 5:44 AM Sanjay Chitroda
<quic_ckantibh@...cinc.com> wrote:
>
> This patch increases the size of the `probe_name` buffer in
> `probe_kern_syscall_wrapper()` from `MAX_EVENT_NAME_LEN` to
> `MAX_EVENT_NAME_LEN * 2`.
>
> The change addresses a build failure in perf builds caused by GCC's
> -Werror=format-truncation warning:
>
>   libbpf.c:11052:45: error: '%s' directive output may be truncated writing up to 63 bytes into a region of size between 34 and 53 [-Werror=format-truncation]
>

We write into that buffer with snprintf() which will handle truncation
and leave properly zero terminated string. GCC being overly aggressive
with its warning? Or am I missing something?

pw-bot: cr


> The warning is triggered by a `snprintf()` call that formats a string
> using syscall names and other identifiers. In some cases, the buffer
> size is insufficient, leading to potential truncation.
>
> Debug builds pass because they do not treat warnings as errors, but
> perf builds fail due to `-Werror`.
>
> Increasing the buffer size ensures that the formatted string fits
> safely, resolving the issue without affecting functionality.
>
> Signed-off-by: Sanjay Chitroda <quic_ckantibh@...cinc.com>
> ---
>  tools/lib/bpf/libbpf.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> index 8f5a81b672e1..9413e86476da 100644
> --- a/tools/lib/bpf/libbpf.c
> +++ b/tools/lib/bpf/libbpf.c
> @@ -11290,7 +11290,7 @@ int probe_kern_syscall_wrapper(int token_fd)
>
>                 return pfd >= 0 ? 1 : 0;
>         } else { /* legacy mode */
> -               char probe_name[MAX_EVENT_NAME_LEN];
> +               char probe_name[MAX_EVENT_NAME_LEN * 2];
>
>                 gen_probe_legacy_event_name(probe_name, sizeof(probe_name), syscall_name, 0);
>                 if (add_kprobe_event_legacy(probe_name, false, syscall_name, 0) < 0)
> --
> 2.34.1
>