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

[Andrii Nakryiko]

On Wed, Sep 17, 2025 at 5:44 AM Sanjay Chitroda
<quic_ckantibh@xxxxxxxxxxx> 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@xxxxxxxxxxx>
> ---
>  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
>