Re: [PATCH bpf-next] libbpf: Fix strncat bounds error in libbpf_prog_type_by_name

[Andrii Nakryiko]

On Tue, Oct 22, 2019 at 7:19 AM KP Singh <kpsingh@xxxxxxxxxxxx> wrote:
>
> From: KP Singh <kpsingh@xxxxxxxxxx>
>
> On compiling samples with this change, one gets an error:
>
>  error: âstrncatâ specified bound 118 equals destination size
>   [-Werror=stringop-truncation]
>
>     strncat(dst, name + section_names[i].len,
>     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>      sizeof(raw_tp_btf_name) - (dst - raw_tp_btf_name));
>      ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>
> strncat requires the destination to have enough space for the
> terminating null byte.
>
> Fixes: f75a697e09137 ("libbpf: Auto-detect btf_id of BTF-based raw_tracepoint")
> Signed-off-by: KP Singh <kpsingh@xxxxxxxxxx>
> ---
>  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 9364e66d755d..5fff3f15d705 100644
> --- a/tools/lib/bpf/libbpf.c
> +++ b/tools/lib/bpf/libbpf.c
> @@ -4666,7 +4666,7 @@ int libbpf_prog_type_by_name(const char *name, enum bpf_prog_type *prog_type,
>                         }
>                         /* prepend "btf_trace_" prefix per kernel convention */
>                         strncat(dst, name + section_names[i].len,
> -                               sizeof(raw_tp_btf_name) - (dst - raw_tp_btf_name));
> +                               sizeof(raw_tp_btf_name) - (dst - raw_tp_btf_name + 1));

Just:

sizeof(raw_tp_btf_name) - sizeof("btf_trace_")

?

>                         ret = btf__find_by_name(btf, raw_tp_btf_name);
>                         btf__free(btf);
>                         if (ret <= 0) {
> --
> 2.20.1
>