Re: [PATCH v3] libbpf: Replace strncpy() with strnlen()+memcpy() in skel_map_create()

[Alexei Starovoitov]

On Tue, Mar 24, 2026 at 9:16 AM Kees Cook <kees@xxxxxxxxxx> wrote:
>
> Replace the deprecated[1] strncpy() with strnlen() on the source
> followed by memcpy(). Normally strscpy() would be used in this case,
> but skel_internal.h is shared between kernel and userspace tools, and
> strscpy() is not available in the userspace build context.
>
> The source map_name is a NUL-terminated C string (the only caller
> passes the "__loader.map" 12 character string literal). The destination
> attr.map_name is char[BPF_OBJ_NAME_LEN] (16 bytes) in union bpf_attr,
> ultimately passed to the bpf() syscall.
>
> The bpf(BPF_MAP_CREATE) syscall, through bpf_obj_name_cpy(), requires a
> NUL terminator within this 16-byte array, rejecting names that use all 16
> bytes. Valid names are therefore at most 15 characters, but this wasn't
> being checked via the skel_map_create() path. Add a matching check and
> refuse 16+ character strings early, as they would be refused later by
> bpf_obj_name_cpy().
>
> The attr is pre-zeroed with memset() at the top of the function, so
> the last byte of attr.map_name is always NUL, meaning the memcpy()
> of just the non-NUL characters from the source will always produce a
> NUL-terminated destination string.
>
> Link: https://github.com/KSPP/linux/issues/90 [1]
> Signed-off-by: Kees Cook <kees@xxxxxxxxxx>
> ---
>  v3: instead of truncation, refuse the long length (test appears to have been a flake)
>  v2: https://lore.kernel.org/lkml/20260324053036.it.906-kees@xxxxxxxxxx/
>  v1: https://lore.kernel.org/lkml/20260324040535.work.851-kees@xxxxxxxxxx/
> Cc: Alexei Starovoitov <ast@xxxxxxxxxx>
> Cc: Jiri Olsa <jolsa@xxxxxxxxxx>
> Cc: sun jian <sun.jian.kdev@xxxxxxxxx>
> Cc: Andrii Nakryiko <andrii@xxxxxxxxxx>
> Cc: Eduard Zingerman <eddyz87@xxxxxxxxx>
> Cc: Daniel Borkmann <daniel@xxxxxxxxxxxxx>
> Cc: Martin KaFai Lau <martin.lau@xxxxxxxxx>
> Cc: Song Liu <song@xxxxxxxxxx>
> Cc: Yonghong Song <yonghong.song@xxxxxxxxx>
> Cc: John Fastabend <john.fastabend@xxxxxxxxx>
> Cc: KP Singh <kpsingh@xxxxxxxxxx>
> Cc: Stanislav Fomichev <sdf@xxxxxxxxxxx>
> Cc: Hao Luo <haoluo@xxxxxxxxxx>
> Cc: <bpf@xxxxxxxxxxxxxxx>
> ---
>  tools/lib/bpf/skel_internal.h | 8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
>
> diff --git a/tools/lib/bpf/skel_internal.h b/tools/lib/bpf/skel_internal.h
> index 6a8f5c7a02eb..2d38c387f43c 100644
> --- a/tools/lib/bpf/skel_internal.h
> +++ b/tools/lib/bpf/skel_internal.h
> @@ -236,6 +236,7 @@ static inline int skel_map_create(enum bpf_map_type map_type,
>  {
>         const size_t attr_sz = offsetofend(union bpf_attr, excl_prog_hash_size);
>         union bpf_attr attr;
> +       size_t map_name_len;
>
>         memset(&attr, 0, attr_sz);
>
> @@ -243,7 +244,12 @@ static inline int skel_map_create(enum bpf_map_type map_type,
>         attr.excl_prog_hash = (unsigned long) excl_prog_hash;
>         attr.excl_prog_hash_size = excl_prog_hash_sz;
>
> -       strncpy(attr.map_name, map_name, sizeof(attr.map_name));
> +       /* attr.map_name must be NUL-terminated, like bpf_obj_name_cpy() */
> +       map_name_len = strnlen(map_name, sizeof(attr.map_name));
> +       if (map_name_len == sizeof(attr.map_name))
> +               return -EINVAL;
> +       memcpy(attr.map_name, map_name, map_name_len);

and now you keep spamming without waiting for replies.
nack.

pw-bot: cr