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

[Kees Cook <kees@kernel.org>]

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@kernel.org>
---
 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@kernel.org/
 v1: https://lore.kernel.org/lkml/20260324040535.work.851-kees@kernel.org/
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: sun jian <sun.jian.kdev@gmail.com>
Cc: Andrii Nakryiko <andrii@kernel.org>
Cc: Eduard Zingerman <eddyz87@gmail.com>
Cc: Daniel Borkmann <daniel@iogearbox.net>
Cc: Martin KaFai Lau <martin.lau@linux.dev>
Cc: Song Liu <song@kernel.org>
Cc: Yonghong Song <yonghong.song@linux.dev>
Cc: John Fastabend <john.fastabend@gmail.com>
Cc: KP Singh <kpsingh@kernel.org>
Cc: Stanislav Fomichev <sdf@fomichev.me>
Cc: Hao Luo <haoluo@google.com>
Cc: <bpf@vger.kernel.org>
---
 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);
+
 	attr.key_size = key_size;
 	attr.value_size = value_size;
 	attr.max_entries = max_entries;
-- 
2.34.1