bpf: Check tail zero of bpf_map_info

Since there're 4 bytes padding at the end of struct bpf_map_info, they
won't be checked by bpf_check_uarg_tail_zero().

pahole -C bpf_map_info ./vmlinux
struct bpf_map_info {
	...
	__u64                      hash __attribute__((__aligned__(8))); /*    88     8 */
	__u32                      hash_size;            /*    96     4 */

	/* size: 104, cachelines: 2, members: 18 */
	/* padding: 4 */
	/* forced alignments: 1 */
	/* last cacheline: 40 bytes */
} __attribute__((__aligned__(8)));

If a future kernel extension adds a new 4-byte field, older userspace
programs allocating this structure on the stack might inadvertently pass
uninitialized stack garbage into the new field, permanently breaking
backward compatibility. -- sashiko [1]

Fix it by changing sizeof(info) to
offsetofend(struct bpf_map_info, hash_size).

And, add "__u32 :32" to the tail of struct bpf_map_info.

[1] https://lore.kernel.org/bpf/20260513224823.6494FC19425@smtp.kernel.org/

Fixes: ea2e6467ac36 ("bpf: Return hashes of maps in BPF_OBJ_GET_INFO_BY_FD")
Acked-by: Mykyta Yatsenko <yatsenko@meta.com>
Signed-off-by: Leon Hwang <leon.hwang@linux.dev>
Link: https://lore.kernel.org/r/20260605155249.20772-2-leon.hwang@linux.dev
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

1 files changed, 1 insertions, 0 deletions

diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
index bed9b1b4d5ef..e1730f449d9e 100644
--- a/include/uapi/linux/bpf.h
+++ b/include/uapi/linux/bpf.h
@@ -6733,6 +6733,7 @@ struct bpf_map_info {
 	__u64 map_extra;
 	__aligned_u64 hash;
 	__u32 hash_size;
+	__u32 :32;
 } __attribute__((aligned(8)));
 
 struct bpf_btf_info {