bpf: Reduce smap->elem_size

'struct bpf_local_storage_elem' has an unused 56 byte padding at the
end due to struct's cache-line alignment requirement. This padding
space is overlapped by storage value contents, so if we use sizeof()
to calculate the total size, we overinflate it by 56 bytes. Use
offsetof() instead to calculate more exact memory use.

Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Yonghong Song <yhs@fb.com>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20221221013036.3427431-1-martin.lau@linux.dev

diff --git a/kernel/bpf/bpf_local_storage.c b/kernel/bpf/bpf_local_storage.c
index b39a46e8fb08d6aa53ab1ce26e7b711fb4c6d45b..373c3c2c75bc0ac07c12fec4837a8e3bf1d702c9 100644 (file)
--- a/kernel/bpf/bpf_local_storage.c
+++ b/kernel/bpf/bpf_local_storage.c
@@ -580,8 +580,8 @@ static struct bpf_local_storage_map *__bpf_local_storage_map_alloc(union bpf_att
                raw_spin_lock_init(&smap->buckets[i].lock);
        }
 
-       smap->elem_size =
-               sizeof(struct bpf_local_storage_elem) + attr->value_size;
+       smap->elem_size = offsetof(struct bpf_local_storage_elem,
+                                  sdata.data[attr->value_size]);
 
        return smap;
 }