[PATCH v2 bpf-next] bpf: Reduce smap->elem_size

[Martin KaFai Lau <martin.lau@linux.dev>]

From: Martin KaFai Lau <martin.lau@kernel.org>

'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
offsetofend() instead to calculate more exact memory use.

Acked-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
---
v2:
  Rephrase the commit message (Andrii and Yonghong)
  Use offsetofend instead of offsetof (Andrii)

 kernel/bpf/bpf_local_storage.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/bpf/bpf_local_storage.c b/kernel/bpf/bpf_local_storage.c
index b39a46e8fb08..e73fc70071b0 100644
--- 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 = offsetofend(struct bpf_local_storage_elem, sdata) +
+		attr->value_size;
 
 	return smap;
 }
-- 
2.30.2