[PATCH 1/2] btf_loader: infer alignment for zero-length arrays

[Antoine Tenart <atenart@kernel.org>]

Zero sized arrays do not go through the bitfield fixup logic, which in
turn makes them to skip the alignment infer logic. The result is
zero-length arrays being incorrectly aligned when an explicit statement
is needed. E.g. this happens for variable length arrays at end of
structures, or for 0-length ones.

Before this patch we can see:

  struct skb_ext {
	refcount_t                 refcnt;               /*     0     4 */
	u8                         offset[4];            /*     4     4 */
	u8                         chunks;               /*     8     1 */

	/* XXX 7 bytes hole, try to pack */

	char                       data[];               /*    16     0 */

	/* size: 16, cachelines: 1, members: 4 */
	/* sum members: 9, holes: 1, sum holes: 7 */
	/* last cacheline: 16 bytes */
  } __attribute__((__aligned__(8)));

  offsetof(struct skb_ext, data) returns 9.

After this patch we get:

  struct skb_ext {
	refcount_t                 refcnt;               /*     0     4 */
	u8                         offset[4];            /*     4     4 */
	u8                         chunks;               /*     8     1 */

	/* XXX 7 bytes hole, try to pack */

	char                       data[] __attribute__((__aligned__(8))); /*    16     0 */

	/* size: 16, cachelines: 1, members: 4 */
	/* sum members: 9, holes: 1, sum holes: 7 */
	/* forced alignments: 1, forced holes: 1, sum forced holes: 7 */
	/* last cacheline: 16 bytes */
  } __attribute__((__aligned__(8)));

  offsetof(struct skb_ext, data) returns 16.

Signed-off-by: Antoine Tenart <atenart@kernel.org>
---
 btf_loader.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/btf_loader.c b/btf_loader.c
index f4f9f65289b5..76771afedd95 100644
--- a/btf_loader.c
+++ b/btf_loader.c
@@ -648,6 +648,10 @@ static int class__fixup_btf_bitfields(const struct conf_load *conf, struct tag *
 		/* if BTF data is incorrect and has size == 0, skip field,
 		 * instead of crashing */
 		if (pos->byte_size == 0) {
+			pos->alignment = class__infer_alignment(conf,
+								pos->byte_offset,
+								tag__natural_alignment(type, cu),
+								smallest_offset);
 			continue;
 		}
 
-- 
2.49.0