[bpf-next,4/6] libbpf: fix btf__align_of() by taking into account field offsets

Commit Message

Andrii Nakryiko Dec. 8, 2022, 6:57 p.m. UTC

btf__align_of() is supposed to be return alignment requirement of
a requested BTF type. For STRUCT/UNION it doesn't always return correct
value, because it calculates alignment only based on field types. But
for packed structs this is not enough, we need to also check field
offsets and struct size. If field offset isn't aligned according to
field type's natural alignment, then struct must be packed. Similarly,
if struct size is not a multiple of struct's natural alignment, then
struct must be packed as well.

This patch fixes this issue precisely by additionally checking these
conditions.

Fixes: 3d208f4ca111 ("libbpf: Expose btf__align_of() API")
Signed-off-by: Andrii Nakryiko <[e-mail]>
---
 tools/lib/bpf/btf.c | 13 +++++++++++++
 1 file changed, 13 insertions(+)

Patch

diff --git a/tools/lib/bpf/btf.c b/tools/lib/bpf/btf.c
index 71e165b09ed5..8cbcef959456 100644
--- a/tools/lib/bpf/btf.c
+++ b/tools/lib/bpf/btf.c
@@ -688,8 +688,21 @@  int btf__align_of(const struct btf *btf, __u32 id)
 			if (align <= 0)
 				return libbpf_err(align);
 			max_align = max(max_align, align);
+
+			/* if field offset isn't aligned according to field
+			 * type's alignment, then struct must be packed
+			 */
+			if (btf_member_bitfield_size(t, i) == 0 &&
+			    (m->offset % (8 * align)) != 0)
+				return 1;
 		}
 
+		/* if struct/union size isn't a multiple of its alignment,
+		 * then struct must be packed
+		 */
+		if ((t->size % max_align) != 0)
+			return 1;
+
 		return max_align;
 	}
 	default: