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libbpf: Fix btf__align_of() by taking into account field offsets

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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 <andrii@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20221212211505.558851-5-andrii@kernel.org
This commit is contained in:
Andrii Nakryiko 2022-12-12 13:15:03 -08:00 committed by Daniel Borkmann
parent 9d2349740e
commit 25a4481b41
1 changed files with 13 additions and 0 deletions
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13
tools/lib/bpf/btf.c
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@ -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: