Commit a463e81c authored by Andrii Nakryiko's avatar Andrii Nakryiko Committed by sanglipeng
Browse files

libbpf: Fix btf_dump's packed struct determination 

stable inclusion
from stable-v5.10.177
commit b4781477f397a9e10f36d035ffc76bfb67aa1639
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I88YNP

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=b4781477f397a9e10f36d035ffc76bfb67aa1639



--------------------------------

[ Upstream commit 4fb877aa ]

Fix bug in btf_dump's logic of determining if a given struct type is
packed or not. The notion of "natural alignment" is not needed and is
even harmful in this case, so drop it altogether. The biggest difference
in btf_is_struct_packed() compared to its original implementation is
that we don't really use btf__align_of() to determine overall alignment
of a struct type (because it could be 1 for both packed and non-packed
struct, depending on specifci field definitions), and just use field's
actual alignment to calculate whether any field is requiring packing or
struct's size overall necessitates packing.

Add two simple test cases that demonstrate the difference this change
would make.

Fixes: ea2ce1ba ("libbpf: Fix BTF-to-C converter's padding logic")
Reported-by: default avatarEduard Zingerman <eddyz87@gmail.com>
Signed-off-by: default avatarAndrii Nakryiko <andrii@kernel.org>
Signed-off-by: default avatarDaniel Borkmann <daniel@iogearbox.net>
Acked-by: default avatarEduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/bpf/20221215183605.4149488-1-andrii@kernel.org


Signed-off-by: default avatarSasha Levin <sashal@kernel.org>
Signed-off-by: default avatarsanglipeng <sanglipeng1@jd.com>
parent 0d26450b

tools/lib/bpf/btf_dump.c

+6 −27
Original line number Diff line number Diff line
@@ -788,47 +788,26 @@ static void btf_dump_emit_type(struct btf_dump *d, __u32 id, __u32 cont_id) 
	}

}



static int btf_natural_align_of(const struct btf *btf, __u32 id)

{

	const struct btf_type *t = btf__type_by_id(btf, id);

	int i, align, vlen;

	const struct btf_member *m;



	if (!btf_is_composite(t))

		return btf__align_of(btf, id);



	align = 1;

	m = btf_members(t);

	vlen = btf_vlen(t);

	for (i = 0; i < vlen; i++, m++) {

		align = max(align, btf__align_of(btf, m->type));

	}



	return align;

}



static bool btf_is_struct_packed(const struct btf *btf, __u32 id,

				 const struct btf_type *t)

{

	const struct btf_member *m;

	int align, i, bit_sz;

	int max_align = 1, align, i, bit_sz;

	__u16 vlen;



	align = btf_natural_align_of(btf, id);

	/* size of a non-packed struct has to be a multiple of its alignment */

	if (align && (t->size % align) != 0)

		return true;



	m = btf_members(t);

	vlen = btf_vlen(t);

	/* all non-bitfield fields have to be naturally aligned */

	for (i = 0; i < vlen; i++, m++) {

		align = btf_natural_align_of(btf, m->type);

		align = btf__align_of(btf, m->type);

		bit_sz = btf_member_bitfield_size(t, i);

		if (align && bit_sz == 0 && m->offset % (8 * align) != 0)

			return true;

		max_align = max(align, max_align);

	}



	/* size of a non-packed struct has to be a multiple of its alignment */

	if (t->size % max_align != 0)

		return true;

	/*

	 * if original struct was marked as packed, but its layout is

	 * naturally aligned, we'll detect that it's not packed

tools/testing/selftests/bpf/progs/btf_dump_test_case_packing.c

+19 −0
Original line number Diff line number Diff line
@@ -116,6 +116,23 @@ struct usb_host_endpoint { 
	long: 0;

};



/* ----- START-EXPECTED-OUTPUT ----- */

struct nested_packed_struct {

	int a;

	char b;

} __attribute__((packed));



struct outer_nonpacked_struct {

	short a;

	struct nested_packed_struct b;

};



struct outer_packed_struct {

	short a;

	struct nested_packed_struct b;

} __attribute__((packed));



/* ------ END-EXPECTED-OUTPUT ------ */



int f(struct {

	struct packed_trailing_space _1;
@@ -128,6 +145,8 @@ int f(struct { 
	union jump_code_union _8;

	struct outer_implicitly_packed_struct _9;

	struct usb_host_endpoint _10;

	struct outer_nonpacked_struct _11;

	struct outer_packed_struct _12;

} *_)

{

	return 0;