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libbpf: Fix btf_dump's packed struct determination 

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: ea2ce1ba99 ("libbpf: Fix BTF-to-C converter's padding logic")
Reported-by: Eduard Zingerman <eddyz87@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/bpf/20221215183605.4149488-1-andrii@kernel.org
This commit is contained in:
Andrii Nakryiko 2022-12-15 10:36:05 -08:00 committed by Daniel Borkmann
parent b148c8b9b9
commit 4fb877aaa1
2 changed files with 25 additions and 27 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

33
tools/lib/bpf/btf_dump.c
View File

@ -830,47 +830,26 @@ 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

19
tools/testing/selftests/bpf/progs/btf_dump_test_case_packing.c
View File

@ -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;