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: ea2ce1ba99aa ("libbpf: Fix BTF-to-C converter's padding logic")
Reported-by: Eduard Zingerman <[email protected]>
Signed-off-by: Andrii Nakryiko <[email protected]>
Signed-off-by: Daniel Borkmann <[email protected]>
Acked-by: Eduard Zingerman <[email protected]>
Link: https://lore.kernel.org/bpf/[email protected]

1 files changed, 19 insertions, 0 deletions

diff --git a/tools/testing/selftests/bpf/progs/btf_dump_test_case_packing.c b/tools/testing/selftests/bpf/progs/btf_dump_test_case_packing.c
index 5c6c62f7ed32..7998f27df7dd 100644
--- a/tools/testing/selftests/bpf/progs/btf_dump_test_case_packing.c
+++ b/tools/testing/selftests/bpf/progs/btf_dump_test_case_packing.c
@@ -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;