[PATCH] libbpf: Replace zero-length array with flexible-array

["Gustavo A. R. Silva" <gustavoars@kernel.org>]

The current codebase makes use of the zero-length array language
extension to the C90 standard, but the preferred mechanism to declare
variable-length types such as these ones is a flexible array member[1][2],
introduced in C99:

struct foo {
        int stuff;
        struct boo array[];
};

By making use of the mechanism above, we will get a compiler warning
in case the flexible array does not occur last in the structure, which
will help us prevent some kind of undefined behavior bugs from being
inadvertently introduced[3] to the codebase from now on.

Also, notice that, dynamic memory allocations won't be affected by
this change:

"Flexible array members have incomplete type, and so the sizeof operator
may not be applied. As a quirk of the original implementation of
zero-length arrays, sizeof evaluates to zero."[1]

sizeof(flexible-array-member) triggers a warning because flexible array
members have incomplete type[1]. There are some instances of code in
which the sizeof operator is being incorrectly/erroneously applied to
zero-length arrays and the result is zero. Such instances may be hiding
some bugs. So, this work (flexible-array member conversions) will also
help to get completely rid of those sorts of issues.

This issue was found with the help of Coccinelle.

[1] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
[2] https://github.com/KSPP/linux/issues/21
[3] commit 76497732932f ("cxgb3/l2t: Fix undefined behaviour")

Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
---
 kernel/bpf/queue_stack_maps.c                        |    2 +-
 tools/lib/bpf/libbpf.c                               |    2 +-
 tools/lib/bpf/libbpf_internal.h                      |    2 +-
 tools/testing/selftests/bpf/progs/core_reloc_types.h |    2 +-
 4 files changed, 4 insertions(+), 4 deletions(-)

diff --git a/kernel/bpf/queue_stack_maps.c b/kernel/bpf/queue_stack_maps.c
index f697647ceb54..30e1373fd437 100644
--- a/kernel/bpf/queue_stack_maps.c
+++ b/kernel/bpf/queue_stack_maps.c
@@ -19,7 +19,7 @@ struct bpf_queue_stack {
 	u32 head, tail;
 	u32 size; /* max_entries + 1 */
 
-	char elements[0] __aligned(8);
+	char elements[] __aligned(8);
 };
 
 static struct bpf_queue_stack *bpf_queue_stack(struct bpf_map *map)
diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index 8f480e29a6b0..b9335c686353 100644
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c
@@ -8035,7 +8035,7 @@ static struct perf_buffer *__perf_buffer__new(int map_fd, size_t page_cnt,
 struct perf_sample_raw {
 	struct perf_event_header header;
 	uint32_t size;
-	char data[0];
+	char data[];
 };
 
 struct perf_sample_lost {
diff --git a/tools/lib/bpf/libbpf_internal.h b/tools/lib/bpf/libbpf_internal.h
index 8c3afbd97747..50d70e90d5f1 100644
--- a/tools/lib/bpf/libbpf_internal.h
+++ b/tools/lib/bpf/libbpf_internal.h
@@ -153,7 +153,7 @@ struct btf_ext_info_sec {
 	__u32	sec_name_off;
 	__u32	num_info;
 	/* Followed by num_info * record_size number of bytes */
-	__u8	data[0];
+	__u8	data[];
 };
 
 /* The minimum bpf_func_info checked by the loader */
diff --git a/tools/testing/selftests/bpf/progs/core_reloc_types.h b/tools/testing/selftests/bpf/progs/core_reloc_types.h
index 6d598cfbdb3e..34d84717c946 100644
--- a/tools/testing/selftests/bpf/progs/core_reloc_types.h
+++ b/tools/testing/selftests/bpf/progs/core_reloc_types.h
@@ -379,7 +379,7 @@ struct core_reloc_arrays___equiv_zero_sz_arr {
 	struct core_reloc_arrays_substruct c[3];
 	struct core_reloc_arrays_substruct d[1][2];
 	/* equivalent to flexible array */
-	struct core_reloc_arrays_substruct f[0][2];
+	struct core_reloc_arrays_substruct f[][2];
 };
 
 struct core_reloc_arrays___fixed_arr {