Re: [PATCH] bpf: queue_stack_maps: Replace zero-length array with flexible-array member
From: Andrii Nakryiko <hidden>
Date: 2020-02-13 17:47:40
Also in:
bpf, lkml
On Thu, Feb 13, 2020 at 7:22 AM Gustavo A. R. Silva [off-list ref] wrote:
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]
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 <redacted>
---Sure, why not, though I don't think that's the only one (e.g., bpf_storage_buffer's data is zero-length as well). Acked-by: Andrii Nakryiko <redacted>
quoted hunk ↗ jump to hunk
kernel/bpf/queue_stack_maps.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)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) --2.25.0