Re: [PATCH] bpf: queue_stack_maps: Replace zero-length array with flexible-array member

From: Daniel Borkmann
Date: Tue Feb 18 2020 - 09:53:33 EST

On 2/13/20 6:47 PM, Andrii Nakryiko wrote:
On Thu, Feb 13, 2020 at 7:22 AM Gustavo A. R. Silva
<gustavo@xxxxxxxxxxxxxx> 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.

[3] commit 76497732932f ("cxgb3/l2t: Fix undefined behaviour")

Signed-off-by: Gustavo A. R. Silva <gustavo@xxxxxxxxxxxxxx>

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 <andriin@xxxxxx>

+1, Gustavo, there are several such instances in the whole BPF subsystem. Please combine
them all into a single patch, including, and