bpf: queue_stack_maps: Replace zero-length array with flexible-array member

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 <[email protected]>

---

 kernel/bpf/queue_stack_maps.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

kernel/bpf/queue_stack_maps.c +1 -2

index f697647ceb54..30e1373fd437 100644
@@ -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

Quoted 29 lines · Gustavo A. R. Silva

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")

Quoted 1 line

Signed-off-by Gustavo A. R. Silva <[email protected]>

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 <[email protected]>