On Thu, May 7, 2020 at 9:15 PM Gustavo A. R. Silva
<gustavoars@xxxxxxxxxx> 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]
>
> 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@xxxxxxxxxx>
> ---
> drivers/block/rbd_types.h | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/drivers/block/rbd_types.h b/drivers/block/rbd_types.h
> index ac98ab6ccd3b..a600e0eb6b6f 100644
> --- a/drivers/block/rbd_types.h
> +++ b/drivers/block/rbd_types.h
> @@ -93,7 +93,7 @@ struct rbd_image_header_ondisk {
> __le32 snap_count;
> __le32 reserved;
> __le64 snap_names_len;
> - struct rbd_image_snap_ondisk snaps[0];
> + struct rbd_image_snap_ondisk snaps[];
> } __attribute__((packed));
>
>
>
Applied (folded into libceph patch).
Thanks,
Ilya
