[PATCH][next] aio: Replace zero-length array with flexible-array member

[PATCH][next] aio: Replace zero-length array with flexible-array member

[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")

Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
---
 fs/aio.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/aio.c b/fs/aio.c
index 94f2b9256c0c..13c4be7f00f0 100644
--- a/fs/aio.c
+++ b/fs/aio.c
@@ -68,7 +68,7 @@ struct aio_ring {
 	unsigned	header_length;	/* size of aio_ring */
 
 
-	struct io_event		io_events[0];
+	struct io_event		io_events[];
 }; /* 128 bytes + ring size */
 
 /*
-- 
2.25.0

Re: [PATCH][next] aio: Replace zero-length array with flexible-array member

[Jeff Moyer]

"Gustavo A. R. Silva" <gustavo@embeddedor.com> writes:

> 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 <gustavo@embeddedor.com>
> ---
>  fs/aio.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/fs/aio.c b/fs/aio.c
> index 94f2b9256c0c..13c4be7f00f0 100644
> --- a/fs/aio.c
> +++ b/fs/aio.c
> @@ -68,7 +68,7 @@ struct aio_ring {
>  	unsigned	header_length;	/* size of aio_ring */
>  
>  
> -	struct io_event		io_events[0];
> +	struct io_event		io_events[];
>  }; /* 128 bytes + ring size */
>  
>  /*

Acked-by: Jeff Moyer <jmoyer@redhat.com>