2020-02-28 13:43:24

by Gustavo A. R. Silva

Subject: [PATCH][next] sunrpc: 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]>
---
include/linux/sunrpc/svc.h | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/linux/sunrpc/svc.h b/include/linux/sunrpc/svc.h
index 1afe38eb33f7..7f0a83451bc0 100644
--- a/include/linux/sunrpc/svc.h
+++ b/include/linux/sunrpc/svc.h
@@ -380,7 +380,7 @@ struct svc_deferred_req {
struct cache_deferred_req handle;
size_t xprt_hlen;
int argslen;
- __be32 args[0];
+ __be32 args[];
};

struct svc_process_info {
--
2.25.0

2020-03-02 19:59:04

by J. Bruce Fields

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

On Fri, Feb 28, 2020 at 07:23:23AM -0600, Gustavo A. R. Silva 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]

I don't understand the quoted sentences at all. But I assume you're
telling me that sizeof(struct svc_deferred_req) won't be changed by this
patch, so, good, applied. Thanks!

--b.

>
> 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]>
> ---
> include/linux/sunrpc/svc.h | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/include/linux/sunrpc/svc.h b/include/linux/sunrpc/svc.h
> index 1afe38eb33f7..7f0a83451bc0 100644
> --- a/include/linux/sunrpc/svc.h
> +++ b/include/linux/sunrpc/svc.h
> @@ -380,7 +380,7 @@ struct svc_deferred_req {
> struct cache_deferred_req handle;
> size_t xprt_hlen;
> int argslen;
> - __be32 args[0];
> + __be32 args[];
> };
>
> struct svc_process_info {
> --
> 2.25.0