linux-kernel - Re: [PATCH] tools/testing: Replace zero-length array with flexible-array

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Message-ID: <94b19470-9ade-5fcc-00c7-324056d049de@linuxfoundation.org>
Date:   Thu, 7 May 2020 13:04:14 -0600
From:   Shuah Khan <skhan@...uxfoundation.org>
To:     "Gustavo A. R. Silva" <gustavoars@...nel.org>,
        Shuah Khan <shuah@...nel.org>
Cc:     linux-kselftest@...r.kernel.org, linux-kernel@...r.kernel.org,
        Shuah Khan <skhan@...uxfoundation.org>
Subject: Re: [PATCH] tools/testing: Replace zero-length array with
 flexible-array

On 5/7/20 12:56 PM, 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]
> 
> 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@...nel.org>
> ---
>   tools/testing/selftests/nsfs/pidns.c |    2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/tools/testing/selftests/nsfs/pidns.c b/tools/testing/selftests/nsfs/pidns.c
> index e0d86e1668c0..e3c772c6a7c7 100644
> --- a/tools/testing/selftests/nsfs/pidns.c
> +++ b/tools/testing/selftests/nsfs/pidns.c
> @@ -27,7 +27,7 @@
>   #define __stack_aligned__	__attribute__((aligned(16)))
>   struct cr_clone_arg {
>   	char stack[128] __stack_aligned__;
> -	char stack_ptr[0];
> +	char stack_ptr[];
>   };
>   
>   static int child(void *args)
> 

Thanks for the patch. I will pull this in for 5.7-rc6

thanks,
-- Shuah