linux-kernel - Re: [PATCH] gcov: gcc_4_7: Replace zero-length array with flexible-array member

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Date:   Wed, 26 Feb 2020 14:38:40 +0100
From:   Peter Oberparleiter <oberpar@...ux.ibm.com>
To:     "Gustavo A. R. Silva" <gustavo@...eddedor.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     linux-kernel@...r.kernel.org
Subject: Re: [PATCH] gcov: gcc_4_7: Replace zero-length array with
 flexible-array member

On 13.02.2020 16:22, 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]
> 
> 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@...eddedor.com>

Acked-by: Peter Oberparleiter <oberpar@...ux.ibm.com>

Andrew, could you pick this patch up?

> ---
>  kernel/gcov/gcc_4_7.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/gcov/gcc_4_7.c b/kernel/gcov/gcc_4_7.c
> index ec37563674d6..908fdf5098c3 100644
> --- a/kernel/gcov/gcc_4_7.c
> +++ b/kernel/gcov/gcc_4_7.c
> @@ -68,7 +68,7 @@ struct gcov_fn_info {
>  	unsigned int ident;
>  	unsigned int lineno_checksum;
>  	unsigned int cfg_checksum;
> -	struct gcov_ctr_info ctrs[0];
> +	struct gcov_ctr_info ctrs[];
>  };
>  
>  /**
> 


-- 
Peter Oberparleiter
Linux on Z Development - IBM Germany