linux-kernel - Re: [PATCH] xfs: Replace zero-length array with flexible-array

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Date:   Tue, 12 May 2020 16:38:43 -0700
From:   "Darrick J. Wong" <darrick.wong@...cle.com>
To:     "Gustavo A. R. Silva" <gustavoars@...nel.org>
Cc:     linux-xfs@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] xfs: Replace zero-length array with flexible-array

On Thu, May 07, 2020 at 02:27:03PM -0500, 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>

/me wonders what weird-arch kbuild robot furies will be let loose by
putting this in, but for now:

Reviewed-by: Darrick J. Wong <darrick.wong@...cle.com>

--D

> ---
>  fs/xfs/libxfs/xfs_format.h |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/fs/xfs/libxfs/xfs_format.h b/fs/xfs/libxfs/xfs_format.h
> index 045556e78ee2..592f1c12ad36 100644
> --- a/fs/xfs/libxfs/xfs_format.h
> +++ b/fs/xfs/libxfs/xfs_format.h
> @@ -1681,7 +1681,7 @@ struct xfs_acl_entry {
>  
>  struct xfs_acl {
>  	__be32			acl_cnt;
> -	struct xfs_acl_entry	acl_entry[0];
> +	struct xfs_acl_entry	acl_entry[];
>  };
>  
>  /*
>