linux-kernel - RE: [PATCH] treewide: Replace zero-length array with flexible-array

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Date:   Thu, 7 May 2020 20:08:40 +0000
From:   "Winkler, Tomas" <tomas.winkler@...el.com>
To:     "Gustavo A. R. Silva" <gustavoars@...nel.org>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH] treewide: Replace zero-length array with flexible-array



> -----Original Message-----
> From: Gustavo A. R. Silva <gustavoars@...nel.org>
> Sent: Thursday, May 07, 2020 21:54
> To: Winkler, Tomas <tomas.winkler@...el.com>
> Cc: linux-kernel@...r.kernel.org
> Subject: [PATCH] treewide: Replace zero-length array with flexible-array


Ack. 

> 
> 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>
> ---
>  samples/mei/mei-amt-version.c |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/samples/mei/mei-amt-version.c b/samples/mei/mei-amt-version.c
> index 32234481ad7d..ad3e56042f96 100644
> --- a/samples/mei/mei-amt-version.c
> +++ b/samples/mei/mei-amt-version.c
> @@ -267,7 +267,7 @@ struct amt_host_if_msg_header {  struct
> amt_host_if_resp_header {
>  	struct amt_host_if_msg_header header;
>  	uint32_t status;
> -	unsigned char data[0];
> +	unsigned char data[];
>  } __attribute__((packed));
> 
>  const uuid_le MEI_IAMTHIF = UUID_LE(0x12f80028, 0xb4b7, 0x4b2d,  \