[PATCH] tty: Replace zero-length array with flexible-array

[PATCH] tty: Replace zero-length array with flexible-array

[Gustavo A. R. Silva]

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@kernel.org>
---
 include/linux/tty.h |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/linux/tty.h b/include/linux/tty.h
index bd5fe0e907e8..a99e9b8e4e31 100644
--- a/include/linux/tty.h
+++ b/include/linux/tty.h
@@ -66,7 +66,7 @@ struct tty_buffer {
 	int read;
 	int flags;
 	/* Data points here */
-	unsigned long data[0];
+	unsigned long data[];
 };
 
 /* Values for .flags field of tty_buffer */

Re: [PATCH] tty: Replace zero-length array with flexible-array

[Jiri Slaby]

On 07. 05. 20, 21:25, 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[];
> };
...

Acked-by: Jiri Slaby <jslaby@suse.cz>

> Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
> ---
>  include/linux/tty.h |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/include/linux/tty.h b/include/linux/tty.h
> index bd5fe0e907e8..a99e9b8e4e31 100644
> --- a/include/linux/tty.h
> +++ b/include/linux/tty.h
> @@ -66,7 +66,7 @@ struct tty_buffer {
>  	int read;
>  	int flags;
>  	/* Data points here */
> -	unsigned long data[0];
> +	unsigned long data[];
>  };
>  
>  /* Values for .flags field of tty_buffer */
> 

thanks,
-- 
js
suse labs