Linux-Fsdevel Archive on lore.kernel.org
help / color / mirror / Atom feed
* [PATCH] fanotify: Replace zero-length array with flexible-array
@ 2020-05-07 18:52 Gustavo A. R. Silva
  2020-05-08  8:38 ` Jan Kara
  0 siblings, 1 reply; 2+ messages in thread
From: Gustavo A. R. Silva @ 2020-05-07 18:52 UTC (permalink / raw)
  To: Jan Kara; +Cc: Amir Goldstein, linux-fsdevel, linux-kernel

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>
---
 fs/notify/fanotify/fanotify.h |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/notify/fanotify/fanotify.h b/fs/notify/fanotify/fanotify.h
index 35bfbf4a7aac..8ce7ccfc4b0d 100644
--- a/fs/notify/fanotify/fanotify.h
+++ b/fs/notify/fanotify/fanotify.h
@@ -89,7 +89,7 @@ struct fanotify_name_event {
 	__kernel_fsid_t fsid;
 	struct fanotify_fh dir_fh;
 	u8 name_len;
-	char name[0];
+	char name[];
 };
 
 static inline struct fanotify_name_event *


^ permalink raw reply	[flat|nested] 2+ messages in thread

* Re: [PATCH] fanotify: Replace zero-length array with flexible-array
  2020-05-07 18:52 [PATCH] fanotify: Replace zero-length array with flexible-array Gustavo A. R. Silva
@ 2020-05-08  8:38 ` Jan Kara
  0 siblings, 0 replies; 2+ messages in thread
From: Jan Kara @ 2020-05-08  8:38 UTC (permalink / raw)
  To: Gustavo A. R. Silva; +Cc: Jan Kara, Amir Goldstein, linux-fsdevel, linux-kernel

On Thu 07-05-20 13:52:30, 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@kernel.org>

Thanks. I've added the patch to my tree.

								Honza

> ---
>  fs/notify/fanotify/fanotify.h |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/fs/notify/fanotify/fanotify.h b/fs/notify/fanotify/fanotify.h
> index 35bfbf4a7aac..8ce7ccfc4b0d 100644
> --- a/fs/notify/fanotify/fanotify.h
> +++ b/fs/notify/fanotify/fanotify.h
> @@ -89,7 +89,7 @@ struct fanotify_name_event {
>  	__kernel_fsid_t fsid;
>  	struct fanotify_fh dir_fh;
>  	u8 name_len;
> -	char name[0];
> +	char name[];
>  };
>  
>  static inline struct fanotify_name_event *
> 
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR

^ permalink raw reply	[flat|nested] 2+ messages in thread

end of thread, other threads:[~2020-05-08  8:38 UTC | newest]

Thread overview: 2+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2020-05-07 18:52 [PATCH] fanotify: Replace zero-length array with flexible-array Gustavo A. R. Silva
2020-05-08  8:38 ` Jan Kara

This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).