* [PATCH][next] ext2: xattr.h: Replace zero-length array with flexible-array member
@ 2020-03-09 18:04 Gustavo A. R. Silva
2020-03-16 14:57 ` Jan Kara
0 siblings, 1 reply; 2+ messages in thread
From: Gustavo A. R. Silva @ 2020-03-09 18:04 UTC (permalink / raw)
To: Jan Kara; +Cc: linux-ext4, linux-kernel, 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]
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@embeddedor.com>
---
fs/ext2/xattr.h | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/fs/ext2/xattr.h b/fs/ext2/xattr.h
index cee888cdc235..16272e6ddcf4 100644
--- a/fs/ext2/xattr.h
+++ b/fs/ext2/xattr.h
@@ -39,7 +39,7 @@ struct ext2_xattr_entry {
__le32 e_value_block; /* disk block attribute is stored on (n/i) */
__le32 e_value_size; /* size of attribute value */
__le32 e_hash; /* hash value of name and value */
- char e_name[0]; /* attribute name */
+ char e_name[]; /* attribute name */
};
#define EXT2_XATTR_PAD_BITS 2
--
2.25.0
^ permalink raw reply related [flat|nested] 2+ messages in thread
* Re: [PATCH][next] ext2: xattr.h: Replace zero-length array with flexible-array member
2020-03-09 18:04 [PATCH][next] ext2: xattr.h: Replace zero-length array with flexible-array member Gustavo A. R. Silva
@ 2020-03-16 14:57 ` Jan Kara
0 siblings, 0 replies; 2+ messages in thread
From: Jan Kara @ 2020-03-16 14:57 UTC (permalink / raw)
To: Gustavo A. R. Silva; +Cc: Jan Kara, linux-ext4, linux-kernel
On Mon 09-03-20 13:04:41, 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@embeddedor.com>
Thanks. I've added the patch to my tree.
Honza
> ---
> fs/ext2/xattr.h | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/fs/ext2/xattr.h b/fs/ext2/xattr.h
> index cee888cdc235..16272e6ddcf4 100644
> --- a/fs/ext2/xattr.h
> +++ b/fs/ext2/xattr.h
> @@ -39,7 +39,7 @@ struct ext2_xattr_entry {
> __le32 e_value_block; /* disk block attribute is stored on (n/i) */
> __le32 e_value_size; /* size of attribute value */
> __le32 e_hash; /* hash value of name and value */
> - char e_name[0]; /* attribute name */
> + char e_name[]; /* attribute name */
> };
>
> #define EXT2_XATTR_PAD_BITS 2
> --
> 2.25.0
>
--
Jan Kara <jack@suse.com>
SUSE Labs, CR
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