[PATCH][next] jfs: Replace zero-length array with flexible-array member

[PATCH][next] jfs: Replace zero-length array with flexible-array member

[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/jfs/jfs_dtree.c | 2 +-
 fs/jfs/jfs_xattr.h | 4 ++--
 2 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/fs/jfs/jfs_dtree.c b/fs/jfs/jfs_dtree.c
index 3acc954f7c04..837d42f61464 100644
--- a/fs/jfs/jfs_dtree.c
+++ b/fs/jfs/jfs_dtree.c
@@ -2964,7 +2964,7 @@ struct jfs_dirent {
 	loff_t position;
 	int ino;
 	u16 name_len;
-	char name[0];
+	char name[];
 };
 
 /*
diff --git a/fs/jfs/jfs_xattr.h b/fs/jfs/jfs_xattr.h
index f0558b3348da..c50167a7bc50 100644
--- a/fs/jfs/jfs_xattr.h
+++ b/fs/jfs/jfs_xattr.h
@@ -17,12 +17,12 @@ struct jfs_ea {
 	u8 flag;	/* Unused? */
 	u8 namelen;	/* Length of name */
 	__le16 valuelen;	/* Length of value */
-	char name[0];	/* Attribute name (includes null-terminator) */
+	char name[];	/* Attribute name (includes null-terminator) */
 };			/* Value immediately follows name */
 
 struct jfs_ea_list {
 	__le32 size;		/* overall size */
-	struct jfs_ea ea[0];	/* Variable length list */
+	struct jfs_ea ea[];	/* Variable length list */
 };
 
 /* Macros for defining maxiumum number of bytes supported for EAs */
-- 
2.25.0

Re: [PATCH][next] jfs: Replace zero-length array with flexible-array member

[Dave Kleikamp]

Looks good. I'll queue this for linux-next.

Shaggy

On 3/9/20 1:23 PM, 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>
> ---
>  fs/jfs/jfs_dtree.c | 2 +-
>  fs/jfs/jfs_xattr.h | 4 ++--
>  2 files changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/fs/jfs/jfs_dtree.c b/fs/jfs/jfs_dtree.c
> index 3acc954f7c04..837d42f61464 100644
> --- a/fs/jfs/jfs_dtree.c
> +++ b/fs/jfs/jfs_dtree.c
> @@ -2964,7 +2964,7 @@ struct jfs_dirent {
>  	loff_t position;
>  	int ino;
>  	u16 name_len;
> -	char name[0];
> +	char name[];
>  };
>  
>  /*
> diff --git a/fs/jfs/jfs_xattr.h b/fs/jfs/jfs_xattr.h
> index f0558b3348da..c50167a7bc50 100644
> --- a/fs/jfs/jfs_xattr.h
> +++ b/fs/jfs/jfs_xattr.h
> @@ -17,12 +17,12 @@ struct jfs_ea {
>  	u8 flag;	/* Unused? */
>  	u8 namelen;	/* Length of name */
>  	__le16 valuelen;	/* Length of value */
> -	char name[0];	/* Attribute name (includes null-terminator) */
> +	char name[];	/* Attribute name (includes null-terminator) */
>  };			/* Value immediately follows name */
>  
>  struct jfs_ea_list {
>  	__le32 size;		/* overall size */
> -	struct jfs_ea ea[0];	/* Variable length list */
> +	struct jfs_ea ea[];	/* Variable length list */
>  };
>  
>  /* Macros for defining maxiumum number of bytes supported for EAs */
>