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[next] ext4: dir: Replace zero-length array with flexible-array member

Message ID 20200309154838.GA31559@embeddedor
State Awaiting Upstream
Headers show
Series [next] ext4: dir: Replace zero-length array with flexible-array member | expand

Commit Message

Gustavo A. R. Silva March 9, 2020, 3:48 p.m. UTC
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/ext4/dir.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

Comments

Theodore Y. Ts'o March 12, 2020, 2:54 p.m. UTC | #1
On Mon, Mar 09, 2020 at 10:48:38AM -0500, 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>

Applied, thanks.

					- Ted
diff mbox series

Patch

diff --git a/fs/ext4/dir.c b/fs/ext4/dir.c
index 9aa1f75409b0..c654205f648d 100644
--- a/fs/ext4/dir.c
+++ b/fs/ext4/dir.c
@@ -392,7 +392,7 @@  struct fname {
 	__u32		inode;
 	__u8		name_len;
 	__u8		file_type;
-	char		name[0];
+	char		name[];
 };
 
 /*