[014/155] ocfs2: replace zero-length array with flexible-array member

Commit Message

Andrew Morton April 2, 2020, 4:03 a.m. UTC

From: "Gustavo A. R. Silva" <[e-mail]>
Subject: ocfs2: replace zero-length array with flexible-array member

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")

Link: http://lkml.kernel.org/r/20200213160244.GA6088@embeddedor
Signed-off-by: Gustavo A. R. Silva <[e-mail]>
Acked-by: Joseph Qi <[e-mail]>
Cc: Mark Fasheh <[e-mail]>
Cc: Joel Becker <[e-mail]>
Cc: Junxiao Bi <[e-mail]>
Cc: Changwei Ge <[e-mail]>
Cc: Gang He <[e-mail]>
Cc: Jun Piao <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
---

 fs/ocfs2/journal.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

Patch

--- a/fs/ocfs2/journal.c~ocfs2-replace-zero-length-array-with-flexible-array-member
+++ a/fs/ocfs2/journal.c
@@ -91,7 +91,7 @@  enum ocfs2_replay_state {
 struct ocfs2_replay_map {
 	unsigned int rm_slots;
 	enum ocfs2_replay_state rm_state;
-	unsigned char rm_replay_slots[0];
+	unsigned char rm_replay_slots[];
 };
 
 static void ocfs2_replay_map_set_state(struct ocfs2_super *osb, int state)