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* [PATCH][next] mtd: ubi: ubi-media.h: Replace zero-length array with flexible-array member
@ 2020-03-19 22:44 Gustavo A. R. Silva
  0 siblings, 0 replies; only message in thread
From: Gustavo A. R. Silva @ 2020-03-19 22:44 UTC (permalink / raw)
  To: Richard Weinberger, Miquel Raynal, Vignesh Raghavendra
  Cc: linux-mtd, 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.

[3] commit 76497732932f ("cxgb3/l2t: Fix undefined behaviour")

Signed-off-by: Gustavo A. R. Silva <>
 drivers/mtd/ubi/ubi-media.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/mtd/ubi/ubi-media.h b/drivers/mtd/ubi/ubi-media.h
index b5fe8f82281b..386db0598e95 100644
--- a/drivers/mtd/ubi/ubi-media.h
+++ b/drivers/mtd/ubi/ubi-media.h
@@ -498,6 +498,6 @@ struct ubi_fm_volhdr {
 struct ubi_fm_eba {
 	__be32 magic;
 	__be32 reserved_pebs;
-	__be32 pnum[0];
+	__be32 pnum[];
 } __packed;
 #endif /* !__UBI_MEDIA_H__ */

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2020-03-19 22:44 [PATCH][next] mtd: ubi: ubi-media.h: Replace zero-length array with flexible-array member Gustavo A. R. Silva

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