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mtd: ubi: ubi-media.h: Replace zero-length array with flexible-array member

PATCH Applied Gustavo A. R. Silva 2 participants 2 messages Mar 19, 2020 – Mar 30, 2020

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Memory technology devices (MTD) Unsorted block images (UBI)
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Gustavo A. R. Silva

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PATCH 1/1 mtd: ubi: ubi-media.h: 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")

Signed-off-by Gustavo A. R. Silva <[email protected]>
---
 drivers/mtd/ubi/ubi-media.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
drivers/mtd/ubi/ubi-media.h +1 -1
index b5fe8f82281b..386db0598e95 100644
@@ -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__ */
Signature · 1 line
2.23.0
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----- Ursprüngliche Mail -----

Quoted 29 lines

Von: "Gustavo A. R. Silva" <[email protected]>
An: "richard" <[email protected]>, "Miquel Raynal" <[email protected]>, "Vignesh Raghavendra" <[email protected]>
CC: "linux-mtd" <[email protected]>, "linux-kernel" <[email protected]>, "Gustavo A. R. Silva" <[email protected]>
Gesendet: Donnerstag, 19. März 2020 23:44:35
Betreff: [PATCH][next] mtd: ubi: ubi-media.h: 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]

Quoted 7 lines

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 <[email protected]>

Applied. Thanks!

Thanks,
//richard