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* [PATCH][next] mtd: maps: sa1100-flash: Replace zero-length array with flexible-array member
@ 2020-03-19 22:42 Gustavo A. R. Silva
  2020-03-24 22:05 ` Miquel Raynal
  0 siblings, 1 reply; 2+ messages in thread
From: Gustavo A. R. Silva @ 2020-03-19 22:42 UTC (permalink / raw)
  To: Miquel Raynal, Richard Weinberger, 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.

[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>
---
 drivers/mtd/maps/sa1100-flash.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/mtd/maps/sa1100-flash.c b/drivers/mtd/maps/sa1100-flash.c
index 47602af4ee34..bb1ef650ffd2 100644
--- a/drivers/mtd/maps/sa1100-flash.c
+++ b/drivers/mtd/maps/sa1100-flash.c
@@ -34,7 +34,7 @@ struct sa_subdev_info {
 struct sa_info {
 	struct mtd_info		*mtd;
 	int			num_subdev;
-	struct sa_subdev_info	subdev[0];
+	struct sa_subdev_info	subdev[];
 };
 
 static DEFINE_SPINLOCK(sa1100_vpp_lock);
-- 
2.23.0


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^ permalink raw reply	[flat|nested] 2+ messages in thread

* Re: [PATCH][next] mtd: maps: sa1100-flash: Replace zero-length array with flexible-array member
  2020-03-19 22:42 [PATCH][next] mtd: maps: sa1100-flash: Replace zero-length array with flexible-array member Gustavo A. R. Silva
@ 2020-03-24 22:05 ` Miquel Raynal
  0 siblings, 0 replies; 2+ messages in thread
From: Miquel Raynal @ 2020-03-24 22:05 UTC (permalink / raw)
  To: Gustavo A. R. Silva, Miquel Raynal, Richard Weinberger,
	Vignesh Raghavendra
  Cc: linux-mtd, linux-kernel

On Thu, 2020-03-19 at 22:42:00 UTC, "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 to https://git.kernel.org/pub/scm/linux/kernel/git/mtd/linux.git mtd/next, thanks.

Miquel

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2020-03-19 22:42 [PATCH][next] mtd: maps: sa1100-flash: Replace zero-length array with flexible-array member Gustavo A. R. Silva
2020-03-24 22:05 ` Miquel Raynal

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