* [PATCH][next] btrfs/rcu-string.h: Replace zero-length array with flexible-array member
@ 2020-03-06 16:51 Gustavo A. R. Silva
2020-03-06 20:02 ` David Sterba
0 siblings, 1 reply; 3+ messages in thread
From: Gustavo A. R. Silva @ 2020-03-06 16:51 UTC (permalink / raw)
To: Chris Mason, Josef Bacik, David Sterba
Cc: linux-btrfs, 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>
---
fs/btrfs/rcu-string.h | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/fs/btrfs/rcu-string.h b/fs/btrfs/rcu-string.h
index a97dc74a4d3d..5c1a617eb25d 100644
--- a/fs/btrfs/rcu-string.h
+++ b/fs/btrfs/rcu-string.h
@@ -8,7 +8,7 @@
struct rcu_string {
struct rcu_head rcu;
- char str[0];
+ char str[];
};
static inline struct rcu_string *rcu_string_strdup(const char *src, gfp_t mask)
--
2.25.0
^ permalink raw reply related [flat|nested] 3+ messages in thread
* Re: [PATCH][next] btrfs/rcu-string.h: Replace zero-length array with flexible-array member
2020-03-06 16:51 [PATCH][next] btrfs/rcu-string.h: Replace zero-length array with flexible-array member Gustavo A. R. Silva
@ 2020-03-06 20:02 ` David Sterba
2020-03-06 21:48 ` Gustavo A. R. Silva
0 siblings, 1 reply; 3+ messages in thread
From: David Sterba @ 2020-03-06 20:02 UTC (permalink / raw)
To: Gustavo A. R. Silva
Cc: Chris Mason, Josef Bacik, David Sterba, linux-btrfs, linux-kernel
On Fri, Mar 06, 2020 at 10:51:05AM -0600, 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>
Added to devel queue, thanks.
^ permalink raw reply [flat|nested] 3+ messages in thread
* Re: [PATCH][next] btrfs/rcu-string.h: Replace zero-length array with flexible-array member
2020-03-06 20:02 ` David Sterba
@ 2020-03-06 21:48 ` Gustavo A. R. Silva
0 siblings, 0 replies; 3+ messages in thread
From: Gustavo A. R. Silva @ 2020-03-06 21:48 UTC (permalink / raw)
To: dsterba, Chris Mason, Josef Bacik, David Sterba, linux-btrfs,
linux-kernel
On 3/6/20 14:02, David Sterba wrote:
>>
>> [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>
>
> Added to devel queue, thanks.
>
Thanks, David.
--
Gustavo
^ permalink raw reply [flat|nested] 3+ messages in thread
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2020-03-06 16:51 [PATCH][next] btrfs/rcu-string.h: Replace zero-length array with flexible-array member Gustavo A. R. Silva
2020-03-06 20:02 ` David Sterba
2020-03-06 21:48 ` Gustavo A. R. Silva
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