* [PATCH] ext4: namei: Replace zero-length array with flexible-array member
@ 2020-02-13 16:06 Gustavo A. R. Silva
2020-03-02 23:57 ` Gustavo A. R. Silva
2020-03-06 1:12 ` Theodore Y. Ts'o
0 siblings, 2 replies; 3+ messages in thread
From: Gustavo A. R. Silva @ 2020-02-13 16:06 UTC (permalink / raw)
To: Theodore Ts'o, Andreas Dilger
Cc: linux-ext4, 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/ext4/namei.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)
diff --git a/fs/ext4/namei.c b/fs/ext4/namei.c
index 129d2ebae00d..77c327d904bf 100644
--- a/fs/ext4/namei.c
+++ b/fs/ext4/namei.c
@@ -233,13 +233,13 @@ struct dx_root
u8 unused_flags;
}
info;
- struct dx_entry entries[0];
+ struct dx_entry entries[];
};
struct dx_node
{
struct fake_dirent fake;
- struct dx_entry entries[0];
+ struct dx_entry entries[];
};
--
2.25.0
^ permalink raw reply related [flat|nested] 3+ messages in thread
* Re: [PATCH] ext4: namei: Replace zero-length array with flexible-array member
2020-02-13 16:06 [PATCH] ext4: namei: Replace zero-length array with flexible-array member Gustavo A. R. Silva
@ 2020-03-02 23:57 ` Gustavo A. R. Silva
2020-03-06 1:12 ` Theodore Y. Ts'o
1 sibling, 0 replies; 3+ messages in thread
From: Gustavo A. R. Silva @ 2020-03-02 23:57 UTC (permalink / raw)
To: Theodore Ts'o, Andreas Dilger; +Cc: linux-ext4, linux-kernel
Hi all,
Friendly ping: who can take this?
Thanks
--
Gustavo
On 2/13/20 10:06, 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>
> ---
> fs/ext4/namei.c | 4 ++--
> 1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/fs/ext4/namei.c b/fs/ext4/namei.c
> index 129d2ebae00d..77c327d904bf 100644
> --- a/fs/ext4/namei.c
> +++ b/fs/ext4/namei.c
> @@ -233,13 +233,13 @@ struct dx_root
> u8 unused_flags;
> }
> info;
> - struct dx_entry entries[0];
> + struct dx_entry entries[];
> };
>
> struct dx_node
> {
> struct fake_dirent fake;
> - struct dx_entry entries[0];
> + struct dx_entry entries[];
> };
>
>
>
^ permalink raw reply [flat|nested] 3+ messages in thread
* Re: [PATCH] ext4: namei: Replace zero-length array with flexible-array member
2020-02-13 16:06 [PATCH] ext4: namei: Replace zero-length array with flexible-array member Gustavo A. R. Silva
2020-03-02 23:57 ` Gustavo A. R. Silva
@ 2020-03-06 1:12 ` Theodore Y. Ts'o
1 sibling, 0 replies; 3+ messages in thread
From: Theodore Y. Ts'o @ 2020-03-06 1:12 UTC (permalink / raw)
To: Gustavo A. R. Silva; +Cc: Andreas Dilger, linux-ext4, linux-kernel
On Thu, Feb 13, 2020 at 10:06:48AM -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>
Thanks, applied.
- Ted
^ permalink raw reply [flat|nested] 3+ messages in thread
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