2020-03-19 18:49:48

by Gustavo A. R. Silva

[permalink] [raw]
Subject: [PATCH] thermal: qcom: tsens.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/thermal/qcom/tsens.h | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/thermal/qcom/tsens.h b/drivers/thermal/qcom/tsens.h
index e24a865fbc34..92503712596a 100644
--- a/drivers/thermal/qcom/tsens.h
+++ b/drivers/thermal/qcom/tsens.h
@@ -488,7 +488,7 @@ struct tsens_priv {
struct dentry *debug_root;
struct dentry *debug;

- struct tsens_sensor sensor[0];
+ struct tsens_sensor sensor[];
};

char *qfprom_read(struct device *dev, const char *cname);
--
2.23.0


2020-03-19 19:57:25

by Amit Kucheria

[permalink] [raw]
Subject: Re: [PATCH] thermal: qcom: tsens.h: Replace zero-length array with flexible-array member

On Fri, Mar 20, 2020 at 12:18 AM Gustavo A. R. Silva
<[email protected]> 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 <[email protected]>

Reviewed-by: Amit Kucheria <[email protected]>

> ---
> drivers/thermal/qcom/tsens.h | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/drivers/thermal/qcom/tsens.h b/drivers/thermal/qcom/tsens.h
> index e24a865fbc34..92503712596a 100644
> --- a/drivers/thermal/qcom/tsens.h
> +++ b/drivers/thermal/qcom/tsens.h
> @@ -488,7 +488,7 @@ struct tsens_priv {
> struct dentry *debug_root;
> struct dentry *debug;
>
> - struct tsens_sensor sensor[0];
> + struct tsens_sensor sensor[];
> };
>
> char *qfprom_read(struct device *dev, const char *cname);
> --
> 2.23.0
>

2020-03-19 20:46:18

by Daniel Lezcano

[permalink] [raw]
Subject: Re: [PATCH] thermal: qcom: tsens.h: Replace zero-length array with flexible-array member

On 19/03/2020 19:48, 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 <[email protected]>

Applied, thanks

> ---
> drivers/thermal/qcom/tsens.h | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/drivers/thermal/qcom/tsens.h b/drivers/thermal/qcom/tsens.h
> index e24a865fbc34..92503712596a 100644
> --- a/drivers/thermal/qcom/tsens.h
> +++ b/drivers/thermal/qcom/tsens.h
> @@ -488,7 +488,7 @@ struct tsens_priv {
> struct dentry *debug_root;
> struct dentry *debug;
>
> - struct tsens_sensor sensor[0];
> + struct tsens_sensor sensor[];
> };
>
> char *qfprom_read(struct device *dev, const char *cname);
>


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