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thermal: qcom: tsens.h: Replace zero-length array with flexible-array member

PATCH Applied Gustavo A. R. Silva 3 participants 3 messages Mar 19, 2020

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Arm Qualcomm mailing list Thermal Qualcomm tsens thermal driver
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Gustavo A. R. Silva

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PATCH 1/1 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(-)
drivers/thermal/qcom/tsens.h +1 -1
index e24a865fbc34..92503712596a 100644
@@ -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);
Signature · 1 line
2.23.0
Reviewed-by on v1 1/1
#1 lore raw
Quoted 23 lines · 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]

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]>
Reviewed-by Amit Kucheria <[email protected]>
Quoted 20 lines
---
 drivers/thermal/qcom/tsens.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
drivers/thermal/qcom/tsens.h +1 -2
index e24a865fbc34..92503712596a 100644
@@ -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

Applied
#2 lore raw
In reply to Gustavo A. R. Silva
Quoted 22 lines · 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]

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

Quoted 18 lines
---
 drivers/thermal/qcom/tsens.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
drivers/thermal/qcom/tsens.h +1 -1
index e24a865fbc34..92503712596a 100644
@@ -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);
Signature · 5 lines
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