[PATCH] tty: hvc-iucv: fix function pointer casts

[PATCH] tty: hvc-iucv: fix function pointer casts

[Arnd Bergmann]

From: Arnd Bergmann <arnd@arndb.de>

clang warns about explicitly casting between incompatible function
pointers:

drivers/tty/hvc/hvc_iucv.c:1100:23: error: cast from 'void (*)(const void *)' to 'void (*)(struct device *)' converts to incompatible function type [-Werror,-Wcast-function-type-strict]
 1100 |         priv->dev->release = (void (*)(struct device *)) kfree;
      |                              ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Add a separate function to handle this correctly.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
---
 drivers/tty/hvc/hvc_iucv.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/drivers/tty/hvc/hvc_iucv.c b/drivers/tty/hvc/hvc_iucv.c
index fdecc0d63731..b1149bc62ca1 100644
--- a/drivers/tty/hvc/hvc_iucv.c
+++ b/drivers/tty/hvc/hvc_iucv.c
@@ -1035,6 +1035,10 @@ static const struct attribute_group *hvc_iucv_dev_attr_groups[] = {
 	NULL,
 };
 
+static void hvc_iucv_free(struct device *data)
+{
+	kfree(data);
+}
 
 /**
  * hvc_iucv_alloc() - Allocates a new struct hvc_iucv_private instance
@@ -1097,7 +1101,7 @@ static int __init hvc_iucv_alloc(int id, unsigned int is_console)
 	priv->dev->bus = &iucv_bus;
 	priv->dev->parent = iucv_root;
 	priv->dev->groups = hvc_iucv_dev_attr_groups;
-	priv->dev->release = (void (*)(struct device *)) kfree;
+	priv->dev->release = hvc_iucv_free;
 	rc = device_register(priv->dev);
 	if (rc) {
 		put_device(priv->dev);
-- 
2.39.2

Re: [PATCH] tty: hvc-iucv: fix function pointer casts

[Segher Boessenkool]

On Tue, Feb 13, 2024 at 11:17:49AM +0100, Arnd Bergmann wrote:
> clang warns about explicitly casting between incompatible function
> pointers:
> 
> drivers/tty/hvc/hvc_iucv.c:1100:23: error: cast from 'void (*)(const void *)' to 'void (*)(struct device *)' converts to incompatible function type [-Werror,-Wcast-function-type-strict]
>  1100 |         priv->dev->release = (void (*)(struct device *)) kfree;
>       |                              ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Such a cast of course is explicitly allowed by 6.3.2.3/8, only calling a
function using a non-compatible type is UB.  This warning message is
quite misleading.  Doubly so because of the -Werror, as always.

Your proposed new code of course is nice and simple (albeit a bit bigger
than it was before, both source and binary).  Such is life ;-)


Segher

RE: [PATCH] tty: hvc-iucv: fix function pointer casts

[David Laight]

From: Segher Boessenkool
> Sent: 13 February 2024 19:13
> 
> On Tue, Feb 13, 2024 at 11:17:49AM +0100, Arnd Bergmann wrote:
> > clang warns about explicitly casting between incompatible function
> > pointers:
> >
> > drivers/tty/hvc/hvc_iucv.c:1100:23: error: cast from 'void (*)(const void *)' to 'void (*)(struct
> device *)' converts to incompatible function type [-Werror,-Wcast-function-type-strict]
> >  1100 |         priv->dev->release = (void (*)(struct device *)) kfree;
> >       |                              ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> 
> Such a cast of course is explicitly allowed by 6.3.2.3/8, only calling a
> function using a non-compatible type is UB.  This warning message is
> quite misleading.  Doubly so because of the -Werror, as always.

But it will get called using the wrong type.
And (is it) fine-ibt will reject the incorrect call.

Has clang/gcc added an attribute to 'seed' the ibt hash yet?
So that functions that are void (*)(void) can be separated?

	David

-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)

Re: [PATCH] tty: hvc-iucv: fix function pointer casts

[Greg Kroah-Hartman]

On Wed, Feb 14, 2024 at 09:46:33AM +0000, David Laight wrote:
> From: Segher Boessenkool
> > Sent: 13 February 2024 19:13
> > 
> > On Tue, Feb 13, 2024 at 11:17:49AM +0100, Arnd Bergmann wrote:
> > > clang warns about explicitly casting between incompatible function
> > > pointers:
> > >
> > > drivers/tty/hvc/hvc_iucv.c:1100:23: error: cast from 'void (*)(const void *)' to 'void (*)(struct
> > device *)' converts to incompatible function type [-Werror,-Wcast-function-type-strict]
> > >  1100 |         priv->dev->release = (void (*)(struct device *)) kfree;
> > >       |                              ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> > 
> > Such a cast of course is explicitly allowed by 6.3.2.3/8, only calling a
> > function using a non-compatible type is UB.  This warning message is
> > quite misleading.  Doubly so because of the -Werror, as always.
> 
> But it will get called using the wrong type.
> And (is it) fine-ibt will reject the incorrect call.

And rightfully so, this type of casting abuse is just that, abuse.

Almost no one should be just calling kfree() on a device pointer, I'll
look at the surrounding code as odds are something odd is going on.  But
for now, this patch is correct.

thanks,

greg k-h

Re: [PATCH] tty: hvc-iucv: fix function pointer casts

[Segher Boessenkool]

On Wed, Feb 14, 2024 at 11:37:21AM +0100, Greg Kroah-Hartman wrote:
> On Wed, Feb 14, 2024 at 09:46:33AM +0000, David Laight wrote:
> > From: Segher Boessenkool
> > > Sent: 13 February 2024 19:13
> > > 
> > > On Tue, Feb 13, 2024 at 11:17:49AM +0100, Arnd Bergmann wrote:
> > > > clang warns about explicitly casting between incompatible function
> > > > pointers:
> > > >
> > > > drivers/tty/hvc/hvc_iucv.c:1100:23: error: cast from 'void (*)(const void *)' to 'void (*)(struct
> > > device *)' converts to incompatible function type [-Werror,-Wcast-function-type-strict]
> > > >  1100 |         priv->dev->release = (void (*)(struct device *)) kfree;
> > > >       |                              ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> > > 
> > > Such a cast of course is explicitly allowed by 6.3.2.3/8, only calling a
> > > function using a non-compatible type is UB.  This warning message is
> > > quite misleading.  Doubly so because of the -Werror, as always.
> > 
> > But it will get called using the wrong type.
> > And (is it) fine-ibt will reject the incorrect call.
> 
> And rightfully so, this type of casting abuse is just that, abuse.
> 
> Almost no one should be just calling kfree() on a device pointer, I'll
> look at the surrounding code as odds are something odd is going on.  But
> for now, this patch is correct.

Yes, and I said so.  My point is that the warning message pretends the
cast is bad or dangerous.  It is not, similar casts are the only way in
C to do certain things (yes, you can always avoid it completely, by
writing completely different code, like the patch does, and that
sometimes is a better idea even).

But the warning message is misleading and does more damage than it helps
avoid.


Segher

Re: [PATCH] tty: hvc-iucv: fix function pointer casts

[Jiri Slaby]

On 13. 02. 24, 11:17, Arnd Bergmann wrote:
> From: Arnd Bergmann <arnd@arndb.de>
> 
> clang warns about explicitly casting between incompatible function
> pointers:
> 
> drivers/tty/hvc/hvc_iucv.c:1100:23: error: cast from 'void (*)(const void *)' to 'void (*)(struct device *)' converts to incompatible function type [-Werror,-Wcast-function-type-strict]
>   1100 |         priv->dev->release = (void (*)(struct device *)) kfree;
>        |                              ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> 
> Add a separate function to handle this correctly.
> 
> Signed-off-by: Arnd Bergmann <arnd@arndb.de>

Reviewed-by: Jiri Slaby <jirislaby@kernel.org>

> diff --git a/drivers/tty/hvc/hvc_iucv.c b/drivers/tty/hvc/hvc_iucv.c
> index fdecc0d63731..b1149bc62ca1 100644
> --- a/drivers/tty/hvc/hvc_iucv.c
> +++ b/drivers/tty/hvc/hvc_iucv.c
> @@ -1035,6 +1035,10 @@ static const struct attribute_group *hvc_iucv_dev_attr_groups[] = {
>   	NULL,
>   };
>   
> +static void hvc_iucv_free(struct device *data)
> +{
> +	kfree(data);
> +}
>   
>   /**
>    * hvc_iucv_alloc() - Allocates a new struct hvc_iucv_private instance
> @@ -1097,7 +1101,7 @@ static int __init hvc_iucv_alloc(int id, unsigned int is_console)
>   	priv->dev->bus = &iucv_bus;
>   	priv->dev->parent = iucv_root;
>   	priv->dev->groups = hvc_iucv_dev_attr_groups;
> -	priv->dev->release = (void (*)(struct device *)) kfree;
> +	priv->dev->release = hvc_iucv_free;
>   	rc = device_register(priv->dev);
>   	if (rc) {
>   		put_device(priv->dev);

-- 
js
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