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

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Date: Wed, 14 Feb 2024 06:36:24 -0600
From: Segher Boessenkool <segher@...nel.crashing.org>
To: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc: David Laight <David.Laight@...lab.com>, Arnd Bergmann <arnd@...nel.org>,
        "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
        Arnd Bergmann <arnd@...db.de>,
        "llvm@...ts.linux.dev" <llvm@...ts.linux.dev>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Nathan Chancellor <nathan@...nel.org>,
        Bill Wendling <morbo@...gle.com>,
        "linux-serial@...r.kernel.org" <linux-serial@...r.kernel.org>,
        Justin Stitt <justinstitt@...gle.com>,
        Jiri Slaby <jirislaby@...nel.org>
Subject: Re: [PATCH] tty: hvc-iucv: fix function pointer casts

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