linux-kernel - RE: [PATCH mm] kfence: fix printk format for ptrdiff

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Message-ID: <4f7becfe2b6e4263be83b5ee461b5732@AcuMS.aculab.com>
Date:   Thu, 18 Mar 2021 09:14:27 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Christophe Leroy' <christophe.leroy@...roup.eu>,
        Segher Boessenkool <segher@...nel.crashing.org>
CC:     Alexander Potapenko <glider@...gle.com>,
        Marco Elver <elver@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Dmitriy Vyukov <dvyukov@...gle.com>,
        Andrey Konovalov <andreyknvl@...gle.com>,
        Jann Horn <jannh@...gle.com>,
        LKML <linux-kernel@...r.kernel.org>,
        "Linux Memory Management List" <linux-mm@...ck.org>,
        kasan-dev <kasan-dev@...glegroups.com>
Subject: RE: [PATCH mm] kfence: fix printk format for ptrdiff_t

From: Christophe Leroy
> Sent: 17 March 2021 17:35
> 
> Le 17/03/2021 à 13:51, David Laight a écrit :
> > From: Christophe Leroy
> >> Sent: 16 March 2021 15:41
> > ...
> >>>> include/linux/types.h:typedef __kernel_ptrdiff_t	ptrdiff_t;
> >>>>
> >>>> And get:
> >>>>
> >>>>     CC      mm/kfence/report.o
> >>>> In file included from ./include/linux/printk.h:7,
> >>>>                    from ./include/linux/kernel.h:16,
> >>>>                    from mm/kfence/report.c:10:
> >>>> mm/kfence/report.c: In function 'kfence_report_error':
> >>>> ./include/linux/kern_levels.h:5:18: warning: format '%td' expects argument
> >>>> of type 'ptrdiff_t', but argument 6 has type 'long int' [-Wformat=]
> >>>
> >>> This is declared as
> >>>           const ptrdiff_t object_index = meta ? meta - kfence_metadata : -1;
> >>> so maybe something with that goes wrong?  What happens if you delete the
> >>> (useless) "const" here?
> >
> > The obvious thing to try is changing it to 'int'.
> > That will break 64bit builds, but if it fixes the 32bit one
> > it will tell you what type gcc is expecting.
> >
> 
> Yes, if defining 'object_index' as int, gcc is happy.
> If removing the powerpc re-definition of ptrdiff_t typedef in
> https://elixir.bootlin.com/linux/v5.12-rc3/source/arch/powerpc/include/uapi/asm/posix_types.h , it
> works great as well.
> 
> So seems like gcc doesn't take into account the typedef behind ptrdiff_t, it just expects it to be
> int on 32 bits ?

gcc never cares how ptrdiff_t (or any of the related types) is defined
it requires int or long for the format depending on the architecture.
The error message will say ptrdiff_t or size_t (etc) - but that is just
in the error message.

So the ppc32 uapi definition of __kernel_ptrdiff_t is wrong.
However it is probably set in stone.

	David

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