Re: [PATCH mm] kfence: fix printk format for ptrdiff_t

[Christophe Leroy]

Le 18/03/2021 à 10:14, David Laight a écrit :
> 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.

Yes it seems to be wrong. It was changed by commit d27dfd3887 ("Import pre2.0.8"), so that's long 
time ago. Before that it was an 'int' for ppc32.

gcc provides ptrdiff_t in stddef.h via __PTRDIFF_TYPE__
gcc defined __PTRDIFF_TYPE__ as 'int' at build time.

Should we fix it in arch/powerpc/include/uapi/asm/posix_types.h ? Anyway 'long' and 'int' makes no 
functionnal difference on 32 bits so there should be no impact for users if any.

Christophe