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

[Christophe Leroy]

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 ?

Christophe