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

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Date:   Tue, 16 Mar 2021 10:33:20 -0500
From:   Segher Boessenkool <segher@...nel.crashing.org>
To:     Christophe Leroy <christophe.leroy@...roup.eu>
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

On Tue, Mar 16, 2021 at 09:32:32AM +0100, Christophe Leroy wrote:
> +segher

I cannot see through the wood of #defines here, sorry.

> Still a problem.
> 
> I don't understand, gcc bug ?

Rule #1: If you do not understand what is happening, it is not a
compiler bug.  I'm not saying that it isn't, just that it is much more
likely something else.

> The offending argument is 'const ptrdiff_t object_index'
> 
> We have:
> 
> arch/powerpc/include/uapi/asm/posix_types.h:typedef long	 
> __kernel_ptrdiff_t;

So this is a 64-bit build.

> 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?


Segher