Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
MIME-Version: 1.0
References: <CGME20230403104617epcms1p383bacbca705c0d7e4fffca6833050e42@epcms1p3>
 <20230403104617epcms1p383bacbca705c0d7e4fffca6833050e42@epcms1p3> <ZCrMMQt2xnnZIyz6@alley>
In-Reply-To: <ZCrMMQt2xnnZIyz6@alley>
From:   Jaewon Kim <jaewon31.kim@gmail.com>
Date:   Mon, 3 Apr 2023 23:39:58 +0900
Message-ID: <CAJrd-Uvzy18SfjjqbqZCPHjw+ujQXsBpvov4b=bfAwZrj=bCAA@mail.gmail.com>
Subject: Re: [RFC] vsprintf: compile error on %09pK
To:     Petr Mladek <pmladek@suse.com>
Cc:     Jaewon Kim <jaewon31.kim@samsung.com>,
        "senozhatsky@chromium.org" <senozhatsky@chromium.org>,
        "linux-mm@kvack.org" <linux-mm@kvack.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        GyeongHwan Hong <gh21.hong@samsung.com>
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable
Precedence: bulk

2023=EB=85=84 4=EC=9B=94 3=EC=9D=BC (=EC=9B=94) =EC=98=A4=ED=9B=84 9:53, Pe=
tr Mladek <pmladek@suse.com>=EB=8B=98=EC=9D=B4 =EC=9E=91=EC=84=B1:
>
> On Mon 2023-04-03 19:46:17, Jaewon Kim wrote:
> > Hello
> >
> > I've just changed %09lx to %09pK on my driver code to hide the address,=
 but I
> > faced compiler error. The %9pK without 0 worked.
>
> What exactly do you want to achieve, please?

Hello

Thank you for your comment.

I wanted to print phys_addr_t  type value only when kptr_restrict sysctl is
allowed. So I thought I could use %pK for that purpose. And the physical
address is not that long. I wanted to make that length short like 9 hex.

>
> Note that printk() hashes pointers by default. It means that %p does not
> print the value but a hash based on the value.
>
> If you print the same pointer twice, you will see the same hash, so
> you know that the pointer is the same. But you do not see the address
> so that you could not use the value for a security attack.
>
> See Documentation/core-api/printk-formats.rst
>
> Anyway, the main question if it makes sense to print the pointer value
> at all. The address is not useful if it can't be compared with
> other pointers or if the data on the address could not be checked.
>
> > Is there restriction on %pK which does now allow %0 ? I've wondered whe=
ther I
> > did wrong or it is a printk problem.
> >
> > To show easily I tried to add pr_info("%09pK\n", nodemask); in page_all=
oc.c
> > Then here's what I did.
> >
> > $ ARCH=3Dx86 make x86_64_defconfig ; make mm/page_alloc.o
> > #
> > # No change to .config
> > #
> >   CALL    scripts/checksyscalls.sh
> >   DESCEND objtool
> >   INSTALL libsubcmd_headers
> >   CC      mm/page_alloc.o
> > In file included from ./include/asm-generic/bug.h:22:0,
> >                  from ./arch/x86/include/asm/bug.h:87,
> >                  from ./include/linux/bug.h:5,
> >                  from ./include/linux/mmdebug.h:5,
> >                  from ./include/linux/mm.h:6,
> >                  from mm/page_alloc.c:19:
> > mm/page_alloc.c: In function =E2=80=98__alloc_pages=E2=80=99:
> > ./include/linux/kern_levels.h:5:18: error: '0' flag used with =E2=80=98=
%p=E2=80=99 gnu_printf format [-Werror=3Dformat=3D]
> >  #define KERN_SOH "\001"  /* ASCII Start Of Header */
>
> As Sergey already wrote. %p does not support any modification flags.

Okay, then we can't use %09pK. I've just wondered because %9pK works.

BR
Jaewon Kim

>
> Best Regards,
> Petr