linux-kernel - Re: [RFC] vsprintf: compile error on %09pK

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Message-ID: <ZCrMMQt2xnnZIyz6@alley>
Date:   Mon, 3 Apr 2023 14:53:05 +0200
From:   Petr Mladek <pmladek@...e.com>
To:     Jaewon Kim <jaewon31.kim@...sung.com>
Cc:     "senozhatsky@...omium.org" <senozhatsky@...omium.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "jaewon31.kim@...il.com" <jaewon31.kim@...il.com>,
        GyeongHwan Hong <gh21.hong@...sung.com>
Subject: Re: [RFC] vsprintf: compile error on %09pK

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?

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 whether I
> did wrong or it is a printk problem.
> 
> To show easily I tried to add pr_info("%09pK\n", nodemask); in page_alloc.c
> Then here's what I did.
> 
> $ ARCH=x86 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 ‘__alloc_pages’:
> ./include/linux/kern_levels.h:5:18: error: '0' flag used with ‘%p’ gnu_printf format [-Werror=format=]
>  #define KERN_SOH "\001"  /* ASCII Start Of Header */

As Sergey already wrote. %p does not support any modification flags.

Best Regards,
Petr