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From: Kees Cook <keescook@chromium.org>
Date: Thu, 20 Jul 2017 22:48:01 -0700
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Subject: Re: [lkp-robot] [include/linux/string.h] 6974f0c455: kernel_BUG_at_lib/string.c
To: Ye Xiaolong <xiaolong.ye@intel.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
        Ananth N Mavinakayanahalli <ananth@linux.vnet.ibm.com>,
        Anil S Keshavamurthy <anil.s.keshavamurthy@intel.com>,
        Masami Hiramatsu <mhiramat@kernel.org>,
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        Mark Rutland <mark.rutland@arm.com>, Daniel Axtens <dja@axtens.net>,
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        Chris Metcalf <cmetcalf@ezchip.com>,
        Thomas Gleixner <tglx@linutronix.de>, "H. Peter Anvin" <hpa@zytor.com>,
        Ingo Molnar <mingo@elte.hu>, Andrew Morton <akpm@linux-foundation.org>,
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On Thu, Jul 20, 2017 at 6:59 PM, Ye Xiaolong <xiaolong.ye@intel.com> wrote:
> On 07/19, Linus Torvalds wrote:
>>Hmm. I wonder why the kernel test robot ends up having that annoying
>>line doubling for the dmesg.
>>
>
> Hmm, this line doubling issue should be caused by we set both
> 'earlyprintk=ttyS0,115200' and 'console=ttyS0,115200' in cmdline, after I
> remove any of it, this issue is gone, is it an inappropriate setting?

This is a weird behavior. Normally the boot console (in this case
"earlyser0") should be disabled when the regular console starts. In
your dmesg I see:

[    0.000000] bootconsole [earlyser0] enabled
...
[    0.003333] console [ttyS0] enabled
[    0.003333] console [ttyS0] enabled

This is where repeating starts, and I would have expected to see:

[    0.000000] bootconsole [earlyser0] enabled
...
[    0.000000] console [ttyS0] enabled
[    0.000000] console [ttyS0] enabled
[    0.000000] bootconsole [earlyser0] disabled
[    0.000000] bootconsole [earlyser0] disabled

And that would be the end of doubling.

Actually, with your command-line ("earlyprintk=ttyS0,115200
console=ttyS0,115200 console=tty0") I would expect:

[    0.000000] console [tty0] enabled
[    0.000000] bootconsole [earlyser0] disabled
[    0.000000] console [ttyS0] enabled

I don't see any mention of tty0, though. I wonder if that is confusing
the disable code. The only way I would normally expect the boot
console not to get unregistered is if ",keep" was appended to it...
The code is in kernel/printk/printk.c, FWIW.

Also, it looks like your kernel command line is cut off:

... earlyprintk=ttyS0,115200 console=ttyS0,115200 console=tty0 vga=

With your script showing "... vga=normal ..."

That's only 968 characters, well shy of x86's 2048 limit. Is this a
line length issue with qemu output, or does /proc/cmdline report the
same truncation?

-Kees

-- 
Kees Cook
Pixel Security