linux-kernel - Re: WARNING in switch_fpu

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Date:   Wed, 8 Jan 2020 09:55:32 +0100
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Dmitry Vyukov <dvyukov@...gle.com>
Cc:     syzbot <syzbot+f2ca20d4aa1408b0385a@...kaller.appspotmail.com>,
        alexander.deucher@....com, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...el.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...hat.com>, nicholas.kazlauskas@....com,
        Rik van Riel <riel@...riel.com>, sunpeng.li@....com,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        the arch/x86 maintainers <x86@...nel.org>, zhan.liu@....com
Subject: Re: WARNING in switch_fpu_return

Hi Dmitry,

On 2020-01-08 05:28:31 [+0100], Dmitry Vyukov wrote:
> > > userspace arch: i386
> >
> > So I tried to reproduce this. syz-prog2c made .c out of the above link.
> > It starts with:
> > |int main(void)
> > | {
> > |   syscall(__NR_mmap, 0x20000000ul, 0x1000000ul, 3ul, 0x32ul, -1, 0);
> 
> Hi Sebastian,
> 
> If you want to generate a C repro for 386 arch, you need to add
> -arch=386 flag to syz-prog2c (then it hopefully should use mmap2).

Ah okay. I've been looking at
	https://github.com/google/syzkaller/blob/master/docs/syzbot.md#crash-does-not-reproduce

and it says
|Note: if the report contains userspace arch: i386, then the program
|needs to be built with -m32 flag.

and with the argument you mentioned it the compiled C code uses mmap2.
Thanks.
Now the 32bit testcase reboots, too :)

> But FWIW syzbot wasn't able to reproduce it with a C program,
> otherwise it would have been provided it. But that may be for various
> reasons.

Yeah, my memory was also that a C-testcase is provided. But there was this
	https://syzkaller.appspot.com/x/repro.syz?x=10cc8971e00000

link so I assumed I should use it myself and I missed the update that
something changed.
So what should I do with the file above? Feed it to `syz-execprog' or is
it a rough idea what the test case should have done?

Sebastian