linux-kernel - Re: BUG: unable to handle kernel paging request from pty

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Date:	Thu, 25 Feb 2016 16:38:39 -0800
From:	Peter Hurley <peter@...leysoftware.com>
To:	Jiri Slaby <jslaby@...e.cz>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Greg KH <gregkh@...uxfoundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	stable <stable@...r.kernel.org>, lwn@....net,
	Steven Rostedt <rostedt@...dmis.org>
Subject: Re: BUG: unable to handle kernel paging request from pty_write [was:
 Linux 4.4.2]

On 02/25/2016 02:33 PM, Peter Hurley wrote:
> On 02/25/2016 01:32 PM, Jiri Slaby wrote:
>> On 02/25/2016, 09:51 PM, Linus Torvalds wrote:
>>> Jiri, can you check your try_to_wake_up() disassembly for some
>>> indirect "jmp" instructions?
>>
>> Nope, there is none.
>>
>> I will reply to all your questions tomorrow.
>>
>> Just quickly, as I have to go (and don't want you to duplicate efforts)
>> the kernel which was used can be obtained here:
>> https://build.opensuse.org/package/binaries/openSUSE:Factory:Staging:I/kernel-default?repository=standard
>>
>> The issue is very weird, indeed, this is what I noted to our bugzilla:
>> The stack trace ends in call of try_to_wake_up. Then, there it has to be
>> some of the indirect calls:
>>
>> callq  *0x40(%rax)
>>   p->sched_class->select_task_rq from select_task_rq
>>
>> RAX is 0x00000000bb37e180, barely can be read with offset 0x40
>>
>> callq  *0xd85656(%rip) # ffffffff81e2aba0 <smp_ops+0x20>
>>   smp_ops.smp_send_reschedule from ttwu_queue_remote
>>
>> Which hardly can be it, given smp_ops is static.
>>
>> So it has to be some other "call *" from a nested function :(.
>>
>>
>>
>>
>> Interestingly, RBP contains address inside try_to_wake_up --
>> ffffffff810a535a (dunno why) which is:
>> ffffffff810a5355:       e8 66 a0 ff ff          callq  ffffffff8109f3c0
>> <ttwu_stat>
>> ffffffff810a535a:       e9 9d fe ff ff          jmpq   ffffffff810a51fc
>> <try_to_wake_up+0x3c>
> 
> That would imply that RSP was off by +8 when the ttwu_stat() epilog was
> executed so that RBP <= ret addr and RIP <= some local var in try_to_wake_up()
> stack frame.
> 
> Looks like R15 in the crash report could be what RBP should have been.
> 
> Now to find out why RSP is +8

Which I would investigate if I could download that kernel.
Unfortunately, OBS doesn't like me so if you could make that
kernel available some other way or send me a mixed listing
of kernel/sched/core.c


>> ttwu_stat does in the begginning:
>> mov    $0x16e80,%r14
>>
>> which is what we actually still have in r14 when it crashes. The first
>> ttwu_stat's "if" has to go through the true branch (otherwise r14 would
>> be overwritten).
>>
>>
>>
>> Another note: we die when jmp/calling to 0xffff88023fd40000.
>> RSI=RDI=0xffff88023fdd6e80. RSI-RIP is 0x96e80, which is R14 + 0x80000.
>> Coincidence?
>>
>> thanks,
>>
>