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

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Date:	Thu, 25 Feb 2016 22:32:26 +0100
From:	Jiri Slaby <jslaby@...e.cz>
To:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Peter Hurley <peter@...leysoftware.com>
Cc:	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, 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>

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,
-- 
js
suse labs