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

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Date:	Thu, 25 Feb 2016 16:38:13 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Jiri Slaby <jslaby@...e.cz>
Cc:	Peter Hurley <peter@...leysoftware.com>,
	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 Thu, Feb 25, 2016 at 1:32 PM, Jiri Slaby <jslaby@...e.cz> wrote:
>
> 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).

Hmm. That does sound very much like it might be ttwu_stat() that has
gotten the stack frame wrong, and when finishes exits, it does

        popq    %rbp
        ret

but in fact it popped the return address, and then returned to a crazy address.

Which sounds like a corrupted stack pointer (not a corrupted stack).

Can you make just the "vmlinux" file available somewhere?

In my own private configuration, ttwu_stat() doesn't actually touch
the stack at all - no stack pointer action anywhere except for the

ttwu_stat:
1:      call    __fentry__
        pushq   %rbp
   ..
        movq    %rsp, %rbp      #,

 .....

        popq    %rbp
        ret

but yeah, as Peter says, maybe an exception screwed up %rsp somehow..

I really don't see how it would happen here - that code doesn't look
particularly odd.

And the fentry code used by the function tracer can certainly screw
things up, but even that would be hard-pressed to screw up %rbp, since
the saving of rbp comes *after* fentry. Old pre-__fentry__ gcc
versions had a much higher likelihood (the whole mcount thing is a
disaster, but I'm assuming you have a compiler that does __fentry__
and have CC_USING_FENTRY set?)

               Linus