linux-kernel - Re: Dcache oops

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Message-ID: <20160603212652.GT14480@ZenIV.linux.org.uk>
Date:	Fri, 3 Jun 2016 22:26:52 +0100
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Oleg Drokin <green@...uxhacker.ru>,
	"<linux-kernel@...r.kernel.org> Mailing List" 
	<linux-kernel@...r.kernel.org>,
	"<linux-fsdevel@...r.kernel.org>" <linux-fsdevel@...r.kernel.org>
Subject: Re: Dcache oops

On Fri, Jun 03, 2016 at 02:18:15PM -0700, Linus Torvalds wrote:

> So something must have corrupted the qstr.
> 
> The remaining length *should* in %edi, judging by the
> 
>    0xffffffff81243b82 <+306>: cmp    $0x7,%edi
> 
> in the __d_lookup() disassembly. And %rdi contains 2, so there were
> supposed to be two more characters at 'ct' (which is %rdx).

... and since r8 and rsi are 0, we couldn't have consumed anything.
> 
> Why would nd->last.name be bogus? I don't see anything.

An interesting part is that it's page-aligned.  Which is impossible for
a short name obtained by getname(), but is quite likely for a symlink body.
So at a guess, we have a page containing a symlink body freed under us.