linux-kernel - Re: [RFC] fixing the UML failure root cause

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Date:	Thu, 13 Oct 2011 23:30:00 -0700
From:	Andrew Lutomirski <luto@....edu>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Ingo Molnar <mingo@...e.hu>,
	richard -rw- weinberger <richard.weinberger@...il.com>,
	Adrian Bunk <bunk@...sta.de>,
	"H. Peter Anvin" <hpa@...ux.intel.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, x86@...nel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC] fixing the UML failure root cause

On Thu, Oct 13, 2011 at 9:46 PM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
> On Thu, Oct 13, 2011 at 8:40 PM, Andrew Lutomirski <luto@....edu> wrote:
>>
>> How does that work?  The tricky case is when one of those three words
>> spans a page boundary if the access to the first page is valid, but
>> the access to the second page is not.  When that happens, if we report
>> the fault as coming from the first page, then UML is likely to get
>> think the fault was spurious and enter an infinite loop.
>
> Hmm. Gaah, I just find that memcpy loop disgusting.
>

Yeah, it's not pretty.

> We already have that ugly "uaccess_error" crap in handle_exception(),
> we might as well do something like the attached and just say "hey, now
> you can catch the page fault information for a get_user/put_user
> fault".
>
> Isn't that much nicer?

I actually tried this.  To really get it right, though, I also need to
either hook the access_ok failure paths (either every single one or
just the ones that matter for those three syscalls, which could be
fragile) or to check access_ok separately in the vsyscall emulation
code.  This also takes up 16 bytes of stack just to support a corner
case of a legacy code path.

Another idea is to have a flag that asks the fault handlers to call
force_sig_info for us.  That's just one bit of per-thread state.  Then
the vsyscall emulation code could check access_ok, force a signal if
access is not ok, then set the flag and do the syscall.  And maybe
some processes would want to opt in to that mode anyway -- arguably
EFAULT is a serious programmer error and should be dealt with more
harshly than other syscall misuses.

Admittedly, UML probably doesn't care about recovering vgettimeofday
pointed at kernel space...

--Andy
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