linux-kernel - Re: [RFC v2] x86-64: Allow emulated vsyscalls from user addresses

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Date:	Thu, 11 Aug 2011 09:27:20 -0400
From:	Andrew Lutomirski <luto@....edu>
To:	Pavel Machek <pavel@....cz>
Cc:	"H. Peter Anvin" <hpa@...or.com>, Andi Kleen <andi@...stfloor.org>,
	x86@...nel.org, linux-kernel@...r.kernel.org,
	torvalds@...ux-foundation.org, lueckintel@...oo.com,
	kimwooyoung@...il.com, Suresh Siddha <suresh.b.siddha@...el.com>
Subject: Re: [RFC v2] x86-64: Allow emulated vsyscalls from user addresses

On Thu, Aug 11, 2011 at 9:16 AM, Pavel Machek <pavel@....cz> wrote:
> Hi!
>
>> They trace control flow through the vsyscall page and recompile that
>> code somewhere else.  Then they expect it to work.  DynamoRIO
>> (http://dynamorio.org/) and Pin (http://www.pintool.org/) are
>> affected.  They crash when tracing programs that use vsyscalls.
>> Valgrind is smart enough not to cause problems.  It crashes on the
>> getcpu vsyscall, but that has nothing to do with emulation.
>>
>> This patch makes each of the three vsyscall entries use a different
>> vector so that they can work when relocated.  It assumes that the
>> code that relocates them is okay with the int instruction acting
>> like ret.  DynamoRIO at least appears to work.
>
> int acting as ret is seriously weird semantics. And no, invalid
> syscall parameters will not cause segfault, just return of -EFAULT. So
> ... can this be changed?

Can which be changed?  int acting as ret already was (in a different
patch, now in tip/x86/vdso), although I still think that user code
should do its best to make no assumptions about the vsyscall page.

invalid syscall parameters do indeed return -EFAULT, but invalid
*v*syscall parameters will segfault in 3.0 and before, since the
vsyscall implementation is just user code and has no exception
handling.

--Andy
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