linux-kernel - Re: New vsyscall emulation breaks JITs

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Message-ID: <20110805204740.GJ5782@one.firstfloor.org>
Date:	Fri, 5 Aug 2011 22:47:40 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Andi Kleen <andi@...stfloor.org>, luto@....edu, x86@...nel.org,
	linux-kernel@...r.kernel.org, torvalds@...ux-foundation.org,
	lueckintel@...oo.com, kimwooyoung@...il.com
Subject: Re: New vsyscall emulation breaks JITs

On Fri, Aug 05, 2011 at 01:36:48PM -0700, H. Peter Anvin wrote:
> On 08/05/2011 01:26 PM, Andi Kleen wrote:
> >> I have to say I believe that trying to JIT the vdso or vsyscall pages is
> >> extremely dubious at best.  They are fundamentally different from normal
> >> user space in that the kernel can muck with them any time, without
> >> notifying userspace about it.  The other aspect of this is that this is
> >> about the legacy vsyscall page, which we're trying to get rid of, partly
> >> because of security problems.
> > 
> > There's clear evidence now you can't: it's used even by new binaries.
> 
> time() is not supported by vdso; this is a problem.  Getting rid of it
> is a long-term thing.

Yes you're right the problem is time. I set a breakpoint on the vsyscalls
and I get:

(gdb) bt
#0  0xffffffffff600400 in ?? ()
#1  0x00007fffe4f703fd in time () at ../sysdeps/unix/sysv/linux/x86_64/time.S:36
...

This is with a very new glibc, in fact a recent git version.

So clearly it's all broken and even outside JITs everyone using time()
will be slow and if they use JITs don't work at all.

-Andi

-- 
ak@...ux.intel.com -- Speaking for myself only.
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