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Date: Sun, 21 Aug 2011 18:19:41 -0700
From: "H. Peter Anvin" <hpa@zytor.com>
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To: Linus Torvalds <torvalds@linux-foundation.org>
CC: Andrew Lutomirski <luto@mit.edu>, Al Viro <viro@zeniv.linux.org.uk>,
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Subject: Re: SYSCALL, ptrace and syscall restart breakages (Re: [RFC] weird
 crap with vdso on uml/i386)
References: <20110820011845.GC2203@ZenIV.linux.org.uk> <4E4FD12F.70508@nod.at> <20110820201406.GF2203@ZenIV.linux.org.uk> <4E501F51.9060905@nod.at> <CAObL_7FoPHu6AKneswjEmiU4UBjUuKpYahakOaTNXM3=ZCpYaA@mail.gmail.com> <CAObL_7H8E1fde-d_Kfjrg2mQ5PdAp9X=fVRTqXJKzzYmQ1vAQQ@mail.gmail.com> <20110821063443.GH2203@ZenIV.linux.org.uk> <20110821084230.GI2203@ZenIV.linux.org.uk> <CAObL_7GXyzAzNjBtgye9BwXLyHm337oBTdjRDjbSPqoKZGWyyA@mail.gmail.com> <20110821144352.GJ2203@ZenIV.linux.org.uk> <20110821164124.GL2203@ZenIV.linux.org.uk> <CAObL_7E1vU8+0UQST6Smf9j40TckTzB=QdY0AuAWo4AX6cs2xg@mail.gmail.com> <CA+55aFztZ=r5wa0x26KJQxvZOaQq8s2v3u50wCyJcA-Sc4g8gQ@mail.gmail.com>
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On 08/21/2011 06:09 PM, Linus Torvalds wrote:
> 
> Indeed. Just add an "int 0x80" instruction to the vsyscall thing, and
> you'd be done.
> 
> In fact, just replace the
> 
>   jmp .Lenter_kernel
> 
> with
> 
>   int 0x80
> 
> and you'd be pretty much all done, no?
> 
> (Ok, that's probably a huge over-simplification, but perhaps "close
> enough" to true that it would be workable)
> 

Hm... I think a jump to something which adjusts %esp and invokes int
$0x80 might just work, but only for SYSENTER.

SYSCALL is different, especially since SYSCALL is legal to execute from
anywhere in userspace (and no, as we have learned already doing EIP
checking is *NOT* acceptable.)

	-hpa

-- 
H. Peter Anvin, Intel Open Source Technology Center
I work for Intel.  I don't speak on their behalf.