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Date: Wed, 15 Mar 2006 01:09:35 -0800
From: Chris Wright <chrisw@sous-sol.org>
To: Zachary Amsden <zach@vmware.com>
Cc: Gerd Hoffmann <kraxel@suse.de>, Chris Wright <chrisw@sous-sol.org>,
       Linus Torvalds <torvalds@osdl.org>,
       Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
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       Leendert van Doorn <leendert@watson.ibm.com>
Subject: Re: [RFC, PATCH 7/24] i386 Vmi memory hole
Message-ID: <20060315090935.GS12807@sorel.sous-sol.org>
References: <200603131804.k2DI4N6s005678@zach-dev.vmware.com> <20060314064107.GK12807@sorel.sous-sol.org> <44166D6B.4090701@vmware.com> <20060314215616.GM12807@sorel.sous-sol.org> <4417454F.2080908@vmware.com> <20060315043108.GP12807@sorel.sous-sol.org> <4417CFDA.1060806@suse.de> <4417D212.20401@vmware.com>
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* Zachary Amsden (zach@vmware.com) wrote:
> ENTRY(sysenter_entry)
>        movl TSS_sysenter_esp0(%esp),%esp
> sysenter_past_esp:
>        STI
>        pushl $(__USER_DS)
>        pushl %ebp
>        pushfl
>        pushl $(__USER_CS)
>        pushl $SYSENTER_RETURN
> 
> SYSENTER_RETURN is a link time constant that is defined based on the 
> location of the vsyscall page.  If the vsyscall page can move, this can 
> not be a constant.  The reason is, this "fake" exception frame is used 
> to return back to the EIP of the call site, and sysenter does not record 
> the EIP of the call site.

It's only real issue for something like execshield.  For this it's easy
to do the fixed math since it's still at fixed address.

+       DEFINE(VSYSCALL_BASE, (PAGE_OFFSET - 2*PAGE_SIZE));

But execshield has to make SYSENTER_RETURN context sensitive to current
since the vdso is mapped at random location.

thanks,
-chris
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