linux-kernel - Re: [uml-devel] SYSCALL, ptrace and syscall restart breakages (Re: [RFC] weird crap with vdso on uml/i386)

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Date:	Tue, 23 Aug 2011 14:08:27 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	Al Viro <viro@...iv.linux.org.uk>,
	Andrew Lutomirski <luto@....edu>,
	Borislav Petkov <bp@...64.org>, Ingo Molnar <mingo@...nel.org>,
	"user-mode-linux-devel@...ts.sourceforge.net" 
	<user-mode-linux-devel@...ts.sourceforge.net>,
	Richard Weinberger <richard@....at>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"mingo@...hat.com" <mingo@...hat.com>
Subject: Re: [uml-devel] SYSCALL, ptrace and syscall restart breakages (Re:
 [RFC] weird crap with vdso on uml/i386)

On 08/23/2011 10:33 AM, Linus Torvalds wrote:
> 
> It would be *nice* if we did the swizzling automatically at setregs()
> time too, but we simply don't have enough information in the kernel to
> do that. Again, exactly because pt_regs doesn't have a "state"
> variable, when user-space does the SETREGS call, we simply don't know
> whether we are in "normal" code or in some system call entry or exit
> state. So the kernel does the swizzling at GETREGS time (by virtue of
> always having the registers in a "canonical" state for system call
> entry), but we fundamentally *cannot* to do the unswizzle, because we
> don't know what the SETREGS caller actually did.
> 

Again, can we steal one of the padding fields to use for that state
variable?  We have two 16-bit padding fields; one for cs and one for ss.

For UML, I agree, let's just not expose the vdso assuming that is
possible, but for other -- possibly future -- users.

	-hpa

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