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

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Message-ID: <CA+55aFwoRO9xxnTyt_gYFdTS9LsaFoCWRuyNeXY7gvrE8CdZ0A@mail.gmail.com>
Date:	Mon, 22 Aug 2011 13:05:34 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Al Viro <viro@...iv.linux.org.uk>
Cc:	Borislav Petkov <bp@...64.org>, Andrew Lutomirski <luto@....edu>,
	Ingo Molnar <mingo@...nel.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	"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 Mon, Aug 22, 2011 at 8:13 AM, Al Viro <viro@...iv.linux.org.uk> wrote:
>
> In __kernel_vsyscall() the problem is possible to deal with; there we control
> the code around that sucker.  It's SYSCALL in 32bit binary outside of
> vdso32 that causes real PITA...

I just checked. 'syscall' (at least on x86-64) is definitely called
outside of __kernel_vsyscall in all the normal cases. It's part of the
fundamental ABI, after all. We don't use "int 0x80" there.

But on x86-32, I think we might be better off. There, we only have
'sysenter', and can perhaps use my suggested "just use int 0x80
instead of the jump back to the sysenter instruction" trick. Plus
people *will* be using __kernel_vsyscall, since on x86-32 you aren't
guaranteed to have a CPU that supports sysenter to begin with.

Or am I missing something else?

                    Linus
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