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: <20110823000314.GW2203@ZenIV.linux.org.uk>
Date:	Tue, 23 Aug 2011 01:03:14 +0100
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	"H. Peter Anvin" <hpa@...or.com>, 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 Mon, Aug 22, 2011 at 04:27:51PM -0700, Linus Torvalds wrote:

> So I think the "let's fix the vdso case for sysenter" + "let's remove
> the 32-bit syscall vdso" is the right solution. If somebody has
> hardcoded syscall instructions, or generates them dynamically with
> some JIT, that's their problem. We'll continue to support it as well
> as we ever have (read: "almost nobody will ever notice").

Umm...  Maybe, but I really wonder if it would be better to do this:
	* check if %ecx is the right one for vdso32 entry.  If it isn't,
act as we act now (and possibly warn).  If it is, increment it by 4.
	* slap 0x90, 0x90, 0xcd, 0x80 right after that syscall insn -
i.e. nop/nop/int 0x80.  Followed by what we currently do there.

	Those who do syscall insn in 32bit mode outside of vdso will get
what they currently get.  __kernel_vsyscall() will keep working and do
that without restart problems.  And the price is comparison + branch not
taken + addition for that path...
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