Re: [PATCH v5 8/9] x86-64: Emulate legacy vsyscalls

From: Andrew Lutomirski <luto@mit.edu>
To: pageexec@freemail.hu
Cc: Brian Gerst <brgerst@gmail.com>, Ingo Molnar <mingo@elte.hu>,
	x86@kernel.org, Thomas Gleixner <tglx@linutronix.de>,
	linux-kernel@vger.kernel.org, Jesper Juhl <jj@chaosbits.net>,
	Borislav Petkov <bp@alien8.de>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Arjan van de Ven <arjan@infradead.org>,
	Jan Beulich <JBeulich@novell.com>,
	richard -rw- weinberger <richard.weinberger@gmail.com>,
	Mikael Pettersson <mikpe@it.uu.se>,
	Andi Kleen <andi@firstfloor.org>,
	Louis Rilling <Louis.Rilling@kerlabs.com>,
	Valdis.Kletnieks@vt.edu
Subject: Re: [PATCH v5 8/9] x86-64: Emulate legacy vsyscalls
Date: Tue, 7 Jun 2011 19:49:02 -0400	[thread overview]
Message-ID: <BANLkTimy2AkWxPAOj3JTzXEDak_othZhZA@mail.gmail.com> (raw)
In-Reply-To: <4DEEB516.4681.19EE05CD@pageexec.freemail.hu>

On Tue, Jun 7, 2011 at 7:32 PM,  <pageexec@freemail.hu> wrote:
>> > do you know what that mucking looks like? ;) prepare for the most complex code
>> > you've ever seen (it's in __bad_area_nosemaphore):
>> >
>> >  779 #ifdef CONFIG_X86_64
>> >  780 »·······if (mm && (error_code & PF_INSTR) && mm->context.vdso) {
>> >  781 »·······»·······if (regs->ip == (unsigned long)vgettimeofday) {
>> >  782 »·······»·······»·······regs->ip = (unsigned long)VDSO64_SYMBOL(mm->context.vdso, gettimeofday);
>> >  783 »·······»·······»·······return;
>> >  784 »·······»·······} else if (regs->ip == (unsigned long)vtime) {
>> >  785 »·······»·······»·······regs->ip = (unsigned long)VDSO64_SYMBOL(mm->context.vdso, clock_gettime);
>> >  786 »·······»·······»·······return;
>> >  787 »·······»·······} else if (regs->ip == (unsigned long)vgetcpu) {
>> >  788 »·······»·······»·······regs->ip = (unsigned long)VDSO64_SYMBOL(mm->context.vdso, getcpu);
>> >  789 »·······»·······»·······return;
>> >  790 »·······»·······}
>> >  791 »·······}
>> >  792 #endif
>>
>> I like this approach, however since we're already in the kernel it
>> makes sense just to run the normal syscall instead of redirecting to
>> the vdso.
>
> it's not that simple as the page fault occurs not at the actual syscall
> insn but at the first insn of the given vsyscall function, so you'd have
> to emulate it carefully to be able to return back the original caller in
> userland.
>
>

My patch (the version that's in tip/x86/vdso) more-or-less does that.
It's something like six lines of code, including error handling.

__bad_area_nosemaphore is not a fast path, and in fact I tried that
for the very first version of this code that I wrote.  My recollection
is that it's noticeably slower than int 0xcc because it has to go
through the whole VMA lookup.

If you want to submit a patch to switch from int 0xcc to
__bad_area_nosemaphore, be my guest :)  It will have almost no effect
on the complexity of the code, and, in fact, you'll probably get a net
deletion of lines because you can remove all the crud in vmlinux.lds.S
and some of the mapping code in vsyscall_64.c.

--Andy