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

[Andrew Lutomirski]

On Tue, Jun 7, 2011 at 7:32 PM,  <pageexec@xxxxxxxxxxx> 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
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