Re: [PATCH] linux-2.5.43_vsyscall_A0

[Elladan]

On Tue, Oct 22, 2002 at 07:27:17AM +0200, Andrea Arcangeli wrote:
> On Tue, Oct 22, 2002 at 12:07:16AM -0500, Jeff Dike wrote:
> 
> > > My problem is that mapping user code into the vsyscall fixmap is
> > > complex and not very clean at all, breaks various concepts in the mm
> > > and last but not the least it is slow
> > 
> > Can you explain, in small words, why mapping user code is so horrible?
> 
> Mapping user code above PAGE_OFFSET is messy yes. We could
> use get_user_pages, pin the page set the fixmap to point to it, set the
> old vsyscall code in some other kernel space, tell userspace the new
> location of the native vsyscalls, keep track of the whole thing in the
> task struct and switch both old vsyscall and new vsyscall during
> switch_to, and cleanup the mess when the task exits tell us nothing.
> 
> Actually the second copy of the vsyscalls could stay mapped all the
> time, we only return to userspace its fixed address so we don't waste
> address space. Plus it would be simpler and cleaner to have the vsyscall
> calling into an user specified address, that sounds a much more usable
> API too infact, you pass an pointer to function rather than a user page
> address to remap, it would be more handy from your part too so you don't
> need to build a magic vsyscall page to remap but you only care about the
> callee.
> 
> So with these new ideas (to keep the second copy constantly mapped above
> the last -ENOSYS and to have userspace specifying the address to call)
> it sounds much simpler than the idea of mapping user code in kernel
> space and not much more complicated than just redirecting the vsyscalls
> to kernel.

This seems somewhat painful all-around, since if I'm reading this right,
you take a switch_to hit to find out whether the user redirected the
vsyscall, and a vsyscall branch hit as well.

A switch_to seems like something to avoid, since it slows down the
system at all times, even when vgettimeofday is not being used very
often.  Does the average system call gettimeofday() more than once per
context switch?  If not, don't change switch_to...

If you don't mind a somewhat nasty looking tactic, another way you could
implement something like this and give a boost to virtualizing programs
would be to do a special case in the syscall_trace handler for
gettimeofday.  

Just do the global flag test in the vgettimeofday code, and when it does
the fallback system call, if the process is being traced, we check to
see if the control process requested some special handling for the
syscall (obviously very easy to do in kernel mode).  If so, do a special
fixup which, say, returns execution back to user space in a
user-specified location without notifying the tracing task of the system
call event.

This way, the main system just sees vsyscalls degrade to normal system
calls, which is ok, and programs that want to virtualize like UML get to
bounce execution into some special user-specified vsyscall code of their
own, with the cost being just one system call transition for UML as
well - a big speedup.

This sort of tactic might be interesting in general for a virtualizing
program, since you could bounce many of the system calls in the traced
process into user-specified code pages (especially if security in the
virtualized program isn't too big a concern).

It also would have the advantage of only mangling the trace path in the
kernel, which isn't the most performance-critical one around, without
overly complicating the vsyscalls in the average case.

Of course, it's kind of... ugly.

-J