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

Al Viro
2011-08-23T17:33:47Z

On Tue, Aug 23, 2011 at 09:20:12AM -0700, Linus Torvalds wrote:

> It's EMULATING A SYSTEM CALL. That original "getregs" value is not
> some "user space state". It's the *system call* state that you got
> after the system call trapped. Setting it back is an insane operation,
> but it would happen to work - if you make no changes.
>
> But UML *does* make changes. It takes that system call state, and then
> EMULATES THE SYSTEM CALL INCORRECTLY.
>
> If you see it that way (which is the correct way), then it's clearly
> an UML problem, and it's not at all "madness" that your
> getregs/setregs pairing doesn't work.
>
> See? Buggy system call emulation. It's really that simple. Of course,
> "simple" in this case is "really really subtle differences in how the
> kernel treats syscall/sysenter/int80", so the *details* are certainly
> not simple, but the concept is.

It's a bit more than that (ptrace changes to syscall arguments *are*
lost on syscall restart), but... as far as I'm concerned, the situation
is simple now:
* SYSCALL is not terminally broken wrt restarts. My apologies for
misreading what was going on.
* SYSENTER with Linus' patch does work just fine wrt restarts + ptrace
* SYSCALL is losing ptrace-made changes to arguments when it restarts.
Might or might not be a problem for somebody.
* UML should not touch SYSCALL for 32bit. Not without serious changes
in UML and I'm not convinced that it won't be worse than what we probably
ought to do there: check if __kernel_vsyscall() does SYSCALL (recognizable
by interaction with POKEUSER) and don't tell about vdso to guest processes.
Anything well-behaving won't step on SYSCALL and the things that do deserve
the subtle bugs they get.
* asm glue is subtle, evil and doesn't have anywhere near enough
documentation ;-/