Re: Again: UML on s390 (31Bit)

[Bodo Stroesser <bstroesser@fujitsu-siemens.com>]

Martin Schwidefsky wrote:
> So (!entryexit & regs->gprs[2] < 0) translates to the debugger changed the
> guest
> system call to something illegal on the first of the two ptrace calls. So
> the
> patch doesn't hurt for normal, non-ptraced operation but it might hurt
> other
> users of ptrace.
I don't think, it hurts. If a debugger willingly sets the syscall number
to -1, what would happen without the patch?
The kernel will set the result -ENOSYS into grps[2]. So, even if trap
still indicates a syscall and a signal is pending, no syscall restarting
will be done.
With the patch, a debugger would observe changed behavior of the kernel
*only*, if it writes the syscall number to -1 on the first syscall
interception and then writes the result to ERESTARTXXXXX on the second,
while at the same time a signal is pending for the debugged process.

I assumed, that non of the current users of ptrace exactly does this.
If I'm wrong here, the patch *really* is bad.

> Ok, I think I've understood the problem now. What you are basically have is
> a process running in a UML guest that happens to have -ERESTARTXXX in grp2
> when it gets interrupted. A signal is delivered and on return from that
> signal
> with sys_(rt_)sigreturn >another< signal might be pending and then
> do_signal
> gets confused because of -ERESTARTXXX in grp2.
This other signal must be pending on the *host*, in UML, this might be
SIGVTALRM.

> For normal, non-uml operation
> restore_sigregs resets regs->trap to -1 which avoids the confusion. With
> UML
> the host intercepts sys_rt_sigreturn and does whatever needs to be done for
> the guest >except< resetting regs->trap to -1. So the problem seems to be
> that you need a ptrace interface to do that. I don't think it is a good
> idea
> to kludge syscall_trace to reset regs->trap under some conditions.
My idea was to enable the existing ptrace interface to do what UML
needs, without changing it in a way observable to other users of ptrace.
I expected my patch to exactly do that, but maybe I missed something.
Any better idea is welcome.

Regards,  Bodo
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