Re: Again: UML on s390 (31Bit)

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

Martin Schwidefsky wrote:
> Bodo Stroesser <bstroesser@fujitsu-siemens.com> wrote on 04/27/2005
> 10:21:58 PM:
> > 1) UML includes some of the subarch's (s390) headers. I had to
> >    change one of them with the following one-liner, to make this
> >    compile. AFAICS, this change doesn't break compilation of s390
> >    itself.
>
>
> This one isn't a problem. I'll add it to the repository.
Thank you!

> > ==============================================================================
> > --- linux-2.6.11.orig/arch/s390/kernel/ptrace.c   2005-04-04 18:57:
> > 38.000000000 +0200
> > +++ linux-2.6.11/arch/s390/kernel/ptrace.c   2005-04-04 19:01:51.
> > 000000000 +0200
> > @@ -726,6 +726,13 @@
> >              ? 0x80 : 0));
> >
> >     /*
> > +    * If debugger has set an invalid syscall number,
> > +    * we prepare to skip syscall restart handling
> > +    */
> > +   if (!entryexit && (long )regs->gprs[2] < 0 )
> > +      regs->trap = -1;
> > +
> > +   /*
> >      * this isn't the same as continuing with a signal, but it will do
> >      * for normal use.  strace only continues with a signal if the
> >      * stopping signal is not SIGTRAP.  -brl
> > ==============================================================================
>
>
> This patch is not good. !entryexit indicates that you want to change the trap
> indication on the first of the two calls of syscall_trace for a system call. The
> second condition is gprs[2] < 0 but that can be true for a normal system call as
> well, like sys_exit(-1).
Sorry, that's not right. At that point, gprs[2] holds the syscall number, while the
first argument of the syscall is in origgpr2. If the debugger sets the syscall number
to -1, which is an invalid syscall, changing trap to -1 will result in a changed
behavior only in case, that the debugger on the second syscall interception also sets
the syscall result to ERESTARTXXXXX (This again is modifying gprs[2]). ERESTARTXXXXX
normally would/could be handled by do_signal(), but with the patch it no longer will.
So, I think the patch doesn't hurt in normal cases, but does the trick for UML.

> It might even be true for user addresses if we really
> extent the virtual address space to full 64 bit one day (and the hardware can do
> it with a 5 level paging table). To change regs->trap to -1 with the current
> condition is definitly wrong.
> Independent from that it do not understand why you need it at all. If the
> uml host intercepted and invalidated the guest system call the system restart
> indication bit _TIF_RESTART_SVC shouldn't be set because the guest didn't
> execute a system call.
Let my explain a bit more. UML invalidates UML-user's syscalls on the host, processes
the syscall itself and inserts the result into gprs[2] on the second syscall
interception. For nearly all syscalls ERESTARTXXXXX is a result not returned to user,
but handled in UML kernel internally. But that's not true for sys_(rt_)sigreturn.
The "result" of those is the original contents of gpr2 of the interrupted routine,
which accidentally also might be ERESTARTXXXXXXX (BTW, that's the reason for
sys_(rt_)sigreturn setting trap to -1 also). We skip UML's syscall restart handling
in this case, but we need to skip it in the host, too.

> blue skies,
>    Martin
>
> Martin Schwidefsky
> Linux for zSeries Development & Services
> IBM Deutschland Entwicklung GmbH

Regards,  Bodo
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