Re: Again: UML on s390 (31Bit)

From: Bodo Stroesser
Date: Thu Apr 28 2005 - 04:55:51 EST

Martin Schwidefsky wrote:
Bodo Stroesser <bstroesser@xxxxxxxxxxxxxxxxxxx> 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

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 Schwidefsky
Linux for zSeries Development & Services
IBM Deutschland Entwicklung GmbH

Regards, Bodo
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