[PATCH] UML on UML breaks when testing SYSEMU

[Renzo Davoli]

It is currently impossible to run a user-mode linux machine inside another user-mode 
linux (UML on UML).

This is the output of a broken attempt:
$ ./linux mem=256m ubd0=cow
Locating the bottom of the address space ... 0x0
Locating the top of the address space ... 0xc0000000
Core dump limits :
        soft - 0
        hard - NONE
Checking that ptrace can change system call numbers...OK
Checking ptrace new tags for syscall emulation...unsupported
Checking syscall emulation patch for ptrace...check_sysemu : expected SIGTRAP, got status = 256
$

The problem is the following:
PTRACE_SYSCALL/SINGLESTEP is currently managed inside arch_ptrace for ARCH=um.

PTRACE_SYSEMU/SUSEMU_SINGLESTEP is not captured in arch_ptrace's switch, therefore
it is erroneously passed back to ptrace_request (in kernel/ptrace).

This simple patch simply forces ptrace to return an error on PTRACE_SYSEMU/SUSEMU_SINGLESTEP
as it is unsupported on ARCH=um, and fixes the problem.

Signed-off-by: Renzo Davoli <renzo@xxxxxxxxxxx>
---
diff -Naur linux-2.6.28.2/arch/um/kernel/ptrace.c linux-2.6.28.2-umluml/arch/um/kernel/ptrace.c
--- linux-2.6.28.2/arch/um/kernel/ptrace.c	2009-01-25 01:42:07.000000000 +0100
+++ linux-2.6.28.2-umluml/arch/um/kernel/ptrace.c	2009-02-02 14:01:52.000000000 +0100
@@ -64,6 +64,11 @@
 		ret = poke_user(child, addr, data);
 		break;
 
+	case PTRACE_SYSEMU:
+	case PTRACE_SYSEMU_SINGLESTEP:
+		ret=-EIO;
+		break;
+
 	/* continue and stop at next (return from) syscall */
 	case PTRACE_SYSCALL:
 	/* restart after signal. */
--
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