[PATCH] uml: fix sysemu test at startup

From: Paolo 'Blaisorblade' Giarrusso <blaisorblade_spam@yahoo.it>
      Bodo Stroesser <bodo.stroesser@fujitsu-siemens.com>
      Jeff Dike <jdike@addtoit.com>

Currently, the test for the SYSEMU support on the host is completely wrong, as
Bodo noticed.  We should change the syscall result (inserting the host pid)
and check if it is received correctly by the guest.  What we actually do,
without this patch, is to overwrite the syscall number.

This went unnoticed because we only check that the getpid() syscall from the
child does not return its pid.  We don't check that it returns the correct
value.

Also, override the result portably, using the PT_SYSCALL_RET_OFFSET macro
which abstract away the host stack frame layout (took from Jeff Dike code).
Signed-off-by: Paolo 'Blaisorblade' Giarrusso <blaisorblade_spam@yahoo.it>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

arch/um/kernel/process.c

@@ -214,8 +214,6 @@ static void __init check_sysemu(void)
 	sysemu_supported = 0;
 	pid = start_ptraced_child(&stack);
 	if(ptrace(PTRACE_SYSEMU, pid, 0, 0) >= 0) {
-		struct user_regs_struct regs;
-
 		CATCH_EINTR(n = waitpid(pid, &status, WUNTRACED));
 		if (n < 0)
 			panic("check_ptrace : wait failed, errno = %d", errno);
@@ -223,18 +221,16 @@ static void __init check_sysemu(void)
 			panic("check_ptrace : expected SIGTRAP, "
 			      "got status = %d", status);
 
-		if (ptrace(PTRACE_GETREGS, pid, 0, &regs) < 0)
-			panic("check_ptrace : failed to read child "
-			      "registers, errno = %d", errno);
-		regs.orig_eax = pid;
-		if (ptrace(PTRACE_SETREGS, pid, 0, &regs) < 0)
-			panic("check_ptrace : failed to modify child "
-			      "registers, errno = %d", errno);
+		n = ptrace(PTRACE_POKEUSER, pid, PT_SYSCALL_RET_OFFSET,
+			   os_getpid());
+		if(n < 0)
+			panic("check_ptrace : failed to modify system "
+			      "call return, errno = %d", errno);
 
 		stop_ptraced_child(pid, stack, 0);
 
 		sysemu_supported = 1;
-		printk("found\n");
+		printk("OK\n");
 	}
 	else
 	{