[PATCH] uml: fix random segfaults at bootup

Don't use printk() where "current_thread_info()" is crap.

Until when we switch to running on init_stack, current_thread_info() evaluates
to crap. Printk uses "current" at times (in detail, &current is evaluated with
CONFIG_DEBUG_SPINLOCK to check the spinlock owner task).

And this leads to random segmentation faults.

Exactly, what happens is that &current = *(current_thread_info()), i.e. round
down $esp and dereference the value. I.e. access the stack below $esp, which
causes SIGSEGV on a VM_GROWSDOWN vma (see arch/i386/mm/fault.c).
Signed-off-by: NPaolo 'Blaisorblade' Giarrusso <blaisorblade@yahoo.it>
Signed-off-by: NLinus Torvalds <torvalds@osdl.org>

arch/um/os-Linux/start_up.c

@@ -116,16 +116,16 @@ static int stop_ptraced_child(int pid, void *stack, int exitcode,
 	if(!WIFEXITED(status) || (WEXITSTATUS(status) != exitcode)) {
 		int exit_with = WEXITSTATUS(status);
 		if (exit_with == 2)
-			printk("check_ptrace : child exited with status 2. "
+			printf("check_ptrace : child exited with status 2. "
 			       "Serious trouble happening! Try updating your "
 			       "host skas patch!\nDisabling SYSEMU support.");
-		printk("check_ptrace : child exited with exitcode %d, while "
+		printf("check_ptrace : child exited with exitcode %d, while "
 		      "expecting %d; status 0x%x", exit_with,
 		      exitcode, status);
 		if (mustpanic)
 			panic("\n");
 		else
-			printk("\n");
+			printf("\n");
 		ret = -1;
 	}
 
@@ -183,7 +183,7 @@ static void __init check_sysemu(void)
 	void *stack;
  	int pid, n, status, count=0;
 
-	printk("Checking syscall emulation patch for ptrace...");
+	printf("Checking syscall emulation patch for ptrace...");
 	sysemu_supported = 0;
 	pid = start_ptraced_child(&stack);
 
@@ -207,10 +207,10 @@ static void __init check_sysemu(void)
 		goto fail_stopped;
 
 	sysemu_supported = 1;
-	printk("OK\n");
+	printf("OK\n");
 	set_using_sysemu(!force_sysemu_disabled);
 
-	printk("Checking advanced syscall emulation patch for ptrace...");
+	printf("Checking advanced syscall emulation patch for ptrace...");
 	pid = start_ptraced_child(&stack);
 
 	if(ptrace(PTRACE_OLDSETOPTIONS, pid, 0,
@@ -246,7 +246,7 @@ static void __init check_sysemu(void)
 		goto fail_stopped;
 
 	sysemu_supported = 2;
-	printk("OK\n");
+	printf("OK\n");
 
 	if ( !force_sysemu_disabled )
 		set_using_sysemu(sysemu_supported);
@@ -255,7 +255,7 @@ static void __init check_sysemu(void)
 fail:
 	stop_ptraced_child(pid, stack, 1, 0);
 fail_stopped:
-	printk("missing\n");
+	printf("missing\n");
 }
 
 static void __init check_ptrace(void)
@@ -263,7 +263,7 @@ static void __init check_ptrace(void)
 	void *stack;
 	int pid, syscall, n, status;
 
-	printk("Checking that ptrace can change system call numbers...");
+	printf("Checking that ptrace can change system call numbers...");
 	pid = start_ptraced_child(&stack);
 
 	if(ptrace(PTRACE_OLDSETOPTIONS, pid, 0, (void *)PTRACE_O_TRACESYSGOOD) < 0)
@@ -292,7 +292,7 @@ static void __init check_ptrace(void)
 		}
 	}
 	stop_ptraced_child(pid, stack, 0, 1);
-	printk("OK\n");
+	printf("OK\n");
 	check_sysemu();
 }
 
@@ -472,6 +472,8 @@ int can_do_skas(void)
 
 int have_devanon = 0;
 
+/* Runs on boot kernel stack - already safe to use printk. */
+
 void check_devanon(void)
 {
 	int fd;