[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: Paolo 'Blaisorblade' Giarrusso <blaisorblade@yahoo.it>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 12 insertions, 10 deletions

diff --git a/arch/um/os-Linux/start_up.c b/arch/um/os-Linux/start_up.c
index 37517d49c4ae..29a9e3f43763 100644
--- a/arch/um/os-Linux/start_up.c
+++ b/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;