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Commit 51694948 authored by Paolo 'Blaisorblade' Giarrusso's avatar Paolo 'Blaisorblade' Giarrusso Committed by Linus Torvalds
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[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: default avatarPaolo 'Blaisorblade' Giarrusso <blaisorblade@yahoo.it>
Signed-off-by: default avatarLinus Torvalds <torvalds@osdl.org>
parent 3603bc8d

arch/um/os-Linux/start_up.c

+12 −10
Original line number Diff line number Diff line
@@ -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;