Re: 2.6.23 regression: accessing invalid mmap'ed memory from gdb causes unkillable spinning

Nick Piggin
2007-10-31T06:42:33Z

On Wed, Oct 31, 2007 at 12:45:35AM +0000, Duane Griffin wrote:
> Accessing a memory mapped region past the last page containing a valid
> file mapping produces a SIGBUS fault (as it should). Running a program
> that does this under gdb, then accessing the invalid memory from gdb,
> causes it to start consuming 100% CPU and become unkillable. Once in
> that state, SysRq-T doesn't show a stack trace for gdb, although it is
> shown as running and stack traces are dumped for other tasks.

BTW. this has come up for me before, and I have found it useful on
a number of occasions to print the stack of running tasks when they
are looping in the kernel...

Any reason we can't do this?

--
Sysrq+T fails to show the stack trace of a running task. Presumably this
is to avoid a garbled stack, however it can often be useful, and besides
there is no guarantee that the task won't start running in the middle of
show_stack(). If there are any correctness issues, then the archietcture
would have to take further steps to ensure the task is not running.

Signed-off-by: Nick Piggin <npiggin@suse.de>

Index: linux-2.6/kernel/sched.c
===================================================================
--- linux-2.6.orig/kernel/sched.c 2007-10-31 06:53:22.000000000 +1100
+++ linux-2.6/kernel/sched.c 2007-10-31 06:56:02.000000000 +1100
@@ -4900,8 +4900,7 @@
printk(KERN_CONT "%5lu %5d %6d\n", free,
task_pid_nr(p), task_pid_nr(p->parent));

- if (state != TASK_RUNNING)
- show_stack(p, NULL);
+ show_stack(p, NULL);
}

void show_state_filter(unsigned long state_filter)