On Wed, 2006-08-16 at 11:41 +0530, Amit S. Kale wrote:
> Hi Kevin,
>
> Thanks for reporting this problem. Looking more into KGDB patches, this seems
> to be present on all platforms.
>
> KGDB shouldn't access user level memory at all. There is a comparison of
> requested address against TASK_SIZE in kernel/kgdb.c functions to take care
> of this. kgdb_set_mem and kgdb_get_mem functions in present kernel/kgdb.c
> contain this check but kgdb_mem2hex, kgdb_ebin2mem, kgdb_hex2mem don't. We
> need to add this check to these three functions also.
I recall having a problem with the 2.6.12/2.6.13 kgdb patch when using
gdb in user space and using a watchpoint. I believe this uses Intel
debug hardware and I think somehow got involved with kgdb. If I see it
again I'll let ya know.
-piet
>
> -Amit
>
> On Thursday 10 August 2006 04:00, Kevin Hilman wrote:
> > There's a problem in handling kgdb-triggered memory faults on ARM when
> > the process in question has a user context.
> >
> > To reproduce, set a breakpoint at a system call (sys_sync for example),
> > once the breakpoint is hit, do an 'info threads'. In the process of
> > doing this, kgdb_mem2hex may trigger a memory fault, but since this
> > process has a user context, it doesn't take the path with the kgdb
> > setjmp/longjmp fixups. Even worse, it eventually ends up in do_no_page
> > which can sleep, and since kgdb has disabled interrupts, a 'scheduling
> > while atomic' BUG appears.
> >
> > It seems to work OK with no preemption, but this bug arises under
> > CONFIG_PREEMPT.
> >
> > Patch below has been verified on a MontaVista 2.6.10 kernel (ARMv5, and
> > v6) but should be same on newer kernels.
> >
> > Kevin
> >
> > Index: linux-2.6.10/arch/arm/mm/fault.c
> > ===================================================================
> > --- linux-2.6.10.orig/arch/arm/mm/fault.c
> > +++ linux-2.6.10/arch/arm/mm/fault.c
> > @@ -217,6 +217,10 @@ out:
> > return fault;
> > }
> >
> > +#ifdef CONFIG_KGDB
> > +extern int kgdb_may_fault;
> > +#endif
> > +
> > static int
> > do_page_fault(unsigned long addr, unsigned int fsr, struct pt_regs *regs)
> > {
> > @@ -227,6 +231,11 @@ do_page_fault(unsigned long addr, unsign
> > tsk = current;
> > mm = tsk->mm;
> >
> > +#ifdef CONFIG_KGDB
> > + if (kgdb_may_fault)
> > + goto no_context;
> > +#endif
> > +
> > /*
> > * If we're in an interrupt or have no user
> > * context, we must not take the fault..
> >
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Piet Delaney
BlueLane Teck
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