Re: [crash] BUG: unable to handle kernel NULL pointer dereferenceat (null), last sysfs file: /sys/devices/pci0000:00/0000:00:01.0/local_cpus

[Jesse Barnes]

On Fri, 18 Sep 2009 08:38:02 -0700 (PDT)
Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:

> 
> 
> On Fri, 18 Sep 2009, Ingo Molnar wrote:
> > 
> > [  158.058140] warning: `dbus-daemon' uses 32-bit capabilities
> > (legacy support in use) [  159.370562] BUG: unable to handle kernel
> > NULL pointer dereference at (null) [  159.372694] IP:
> > [<ffffffff8143b722>] bitmap_scnprintf+0x72/0xd0
> 
> Hmm. The code is
> 
>    a:	49 63 fc             	movslq %r12d,%rdi
>    d:	0f 49 d3             	cmovns %ebx,%edx
>   10:	c1 f8 1f             	sar    $0x1f,%eax
>   13:	4c 01 ff             	add    %r15,%rdi
>   16:	c1 e8 1a             	shr    $0x1a,%eax
>   19:	c1 fa 06             	sar    $0x6,%edx
>   1c:	41 c1 e8 02          	shr    $0x2,%r8d
>   20:	8d 0c 03             	lea    (%rbx,%rax,1),%ecx
>   23:	48 63 d2             	movslq %edx,%rdx
>   26:	83 e1 3f             	and    $0x3f,%ecx
>   29:	29 c1                	sub    %eax,%ecx
>   2b:*	49 8b 44 d5 00       	mov
> 0x0(%r13,%rdx,8),%rax     <-- trapping instruction 30:	48 c7
> c2 8c 37 16 82 	mov    $0xffffffff8216378c,%rdx 37:	48
> d3 e8             	shr    %cl,%rax 3a:	89
> f1                	mov    %esi,%ecx 3c:	44 89
> f6             	mov    %r14d,%esi
> 
> and the obvious reason seems to be that 'maskp' is NULL (that
> faulting thing is the code for "val = (maskp[word] >> bit) &
> chunkmask;" with the actual fault being the access of "maskp[word]".
> 
> Now, the caller does
> 
> 	mask = cpumask_of_pcibus(to_pci_dev(dev)->bus);
> 
> and then uses cpumask_scnprintf() that is just a wrapper that does
> 
> 	bitmap_scnprintf(buf, len, cpumask_bits(srcp),
> nr_cpumask_bits);
> 
> So clearly we have "cpumask_of_pcibus()" being NULL (cpumask_bits()
> would not change it).
> 
> I assume this is the NUMA case? The non-NUMA case has just
> 
> 	static inline const struct cpumask *cpumask_of_node(int node)
> 	{
> 	        return cpu_online_mask;
> 	}
> 
> so I don't think you can ever get NULL (if we have a NULL
> cpu_online_mask we have bigger problems). 
> 
> [ Side note: looking closer, I think our headers are buggy, and I
> _know_ they are confusing. The above inline declaration of
> cpumask_of_node() seems to be then later overridden in
> <asm-generic/topology.h> by a #define! 
> 
>   And if I read that right, that will also override the debugging 
>   versions that we declared if CONFIG_DEBUG_PER_CPU_MAPS is on. Ingo? 
>   Rusty? Am I missing something?
> 
>   That said, those overrides should only happen for non-NUMA ]
> 
> The NUMA version of 'cpumask_of_node()' has all the debug code for
> show it's not returning NULL, but only when CONFIG_DEBUG_PER_CPU_MAPS
> is enabled. Otherwise it all seems to boil down to (through
> cpumask_of_pcibus and __pcibus_to_node):
> 
> 	node_to_cpumask_map[bus->sysdata->node]
> 
> and it can fail either because "node" isn't initialized, or 
> node_to_cpumask_map[] isn't.
> 
> Probably 'node' is still -1, and it gets the NULL by going off the
> array into la-la-land.

Yeah David posted a fix for this

diff --git a/arch/x86/include/asm/pci.h b/arch/x86/include/asm/pci.h
--- a/arch/x86/include/asm/pci.h
+++ b/arch/x86/include/asm/pci.h
@@ -143,7 +143,11 @@ static inline int __pcibus_to_node(const struct
pci_bus *bus) static inline const struct cpumask *
 cpumask_of_pcibus(const struct pci_bus *bus)
 {
-	return cpumask_of_node(__pcibus_to_node(bus));
+	int node;
+
+	node = __pcibus_to_node(bus);
+	return (node == -1) ? cpu_online_mask :
+			      cpumask_of_node(node);
 }
 #endif


Looks like it should fix this issue.  He's also right that we should
probably have a NUMA_NO_NODE define for this case...

I'll pick it up and put it in my tree.

Jesse
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/