Re: perf/ring-buffer: Undefined behaviour in kernel/events/ring_buffer.c:685:22

[Sasha Levin]

On 01/19/2016 09:31 AM, Peter Zijlstra wrote:
> On Sun, Jan 10, 2016 at 03:55:13PM -0500, Sasha Levin wrote:
>> Hi all,
>>
>> While fuzzing with trinity inside a KVM tools guest, running the latest -next
>> kernel, I've hit the following warning:
>>
>> [ 3494.030114] UBSAN: Undefined behaviour in kernel/events/ring_buffer.c:685:22
>> [ 3494.030647] shift exponent -1 is negative
> 
> That's rb->page_order == -1, which should 'never' happen, curious!
> 
> Funny though that rb::page_order is the exact field _after_ rb::work, ho
> humm.
> 

I've tested your theory using:

diff --git a/kernel/events/internal.h b/kernel/events/internal.h
index 2bbad9c..f627a40 100644
--- a/kernel/events/internal.h
+++ b/kernel/events/internal.h
@@ -14,6 +14,7 @@ struct ring_buffer {
        struct irq_work                 irq_work;
 #ifdef CONFIG_PERF_USE_VMALLOC
        struct work_struct              work;
+       unsigned long dummy;
        int                             page_order;     /* allocation order  */
 #endif
        int                             nr_pages;       /* nr of data pages  */
diff --git a/kernel/events/ring_buffer.c b/kernel/events/ring_buffer.c
index adfdc05..65346f8 100644
--- a/kernel/events/ring_buffer.c
+++ b/kernel/events/ring_buffer.c
@@ -682,6 +682,8 @@ void rb_free(struct ring_buffer *rb)
 #else
 static int data_page_nr(struct ring_buffer *rb)
 {
+       if (page_order(rb) < 0 || rb->dummy)
+               pr_emerg("*** %lx\n", rb->dummy);
        return rb->nr_pages << page_order(rb);
 }

But the output I'm seeing indicates that dummy isn't corrupted:

[  758.806091] *** 0
[  758.806821] ================================================================================
[  758.807961] UBSAN: Undefined behaviour in kernel/events/ring_buffer.c:687:22
[  758.808833] shift exponent -1 is negative
[...]


Thanks,
Sasha