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