Messages in this thread Patch in this message | | | Date | Fri, 29 Jan 2016 15:17:51 +0100 | From | Peter Zijlstra <> | Subject | Re: perf/ring-buffer: Undefined behaviour in kernel/events/ring_buffer.c:685:22 |
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On Fri, Jan 22, 2016 at 03:48:55PM +0300, Andrey Ryabinin wrote: > 2016-01-19 17:31 GMT+03:00 Peter Zijlstra <peterz@infradead.org>: > > 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! > > > > It happens if nr_pages = 0: > rb->page_order = ilog2(nr_pages);
Ah indeed.
Something like so then. When !nr_pages, both variables should be 0 and are due to the kzalloc() of rb slightly above.
--- kernel/events/ring_buffer.c | 6 ++++-- 1 file changed, 4 insertions(+), 2 deletions(-)
diff --git a/kernel/events/ring_buffer.c b/kernel/events/ring_buffer.c index adfdc0536117..345130705c49 100644 --- a/kernel/events/ring_buffer.c +++ b/kernel/events/ring_buffer.c @@ -746,8 +746,10 @@ struct ring_buffer *rb_alloc(int nr_pages, long watermark, int cpu, int flags) rb->user_page = all_buf; rb->data_pages[0] = all_buf + PAGE_SIZE; - rb->page_order = ilog2(nr_pages); - rb->nr_pages = !!nr_pages; + if (nr_pages) { + rb->nr_pages = 1; + rb->page_order = ilog2(nr_pages); + } ring_buffer_init(rb, watermark, flags);
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