linux-kernel - perf/ring-buffer: Undefined behaviour in kernel/events/ring

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Date:	Sun, 10 Jan 2016 15:55:13 -0500
From:	Sasha Levin <sasha.levin@...cle.com>
To:	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	Arnaldo Carvalho de Melo <acme@...nel.org>
Cc:	LKML <linux-kernel@...r.kernel.org>
Subject: perf/ring-buffer: Undefined behaviour in
 kernel/events/ring_buffer.c:685:22

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
[ 3494.031025] CPU: 1 PID: 28506 Comm: kworker/1:2 Not tainted 4.4.0-rc8-next-20160108-sasha-00024-gaaecb9a #2780
[ 3494.032861] Workqueue: events rb_free_work
[ 3494.033195]  1ffff10018bb2f1b 0000000025cc19e7 ffff8800c5d97958 ffffffffa101a182
[ 3494.033871]  0000000041b58ab3 ffffffffac1b3838 ffffffffa101a0b7 ffff8800c5d97920
[ 3494.034469]  ffffffffae01db80 ffff8800c5d97940 0000000025cc19e7 ffffffffae01db80
[ 3494.035097] Call Trace:
[ 3494.035336]  [<ffffffffa101a182>] dump_stack+0xcb/0x149
[ 3494.036171]  [<ffffffffa1114296>] ubsan_epilogue+0x12/0x8f
[ 3494.036568]  [<ffffffffa1114e9a>] __ubsan_handle_shift_out_of_bounds+0x2a3/0x308
[ 3494.038670]  [<ffffffff9f6274ee>] rb_free_work+0xae/0x1a0
[ 3494.039491]  [<ffffffff9f3ac74c>] process_one_work+0xb6c/0x13d0
[ 3494.042924]  [<ffffffff9f3adce3>] worker_thread+0xd33/0x1150
[ 3494.043923]  [<ffffffff9f3c4239>] kthread+0x2f9/0x310


Thanks,
Sasha