From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755997AbeEaSwd (ORCPT ); Thu, 31 May 2018 14:52:33 -0400 Received: from relay1.mentorg.com ([192.94.38.131]:39194 "EHLO relay1.mentorg.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755877AbeEaSwc (ORCPT ); Thu, 31 May 2018 14:52:32 -0400 Subject: Re: [PATCH] clk: vc5: Fix div-by-0 when rounding a rate of zero To: Marek Vasut , Steve Longerbeam , Michael Turquette , Stephen Boyd CC: , , Laurent Pinchart , Kieran Bingham References: <1527791036-11251-1-git-send-email-steve_longerbeam@mentor.com> <1084deae-ca55-aba1-ec08-7e1d976de7b7@gmail.com>

From: Steve Longerbeam Message-ID: <59591484-b76d-6764-73c3-637bb4e5178f@mentor.com> Date: Thu, 31 May 2018 11:52:26 -0700 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.8.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset="utf-8"; format=flowed Content-Transfer-Encoding: 8bit Content-Language: en-US X-ClientProxiedBy: svr-orw-mbx-08.mgc.mentorg.com (147.34.90.208) To svr-orw-mbx-02.mgc.mentorg.com (147.34.90.202) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 05/31/2018 11:35 AM, Marek Vasut wrote: > On 05/31/2018 08:32 PM, Steve Longerbeam wrote: >> Hi Marek, >> >> >> On 05/31/2018 11:25 AM, Marek Vasut wrote: >>> On 05/31/2018 08:23 PM, Steve Longerbeam wrote: >>>> Just return zero for a rounded rate if requested rate is zero. >>>> >>>> This was caught by CONFIG_UBSAN: >>>> >>>> [ 192.266748] UBSAN: Undefined behaviour in >>>> drivers/clk/clk-versaclock5.c:513:17 >>>> [ 192.274050] division by zero >>>> [ 192.276976] CPU: 0 PID: 2579 Comm: vsp-unit-test-0 Tainted: G >>>> B WC 4.14.17-02752-g13fb96f #1 >>>> [ 192.286378] Hardware name: Renesas Salvator-X board based on >>>> r8a7795 ES2.0+ (DT) >>>> [ 192.293852] Call trace: >>>> [ 192.296343] [] dump_backtrace+0x0/0x390 >>>> [ 192.301807] [] show_stack+0x14/0x1c >>>> [ 192.306920] [] dump_stack+0x134/0x1a8 >>>> [ 192.312213] [] ubsan_epilogue+0x14/0x60 >>>> [ 192.317677] [] >>>> __ubsan_handle_divrem_overflow+0x11c/0x170 >>>> [ 192.324720] [] vc5_fod_round_rate+0x68/0x148 >>>> [ 192.330620] [] clk_calc_new_rates+0x238/0x3fc >>>> [ 192.336607] [] clk_calc_new_rates+0x29c/0x3fc >>>> [ 192.342595] [] clk_core_set_rate_nolock+0x48/0x11c >>>> [ 192.349019] [] clk_set_rate+0x34/0x4c >>>> [ 192.354307] [] rcar_du_pm_suspend+0x274/0x2f4 >>>> [ 192.360297] [] platform_pm_suspend+0x78/0xb8 >>>> [ 192.366198] [] dpm_run_callback+0x584/0xa18 >>>> [ 192.372010] [] __device_suspend+0x1a8/0x534 >>>> [ 192.377822] [] dpm_suspend+0x130/0xea0 >>>> [ 192.383197] [] dpm_suspend_start+0x130/0x138 >>>> [ 192.389099] [] >>>> suspend_devices_and_enter+0xf0/0x1778 >>>> [ 192.395700] [] pm_suspend+0x2408/0x245c >>>> [ 192.401162] [] state_store+0xf0/0x130 >>>> [ 192.406451] [] kobj_attr_store+0x5c/0x6c >>>> [ 192.412002] [] sysfs_kf_write+0xe8/0xfc >>>> [ 192.417466] [] kernfs_fop_write+0x22c/0x2e4 >>>> [ 192.423281] [] __vfs_write+0x104/0x34c >>>> [ 192.428656] [] vfs_write+0x134/0x2d8 >>>> [ 192.433857] [] SyS_write+0xbc/0x12c >>>> [ 192.438967] Exception stack(0xffff8006cd1cfec0 to 0xffff8006cd1d0000) >>>> [ 192.445480] fec0: 0000000000000001 000000001e303f00 >>>> 0000000000000004 0000ffff959a5000 >>>> [ 192.453397] fee0: 0000000000000000 0000000155510004 >>>> 0000000000000003 000000000000006d >>>> [ 192.461314] ff00: 0000000000000040 0000000000000000 >>>> 0000ffffcc304800 0000000000000020 >>>> [ 192.469230] ff20: 0000000000000000 0000000000000000 >>>> 0000000000000001 0000000000000008 >>>> [ 192.477148] ff40: 00000000004eb3b8 0000ffff958bb840 >>>> 000000000000003d 0000000000000001 >>>> [ 192.485065] ff60: 000000001e303f00 0000ffff959a1508 >>>> 0000000000000004 000000001e303f00 >>>> [ 192.492982] ff80: 0000000000000004 00000000004d4c68 >>>> 0000000000000001 0000000000000000 >>>> [ 192.500899] ffa0: 000000001e30d5c0 0000ffffcc304820 >>>> 0000ffff958bec64 0000ffffcc304820 >>>> [ 192.508816] ffc0: 0000ffff95912898 0000000020000000 >>>> 0000000000000001 0000000000000040 >>>> [ 192.516733] ffe0: 0000000000000000 0000000000000000 >>>> 0000000000000000 0000000000000000 >>>> [ 192.524650] [] el0_svc_naked+0x24/0x28 >>>> >>>> Signed-off-by: Steve Longerbeam >>>> --- >>>> drivers/clk/clk-versaclock5.c | 4 ++++ >>>> 1 file changed, 4 insertions(+) >>>> >>>> diff --git a/drivers/clk/clk-versaclock5.c >>>> b/drivers/clk/clk-versaclock5.c >>>> index decffb3..113523d 100644 >>>> --- a/drivers/clk/clk-versaclock5.c >>>> +++ b/drivers/clk/clk-versaclock5.c >>>> @@ -509,6 +509,10 @@ static long vc5_fod_round_rate(struct clk_hw >>>> *hw, unsigned long rate, >>>> u32 div_int; >>>> u64 div_frc; >>>> + /* prevent div-by-0 */ >>>> + if (rate == 0) >>>> + return 0; >>>> + >>>> /* Determine integer part, which is 12 bit wide */ >>>> div_int = f_in / rate; >>>> /* >>>> >>> Can this actually happen ? >> We caught this using the Renesas 3.6.0 BSP release, when performing >> a suspend of rcar-du driver. The rcar_du_pm_suspend() in 3.6.0 BSP is >> modified >> from mainline version, including calling clk_set_rate() on the crtc >> clocks with a >> rate of zero. So this is not actually reproducible (yet) in mainline. > So it sets clock to 0 ? Yep, see https://kernel.googlesource.com/pub/scm/linux/kernel/git/horms/renesas-bsp/+/rcar-3.6.0/drivers/gpu/drm/rcar-du/rcar_du_drv.c#359 > Anyway, this looks sane, although maybe the > whole driver could use a once-over to see if there could be more of this. Actually I do see more potential divide-by-zeros due to a passed rate of zero, including vc5_pfd_round_rate() and vc5_pfd_set_rate(). I can resubmit this patch fixing all cases in clk-versaclock5.c if you like (and probably remove the misleading backtrace in the commit message since it is a Renesas 3.6.0 kernel backtrace not a mainline backtrace). Or perhaps just treat this as a heads-up, I'll leave it up to you. > > Or should the clock framework even let us set clock to 0 Hz ? That is a good question, it might make sense for the core clock framework to not allow passing a rate of 0 on to the clock ops, and instead treat it generically. Steve