linux-kernel - Re: [PATCH] clk: vc5: Fix div-by-0 when rounding a rate of zero

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Date:   Thu, 31 May 2018 18:46:21 -0700
From:   Steve Longerbeam <slongerbeam@...il.com>
To:     Marek Vasut <marek.vasut@...il.com>,
        Steve Longerbeam <steve_longerbeam@...tor.com>,
        Michael Turquette <mturquette@...libre.com>,
        Stephen Boyd <sboyd@...nel.org>
Cc:     linux-clk@...r.kernel.org, linux-kernel@...r.kernel.org,
        Laurent Pinchart <laurent.pinchart+renesas@...asonboard.com>,
        Kieran Bingham <kieran.bingham@...asonboard.com>
Subject: Re: [PATCH] clk: vc5: Fix div-by-0 when rounding a rate of zero



On 05/31/2018 12:37 PM, Marek Vasut wrote:
> On 05/31/2018 08:52 PM, Steve Longerbeam wrote:
>>
>> 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] [<ffff2000080900dc>] dump_backtrace+0x0/0x390
>>>>>> [  192.301807] [<ffff200008090480>] show_stack+0x14/0x1c
>>>>>> [  192.306920] [<ffff200008f66574>] dump_stack+0x134/0x1a8
>>>>>> [  192.312213] [<ffff2000087aaa30>] ubsan_epilogue+0x14/0x60
>>>>>> [  192.317677] [<ffff2000087ab4d0>]
>>>>>> __ubsan_handle_divrem_overflow+0x11c/0x170
>>>>>> [  192.324720] [<ffff200008852120>] vc5_fod_round_rate+0x68/0x148
>>>>>> [  192.330620] [<ffff20000884567c>] clk_calc_new_rates+0x238/0x3fc
>>>>>> [  192.336607] [<ffff2000088456e0>] clk_calc_new_rates+0x29c/0x3fc
>>>>>> [  192.342595] [<ffff2000088483ac>]
>>>>>> clk_core_set_rate_nolock+0x48/0x11c
>>>>>> [  192.349019] [<ffff2000088484b4>] clk_set_rate+0x34/0x4c
>>>>>> [  192.354307] [<ffff20000895e304>] rcar_du_pm_suspend+0x274/0x2f4
>>>>>> [  192.360297] [<ffff20000898feac>] platform_pm_suspend+0x78/0xb8
>>>>>> [  192.366198] [<ffff2000089a5604>] dpm_run_callback+0x584/0xa18
>>>>>> [  192.372010] [<ffff2000089a69e0>] __device_suspend+0x1a8/0x534
>>>>>> [  192.377822] [<ffff2000089adc48>] dpm_suspend+0x130/0xea0
>>>>>> [  192.383197] [<ffff2000089b0344>] dpm_suspend_start+0x130/0x138
>>>>>> [  192.389099] [<ffff20000817f584>]
>>>>>> suspend_devices_and_enter+0xf0/0x1778
>>>>>> [  192.395700] [<ffff200008183014>] pm_suspend+0x2408/0x245c
>>>>>> [  192.401162] [<ffff20000817c0a4>] state_store+0xf0/0x130
>>>>>> [  192.406451] [<ffff200008f6f19c>] kobj_attr_store+0x5c/0x6c
>>>>>> [  192.412002] [<ffff2000084f4c94>] sysfs_kf_write+0xe8/0xfc
>>>>>> [  192.417466] [<ffff2000084f30b0>] kernfs_fop_write+0x22c/0x2e4
>>>>>> [  192.423281] [<ffff2000083e46d4>] __vfs_write+0x104/0x34c
>>>>>> [  192.428656] [<ffff2000083e4cc4>] vfs_write+0x134/0x2d8
>>>>>> [  192.433857] [<ffff2000083e5150>] 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] [<ffff200008083ef0>] el0_svc_naked+0x24/0x28
>>>>>>
>>>>>> Signed-off-by: Steve Longerbeam <steve_longerbeam@...tor.com>
>>>>>> ---
>>>>>>     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.
> It'd be nice if you resubmitted it fixing all the cases.
>

Ok. Please disregard this patch since there won't be a v2, the
new patch will have a different title.

Steve