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Subject: Re: [PATCH 2/2] rtc: class: return error code when cdev_device_add()
 failed
To:     Alexandre Belloni <alexandre.belloni@bootlin.com>
CC:     <linux-kernel@vger.kernel.org>, <linux-rtc@vger.kernel.org>,
        <a.zummo@towertech.it>
References: <20211011080302.1982894-1-yangyingliang@huawei.com>
 <20211011080302.1982894-2-yangyingliang@huawei.com>
 <YWP3f1wPbim8VG6K@piout.net>
From:   Yang Yingliang <yangyingliang@huawei.com>
Message-ID: <6fcea87c-b44a-2045-9644-dce4eb968c68@huawei.com>
Date:   Mon, 11 Oct 2021 17:40:34 +0800
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Hi,

On 2021/10/11 16:36, Alexandre Belloni wrote:
> On 11/10/2021 16:03:02+0800, Yang Yingliang wrote:
>> I got a null-ptr-deref report when doing fault injection test:
>>
>> general protection fault, probably for non-canonical address 0xdffffc0000000022: 0000 [#1] SMP KASAN PTI
>> KASAN: null-ptr-deref in range [0x0000000000000110-0x0000000000000117]
>> CPU: 1 PID: 1028 Comm: 33 Not tainted 5.15.0-rc3-00111-gf5dad42ed4fe-dirty #481 2a70b3e6ca240b8638beac7ef491cce6183bbec7
>> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.13.0-1ubuntu1.1 04/01/2014
>> RIP: 0010:device_del+0x132/0xdc0
>> Code: 48 c1 ea 03 80 3c 02 00 0f 85 4f 0c 00 00 48 b8 00 00 00 00 00 fc ff df 4c 8b 7b 48 4d 8d a7 10 01 00 00 4c 89 e2 48 c1 ea 03 <0f> b6 04 02 84 c0 74 06 0f 8e 7f 0a 00 00 45 0f b6 b7 10 01 00 00
>> RSP: 0018:ffffc90002e876b8 EFLAGS: 00010206
>> RAX: dffffc0000000000 RBX: ffff88801eb84000 RCX: ffffffff97227644
>> RDX: 0000000000000022 RSI: ffff8880146a0000 RDI: 0000000000000002
>> RBP: ffff88801eb84120 R08: fffffbfff349a60d R09: fffffbfff349a60d
>> R10: ffffc90002e876b8 R11: fffffbfff349a60c R12: 0000000000000110
>> R13: 0000000000000001 R14: ffffc90002e87848 R15: 0000000000000000
>> FS:  00007fa514973500(0000) GS:ffff888104600000(0000) knlGS:0000000000000000
>> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>> CR2: 00007fa51474ccb0 CR3: 000000002bcb0001 CR4: 0000000000770ee0
>> DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
>> DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
>> PKRU: 55555554
>> Call Trace:
>>   ? write_comp_data+0x2a/0x90
>>   ? cleanup_glue_dir+0x260/0x260
>>   ? is_rtc_hctosys.isra.0+0xb9/0xf0
>>   ? rtc_proc_show+0x440/0x440
>>   ? rcu_read_lock_held_common+0xe/0xa0
>>   ? rcu_read_lock_sched_held+0x62/0xe0
>>   cdev_device_del+0x1a/0x80
>>   devm_rtc_unregister_device+0x37/0x80
>>   release_nodes+0xc3/0x3b0
>>
>> If cdev_device_add() fails, 'dev->p' is not set, it causes
>> null-ptr-deref when calling cdev_device_del(), return error
>> code when cdev_device_add() failed to fix this.
>>
>> Reported-by: Hulk Robot <hulkci@huawei.com>
>> Fixes: 3068a254d5519 ("rtc: introduce new registration method")
>> Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
>> ---
>>   drivers/rtc/class.c | 6 ++++--
>>   1 file changed, 4 insertions(+), 2 deletions(-)
>>
>> diff --git a/drivers/rtc/class.c b/drivers/rtc/class.c
>> index 1f18c39a4b82..76422faee05b 100644
>> --- a/drivers/rtc/class.c
>> +++ b/drivers/rtc/class.c
>> @@ -399,12 +399,14 @@ int __devm_rtc_register_device(struct module *owner, struct rtc_device *rtc)
>>   	rtc_dev_prepare(rtc);
>>   
>>   	err = cdev_device_add(&rtc->char_dev, &rtc->dev);
>> -	if (err)
>> +	if (err) {
>>   		dev_warn(rtc->dev.parent, "failed to add char device %d:%d\n",
>>   			 MAJOR(rtc->dev.devt), rtc->id);
>> -	else
>> +		return err;
> As the checkpatch warning indicates, registering the character device as
> always been optional. d5ed9177f64fe95d9de79e6504d41612d9127e8a is the
> commit you want to fix.
So we can keep doing rtc register, even if cdev_device_add() failed ?

Thanks,
Yang
>
>> +	} else {
>>   		dev_dbg(rtc->dev.parent, "char device (%d:%d)\n",
>>   			MAJOR(rtc->dev.devt), rtc->id);
>> +	}
>>   
>>   	rtc_proc_add_device(rtc);
>>   
>> -- 
>> 2.25.1
>>