netdev - Re: [PATCH net-next v1] hinic: fix strncpy output truncated compile warnings

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Date:   Sun, 9 Aug 2020 10:59:54 +0800
From:   "luobin (L)" <luobin9@...wei.com>
To:     David Miller <davem@...emloft.net>
CC:     <David.Laight@...LAB.COM>, <linux-kernel@...r.kernel.org>,
        <netdev@...r.kernel.org>, <luoxianjun@...wei.com>,
        <yin.yinshi@...wei.com>, <cloud.wangxiaoyun@...wei.com>,
        <chiqijun@...wei.com>
Subject: Re: [PATCH net-next v1] hinic: fix strncpy output truncated compile
 warnings

On 2020/8/8 11:42, David Miller wrote:
> From: "luobin (L)" <luobin9@...wei.com>
> Date: Sat, 8 Aug 2020 11:36:42 +0800
> 
>> On 2020/8/7 17:32, David Laight wrote:
>>>> diff --git a/drivers/net/ethernet/huawei/hinic/hinic_devlink.c
>>>> b/drivers/net/ethernet/huawei/hinic/hinic_devlink.c
>>>> index c6adc776f3c8..1ec88ebf81d6 100644
>>>> --- a/drivers/net/ethernet/huawei/hinic/hinic_devlink.c
>>>> +++ b/drivers/net/ethernet/huawei/hinic/hinic_devlink.c
>>>> @@ -342,9 +342,9 @@ static int chip_fault_show(struct devlink_fmsg *fmsg,
>>>>
>>>>  	level = event->event.chip.err_level;
>>>>  	if (level < FAULT_LEVEL_MAX)
>>>> -		strncpy(level_str, fault_level[level], strlen(fault_level[level]));
>>>> +		strncpy(level_str, fault_level[level], strlen(fault_level[level]) + 1);
>>>
>>> Have you even considered what that code is actually doing?
>  ...
>> I'm sorry that I haven't got what you mean and I haven't found any defects in that code. Can you explain more to me?
> 
> David is trying to express the same thing I was trying to explain to
> you, you should use sizeof(level_str) as the third argument because
> the code is trying to make sure that the destination buffer is not
> overrun.
> 
> If you use the strlen() of the source buffer, the strncpy() can still
> overflow the destination buffer.
> 
> Now do you understand?
> .
> 
Thanks for your explanation. I explained that why I didn't use sizeof(level_str) as the third argument in my previous reply e-mail to you.
Because using sizeof(level_str) as the third argument will still cause the following compile warning:

In function ‘strncpy’,
    inlined from ‘chip_fault_show’ at drivers/net/ethernet/huawei/hinic/hinic_devlink.c:345:3:
./include/linux/string.h:297:30: warning: ‘__builtin_strncpy’ specified bound 17 equals destination size [-Wstringop-truncation]
  297 | #define __underlying_strncpy __builtin_strncpy

Now I know that using strncpy() on NUL-terminated strings is deprecated as Kees Cook points out and actually there is no need to use it
in my code.