netdev - Re: [PATCH net-next] dev: Correctly get length of alias string in dev_set

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Message-ID: <de0b307c-63e7-b786-e9d1-8b4a94f61d9a@gmail.com>
Date:   Wed, 20 Dec 2017 21:44:46 +0200
From:   Serhey Popovich <serhe.popovych@...il.com>
To:     David Miller <davem@...emloft.net>
Cc:     netdev@...r.kernel.org
Subject: Re: [PATCH net-next] dev: Correctly get length of alias string in
 dev_set_alias()

David Miller wrote:
> From: Serhey Popovych <serhe.popovych@...il.com>
> Date: Mon, 18 Dec 2017 23:38:35 +0200
> 
>> We supply number of bytes available in @alias via @len
>> parameter to dev_set_alias() which is not the same
>> as zero terminated string length that can be shorter.
>>
>> Both dev_set_alias() users (rtnetlink and sysfs) can
>> submit number of bytes up to IFALIASZ with actual string
>> length slightly shorter by putting '\0' not at @len - 1.
>>
>> Use strnlen() to get length of zero terminated string
>> and not access beyond @len. Correct comment about @len
>> and explain how to unset alias (i.e. use zero for @len).
>>
>> Signed-off-by: Serhey Popovych <serhe.popovych@...il.com>
> 
> I don't really see this as useful, really.
> 
> In the sysfs case, we are not presented with a NULL terminated string.
> Instead, the net sysfs code gives us a length that goes up until the
> trailing newline character.  The sysfs case is never larger than the
> actual string size + 1.
> 
> The netlink attribute is usually sized appropriately for whatever the
> string length actually is.

Sorry but I do not mean larger. I mean shorter. When nla_len() >
strlen() we allocate extra space up to IFALIASZ - 1.

This is definitely fix nothing: we never get above the bounds, but
in case if NULL terminator is in the middle of string with nla_len()
we might allocate unused extra space.

Sorry again if I'm not correct with above assumption.

> 
> This therefore just seems to add an new strnlen() unnecessarily to
> this code path, which rarely does anything helpful.
> 
> Thanks.
> 




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