netdev - Re: [PATCH net 1/2] netlink: add NLA_U8

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Date:   Tue, 5 Dec 2017 09:41:21 -0700
From:   David Ahern <dsahern@...il.com>
To:     Johannes Berg <johannes@...solutions.net>,
        David Miller <davem@...emloft.net>
Cc:     linux-wireless@...r.kernel.org, netdev@...r.kernel.org, j@...fi
Subject: Re: [PATCH net 1/2] netlink: add NLA_U8_BUGGY attribute type

On 12/5/17 9:34 AM, Johannes Berg wrote:
> On Tue, 2017-12-05 at 11:31 -0500, David Miller wrote:
>>
>>> We could try to fix up the big endian problem here, but we
>>> don't know *how* userspace misbehaved - if using nla_put_u32
>>> then we could, but we also found a debug tool (which we'll
>>> ignore for the purposes of this regression) that was putting
>>> the padding into the length.
> 
>> We're stuck with this thing forever... I'd like to consider other
>> options.
>>
>> I've seen this problem at least one time before, therefore I
>> suggest when we see a U8 attribute with a U32's length:
>>
>> 1) We access it as a u32, this takes care of all endianness
>>    issues.
> 
> Possible, but as I said above, I've seen at least one tool (a debug
> only script) now that will actually emit a U8 followed by 3 bytes of
> padding to make it netlink-aligned, but set the length to 4. That would
> be broken by making this change.
> 
> I'm not saying this is bad - but there are different levels of
> compatibility and I'd probably go for "bug compatibility" here rather
> than "fix-it-up compatibility".
> 
> Your call, ultimately - I've already fixed the tool I had found :-)
> 
>> 2) We emit a warning so that the app gets fixes.
> 

The attached is my proposal. Basically, allow it the length mismatch but
print a warning. This restores previous behavior and tells users of bad
commands.

View attachment "0001-netlink-Relax-attr-validation-for-fixed-length-types.patch" of type "text/plain" (2222 bytes)