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

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Date:   Tue, 05 Dec 2017 17:34:21 +0100
From:   Johannes Berg <johannes@...solutions.net>
To:     David Miller <davem@...emloft.net>
Cc:     linux-wireless@...r.kernel.org, netdev@...r.kernel.org, j@...fi,
        dsahern@...il.com
Subject: Re: [PATCH net 1/2] netlink: add NLA_U8_BUGGY attribute type

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.

For sure.

johannes