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

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Date:   Tue, 05 Dec 2017 11:31:45 -0500 (EST)
From:   David Miller <davem@...emloft.net>
To:     johannes@...solutions.net
Cc:     linux-wireless@...r.kernel.org, netdev@...r.kernel.org, j@...fi,
        dsahern@...il.com, johannes.berg@...el.com
Subject: Re: [PATCH net 1/2] netlink: add NLA_U8_BUGGY attribute type

From: Johannes Berg <johannes@...solutions.net>
Date: Sat,  2 Dec 2017 21:23:31 +0100

> From: Johannes Berg <johannes.berg@...el.com>
> 
> This netlink type is used only for backwards compatibility
> with broken userspace that used the wrong size for a given
> u8 attribute, which is now rejected. It would've been wrong
> before already, since on big endian the wrong value (always
> zero) would be used by the kernel, but we can't break the
> existing deployed userspace - hostapd for example now fails
> to initialize entirely.
> 
> 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.
> 
> Fixes: 28033ae4e0f5 ("net: netlink: Update attr validation to require exact length for some types")
> Signed-off-by: Johannes Berg <johannes.berg@...el.com>

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.

2) We emit a warning so that the app gets fixes.

Thanks.