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Subject: Re: [RFCv2 1/4] nl80211: Fix broken non-split wiphy dumps
To:     Johannes Berg <johannes@sipsolutions.net>,
        linux-wireless@vger.kernel.org
References: <20190816192703.12445-1-denkenz@gmail.com>
 <f7c98da178677cbb0cad3568f4ea4ab85171edd8.camel@sipsolutions.net>
From:   Denis Kenzior <denkenz@gmail.com>
Message-ID: <5fc07547-80fe-99ee-a83d-5fabe3c92a1b@gmail.com>
Date:   Fri, 30 Aug 2019 11:32:57 -0500
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Hi Johannes,

On 8/30/19 4:03 AM, Johannes Berg wrote:
> On Fri, 2019-08-16 at 14:27 -0500, Denis Kenzior wrote:
>> If a (legacy) client requested a wiphy dump but did not provide the
>> NL80211_ATTR_SPLIT_WIPHY_DUMP attribute, the dump was supposed to be
>> composed of purely non-split NEW_WIPHY messages, with 1 wiphy per
>> message.  At least this was the intent after commit:
>> 3713b4e364ef ("nl80211: allow splitting wiphy information in dumps")
>>
>> However, in reality the non-split dumps were broken very shortly after.
>> Perhaps around commit:
>> fe1abafd942f ("nl80211: re-add channel width and extended capa advertising")
> 
> Fun. I guess we updated all userspace quickly enough to not actually
> have any issues there. As far as I remember, nobody ever complained, so
> I guess people just updated their userspace.
> 
> Given that it's been 6+ years, maybe we're better off just removing the
> whole non-split thing then, instead of fixing it. Seems even less likely
> now that somebody would run a 6+yo supplicant (from before its commit
> c30a4ab045ce ("nl80211: Fix mode settings with split wiphy dump")).
> 

That would be my vote, given that we're probably one of a handful of 
people in this world that understand that code path.

But...  How would we handle non-dump versions of GET_WIPHY?  To this day 
I have dhcpcd issuing fun stuff like:

< Request: Get Wiphy (0x01) len 8 [ack] 
0.374832
     Interface Index: 59 (0x0000003b)

> OTOH, this is a simple fix, would removing the non-split mode result in
> any appreciable cleanups? Perhaps not, and we'd have to insert something
> instead to reject non-split and log a warning, or whatnot.
> 

Getting rid of the legacy non-split case would simplify things.  We 
could also be a-lot smarter about how we split up the messages in order 
to utilize buffer space more efficiently.  I think you cover this in 
your other replies, but I haven't processed those yet.

Regards,
-Denis