Re: [RFCv2 1/4] nl80211: Fix broken non-split wiphy dumps

[Johannes Berg <johannes@sipsolutions.net>]

On Fri, 2019-08-30 at 11:40 +0200, Johannes Berg wrote:
> On Fri, 2019-08-30 at 11:10 +0200, Johannes Berg wrote:
> > On Fri, 2019-08-30 at 11:03 +0200, 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.
> > 
> > Actually, going back in time to the code there (e.g. iw and hostap), it
> > seems that it quite possibly never was a userspace issue, just an issue
> > with netlink allocating a 4k SKB by default for dumps.
> > 
> > Even then, libnl would've defaulted to a 16k recvmsg() buffer size, and
> > we didn't override that anywhere.
> 
> Ah, also not quite true, at the time it still had a 4k default, until
> commit 807fddc4cd9e ("nl: Increase receive buffer size to 4 pages")
> dated May 8, 2013.

However, even before that, it would have supported responding to
MSG_TRUNC by retrying the recvmsg(), but hostap/iw wouldn't have set
nl_socket_enable_msg_peek()... oh well.

johannes