Re: [PATCH] mac80211: fix race in ieee80211_register_hw()

On Mon, 6 Apr 2020 at 18:17, Johannes Berg johannes@sipsolutions.net wrote:

On Mon, 2020-04-06 at 15:44 +0300, Kalle Valo wrote:

...

...

user-space  ieee80211_register_hw()  RX IRQ
+++++++++++++++++++++++++++++++++++++++++++++
   |                    |             |
   |<---wlan0---wiphy_register()      |
   |----start wlan0---->|             |
   |                    |<---IRQ---(RX packet)
   |              Kernel crash        |
   |              due to unallocated  |
   |              workqueue.          |

[snip]

...

I have understood that no frames should be received until mac80211 calls struct ieee80211_ops::start:

• @start: Called before the first netdevice attached to the hardware

•     is enabled. This should turn on the hardware and must turn on
  

•     frame reception (for possibly enabled monitor interfaces.)
  

True, but I think he's saying that you can actually add and configure an interface as soon as the wiphy is registered?

Indeed, it's a call to "struct ieee80211_ops::start" just after wiphy is registered that causes the frame to be received leading to RX IRQ.

The "wlan0" is kinda wrong there, should be "phy0" I guess, and then interface added from iw?

Okay, will update the sequence diagram.

-Sumit

johannes