[riot-notifications] [RIOT-OS/RIOT] gnrc_ipv6_nib/nc: remove from _next_removable on remove (#10975)

Martine Lenders notifications at github.com
Fri Feb 8 20:25:47 CET 2019

Ok, I think I got it now! The state corruption happens definitely within the thread, but not due to a remove happening just as something is added to the neighbor cache, but much earlier. It does not lead to a problem immediately due to a quirk of `clist` where the first element is not `list` or `list.next`, but `list.next->next`, so that most of its operations can be done in *O(1)*: `list` is just the entry point into the list, so it never appears in it itself, `list.next` is the last element of the list, so things can be appended in *O(1)* and `list.next->next` is the first.

It's pretty hard to explain, but when the last two **elements** are by change removed directly after each other when the NC is full (which introduces NULL pointers at the last and first **position**) and then two new NC entries are added (which have the same pointer as the removed ones), the NULL pointer at the **position** is first moved to the second **position** and then kept there, due to 


When another entry is tried to be added, the cache-out mechanism starts running (since the NC is full again) and tries to pop the first **element** (which is the NULL pointer) and it crashes.

When just the **last** element is removed the situation "fixes" itself, since the entry is still referred to by the first position, so re-adding it just leads to a loop of one (breaking the list, but not the system ;-)).

I provided a unittest to reproduce and illustrate the crash (2451fb36f8d5abea2cbb10cc61d60f597a0518e2 while writing this message).

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