Messages in this thread | | | From | Lorenzo Colitti <> | Date | Wed, 11 Jul 2018 11:46:19 +0900 | Subject | Re: [RFC] Add BPF_SYNCHRONIZE bpf(2) command |
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On Wed, Jul 11, 2018 at 8:52 AM Alexei Starovoitov <alexei.starovoitov@gmail.com> wrote: > > we need to make sure we have detailed description of BPF_SYNC_MAP_ACCESS > in uapi/bpf.h, since I feel the confusion regarding its usage is starting already. > This new cmd will only make sense for map-in-map type of maps. > Expecting that BPF_SYNC_MAP_ACCESS is somehow implies the end of > the program or doing some other map synchronization is not correct. > Commit log of this patch got it right: > """ > For example, userspace can update a map->map entry to point to a new map, > use BPF_SYNCHRONIZE to wait for any BPF programs using the old map to > complete, and then drain the old map without fear that BPF programs > may still be updating it. > """
+1 for detailed documentation. For example, consider what happens if we have two map fds, one active and one standby, and a map-in-map with one element that contains a pointer to the currently-active map fd. The kernel program might do:
===== const int current_map_key = 1; void *current_map = bpf_map_lookup_elem(outer_map, ¤t_map_key);
int stats_key = 42; uint64_t *stats_value = bpf_map_lookup_elem(current_map, &stats_key); __sync_fetch_and_add(&stats_value, 1); =====
If a userspace does:
1. Write new fd to outer_map[1]. 2. Call BPF_SYNC_MAP_ACCESS. 3. Start deleting everything in the old map.
How can we guarantee that the __sync_fetch_and_add will not add to the old map? If it does, we'll lose data. Will the verifier automatically hold the RCU lock for as long as a pointer to an inner map is valid?
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