[CRIU] Dumping BPF Map Data

[Abhishek Vijeev]

Hi,

I've been working on enabling CRIU to c/r BPF map files. The following PR
reads
information about BPF maps from procfs, dumps it and creates new maps with
the
same parameters upon restoration:

https://github.com/checkpoint-restore/criu/pull/1044

For my next goal, I'd like to c/r the data stored in these maps. However,
the BPF
system call does not provide an interface to retrieve all data stored in
BPF maps
without prior information about the data stored.

I shall now describe the problem I'm faced with. Let's take the example of
a BPF
map that stores key-value pairs in the form of a hash table. Here are its
contents:

Key            Value
  a                  1
  b                  2
  c                  3

To extract all data stored in this map, I must iterate through all
key-value pairs using
BPF_MAP_GET_NEXT_KEY, starting with a lookup key that doesn't exist in the
map.
(i.e. anything other than 'a', 'b' or 'c'). Given that I don't know the
data type of the
keys or the contents of the map, how must I generate a key that doesn't
exist in the
map? It is important that the lookup key is not present in the map because
in that
case, I will only be able to iterate through keys that occur after this
lookup key. For
example, using 'b' as the lookup key would miss out the first key-value
pair ('a'->1).

I'm looking for guidance to solve this problem, and would appreciate any
help.

Thank you,
Abhishek Vijeev.
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