Bounded loops in BPF for the 5.3 kernel

Posted Aug 5, 2019 17:21 UTC (Mon) by ecree (guest, #95790)
Parent article: Bounded loops in BPF for the 5.3 kernel

So the verifier does not incur the cost of tracking all registers precisely; instead, when the need arises, it backtracks through the use of a register in the code to generate a precise value if possible.

Alexei can correct me, but I don't think this is quite accurate. AIUI, the verifier always calculates precise value bounds, but normally that precision is ignored for pruning. Only if the path from a pruning point to an exit uses the precise value does the precise value need to be checked for pruning safety. So the backtracking is marking parent states as "needs precision", not actually generating new values.