On 21/08/17 21:27, Daniel Borkmann wrote: > On 08/21/2017 08:36 PM, Edward Cree wrote: >> On 19/08/17 00:37, Alexei Starovoitov wrote: > [...] >> I'm tempted to just rip out env->varlen_map_value_access and always check >> the whole thing, because honestly I don't know what it was meant to do >> originally or how it can ever do any useful pruning. While drastic, it >> does cause your test case to pass. > > Original intention from 484611357c19 ("bpf: allow access into map > value arrays") was that it wouldn't potentially make pruning worse > if PTR_TO_MAP_VALUE_ADJ was not used, meaning that we wouldn't need > to take reg state's min_value and max_value into account for state > checking; this was basically due to min_value / max_value is being > adjusted/tracked on every alu/jmp ops for involved regs (e.g. > adjust_reg_min_max_vals() and others that mangle them) even if we > have the case that no actual dynamic map access is used throughout > the program. To give an example on net tree, the bpf_lxc.o prog's > section increases from 36,386 to 68,226 when env->varlen_map_value_access > is always true, so it does have an effect. Did you do some checks > on this on net-next? I tested with the cilium progs and saw no change in insn count. I suspect that for the normal case I already killed this optimisation when I did my unification patch, it was previously about ignoring min/max values on all regs (including scalars), whereas on net-next it only ignores them on map_value pointers; in practice this is useless because we tend to still have the offset scalar sitting in a register somewhere. (Come to think of it, this may have been behind a large chunk of the #insn increase that my patches caused.) Since we use umax_value in find_good_pkt_pointers() now (to check against MAX_PACKET_OFF and ensure our reg->range is really ok), we can't just stop caring about all min/max values just because we haven't done any variable map accesses. I don't see a way around this.
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