On Thu, Jul 17, 2014 at 04:59:10PM +0100, Alexei Starovoitov wrote: > On Thu, Jul 17, 2014 at 2:19 AM, Will Deacon <will.dea...@arm.com> wrote: > > On Wed, Jul 16, 2014 at 10:19:31PM +0100, Zi Shen Lim wrote: > >> > > >> > Is a BUG_ON justifiable here? Is there not a nicer way to fail? > >> > >> In general, it'd be nice if we returned something like -EINVAL and > >> have all callers handle failures. Today all code gen functions return > >> the u32 instruction and there's no error handling by callers. > >> I think following the precedence (aarch64_insn_gen_branch_imm()) > >> of failing with BUG_ON is a reasonable tradeoff. > > > > Well, I don't necessarily agree with that BUG_ON, either :) > > I take it eBPF doesn't have a `trap' instruction or similar? Otherwise, we > > could generate that and avoid having to propagate errors directly to the > > caller. > > > >> In this case here, when we hit the default (failure) case, that means > >> there's a serious error of attempting to use an unsupported > >> variant. I think we're better off failing hard here than trying to > >> arbitrarily "fallback" on a default choice. > > > > It might be a serious error for BPF, but a BUG_ON brings down the entire > > machine, which I think is unfortunate. > > There is some misunderstanding here. Here BUG_ON will trigger > only on actual bug in JIT implementation, it cannot be triggered by user. > eBPF program is verified before it reaches JIT, so all instructions are > valid and input to JIT is proper. Two instruction are not yet > implemented in this JIT and they trigger pr_.._once(). > So I don't see any issue with this usage of BUG_ON > imo living with silent bugs in JIT is more dangerous. > > For the same reason there is no 'trap' instruction in eBPF. > Static verifier checks that program is valid. If there was a 'trap' > insn the program would be rejected. Like programs with > 'div by zero' are rejected. There is normal 'bpf_exit' insn to > return from the program.
Ok, so assuming that BPF doesn't have any issues, I take your point. However, we could very easily re-use these functions for things like SMP alternatives and kprobes, where simply failing the instruction generation might be acceptable. It just feels like a bit hammer to me, when the machine is probably happily scheduling user tasks, responding to interrupts, writing data to disk etc. Will -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
