Re: [PATCH RFCv3 01/14] arm64: introduce aarch64_insn_gen_comp_branch_imm()

[Z Lim]

(resending this email in case the first one got caught in your spam
filter. sorry.)

On Thu, Jul 17, 2014 at 06:25:26PM +0100, Will Deacon wrote:
> 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.deacon@xxxxxxx> 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.

Yes I agree with you Will, it'd be truly unfortunate if we inadvertently
allow the entire system to be brought down.

Alexei accurately pointed out that if we ever hit such a case, it'd be a bug
in the BPF JIT implementation (or bug in other in-kernel implementations).

Our BPF JIT implementation actually handles this, making sure that input
to the codegen function is valid, or gracefully fail by not JITing and
falling back on the core BPF interpreter. This way our JIT will not trigger
the BUG_ON.

IMO, other future users of these codegen functions should do the same.

An alternative would be to throw away all the BUG_ON and have callers
check for and handle error conditions. I think this is actually more
dangerous as callers who don't handle the error conditions properly may
end up causing system crash later with subtle (and quite possibly hard to
debug) bugs.

>
> Will
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