Re: Help with verifier failure

[Yonghong Song]

On 4/22/21 6:55 AM, Brendan Jackman wrote:
> On Wed, 21 Apr 2021 at 18:59, Yonghong Song <yhs@xxxxxx> wrote:
> > On 4/21/21 8:06 AM, Yonghong Song wrote:
> > > On 4/21/21 5:23 AM, Brendan Jackman wrote:
> > > Thanks, Brendan. Looks at least the verifier failure is triggered
> > > by recent clang changes. I will take a look whether we could
> > > improve verifier for such a case and whether we could improve
> > > clang to avoid generate such codes the verifier doesn't like.
> > > Will get back to you once I had concrete analysis.
> > >
> > > > This seems like it must be a common pitfall, any idea what we can do
> > > > to fix it
> > > > and avoid it in future? Am I misunderstanding the issue?
> >
> > First, for the example code you provided, I checked with llvm11, llvm12
> > and latest trunk llvm (llvm13-dev) and they all generated similar codes,
> > which may trigger verifier failure. Somehow you original code could be
> > different may only show up with a recent llvm, I guess.
> >
> > Checking llvm IR, the divergence between "w2 = w8" and "if r8 < 0x1000"
> > appears in insn scheduling phase related handling PHIs. Need to further
> > check whether it is possible to prevent the compiler from generating
> > such codes.
> >
> > The latest kernel already had the ability to track register equivalence.
> > However, the tracking is conservative for 32bit mov like "w2 = w8" as
> > you described in the above. if we have code like "r2 = r8; if r8 <
> > 0x1000 ...", we will be all good.
> >
> > The following hack fixed the issue,
> >
> > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> > index 58730872f7e5..54f418fd6a4a 100644
> > --- a/kernel/bpf/verifier.c
> > +++ b/kernel/bpf/verifier.c
> > @@ -7728,12 +7728,20 @@ static int check_alu_op(struct bpf_verifier_env
> > *env, struct bpf_insn *insn)
> >                                                   insn->src_reg);
> >                                           return -EACCES;
> >                                   } else if (src_reg->type == SCALAR_VALUE) {
> > +                                       /* If src_reg is in 32bit range,
> > there is
> > +                                        * no need to reset the ID.
> > +                                        */
> > +                                       bool is_32bit_src =
> > src_reg->umax_value <= 0x7fffffff;
> > +
> > +                                       if (is_32bit_src && !src_reg->id)
> > +                                               src_reg->id = ++env->id_gen;
> >                                           *dst_reg = *src_reg;
> >                                           /* Make sure ID is cleared
> > otherwise
> >                                            * dst_reg min/max could be
> > incorrectly
> >                                            * propagated into src_reg by
> > find_equal_scalars()
> >                                            */
> > -                                       dst_reg->id = 0;
> > +                                       if (!is_32bit_src)
> > +                                               dst_reg->id = 0;
> >                                           dst_reg->live |= REG_LIVE_WRITTEN;
> >                                           dst_reg->subreg_def =
> > env->insn_idx + 1;
> >                                   } else {
> >
> > Basically, for a 32bit mov insn like "w2 = w8", if we can ensure
> > that "w8" is 32bit and has no possibility that upper 32bit is set
> > for r8, we can declare them equivalent. This fixed your issue.
> >
> > Will try to submit a formal patch later.
>
> Ah.. I did not realise this equivalence tracking with reg.id was there
> for scalar values! I also didn't take any notice of the use of 32-bit
> operations in the assembly, thanks for pointing that out.
>
> Yes it sounds like this is certainly worth fixing in the kernel - even
> if Clang stops generating the code today it will probably start doing
> so again in the future. I can also help with the verifier work if
> needed.

I won't have time for this in the next few days.
Considering the current upstream merge window is close, yes, please
go head to work on this. Thanks!