Re: [PATCH v4 bpf-next] bpf: Explicitly zero-extend R0 after 32-bit cmpxchg

[Brendan Jackman <jackmanb@google.com>]

On Wed, 24 Feb 2021 at 23:14, Martin KaFai Lau <kafai@fb.com> wrote:
>
> On Wed, Feb 24, 2021 at 10:32:28AM +0100, Brendan Jackman wrote:
> > On Wed, 24 Feb 2021 at 06:48, Martin KaFai Lau <kafai@fb.com> wrote:
> > >
> > > On Tue, Feb 23, 2021 at 03:08:45PM +0000, Brendan Jackman wrote:
> > > [ ... ]
> > >
> > > > diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
> > > > index 0ae015ad1e05..dcf18612841b 100644
> > > > --- a/kernel/bpf/core.c
> > > > +++ b/kernel/bpf/core.c
> > > > @@ -2342,6 +2342,10 @@ bool __weak bpf_helper_changes_pkt_data(void *func)
> > > >  /* Return TRUE if the JIT backend wants verifier to enable sub-register usage
> > > >   * analysis code and wants explicit zero extension inserted by verifier.
> > > >   * Otherwise, return FALSE.
> > > > + *
> > > > + * The verifier inserts an explicit zero extension after BPF_CMPXCHGs even if
> > > > + * you don't override this. JITs that don't want these extra insns can detect
> > > > + * them using insn_is_zext.
> > > >   */
> > > >  bool __weak bpf_jit_needs_zext(void)
> > > >  {
> > > > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> > > > index 3d34ba492d46..ec1cbd565140 100644
> > > > --- a/kernel/bpf/verifier.c
> > > > +++ b/kernel/bpf/verifier.c
> > > > @@ -11061,8 +11061,16 @@ static int opt_subreg_zext_lo32_rnd_hi32(struct bpf_verifier_env *env,
> > > >                        */
> > > >                       if (WARN_ON(!(insn.imm & BPF_FETCH)))
> > > >                               return -EINVAL;
> > > > -                     load_reg = insn.imm == BPF_CMPXCHG ? BPF_REG_0
> > > > -                                                        : insn.src_reg;
> > > > +                     /* There should already be a zero-extension inserted after BPF_CMPXCHG. */
> > > > +                     if (insn.imm == BPF_CMPXCHG) {
> > > > +                             struct bpf_insn *next = &insns[adj_idx + 1];
> > > > +
> > > > +                             if (WARN_ON(!insn_is_zext(next) || next->dst_reg != insn.src_reg))
> > > > +                                     return -EINVAL;
> > > > +                             continue;
> > > This is to avoid zext_patch again for the JITs with
> > > bpf_jit_needs_zext() == true.
> > >
> > > IIUC, at this point, aux[adj_idx].zext_dst == true which
> > > means that the check_atomic() has already marked the
> > > reg0->subreg_def properly.
> >
> > That's right... sorry I'm not sure if you're implying something here
> > or just checking understanding?
> >
> > > > +                     }
> > > > +
> > > > +                     load_reg = insn.src_reg;
> > > >               } else {
> > > >                       load_reg = insn.dst_reg;
> > > >               }
> > > > @@ -11666,6 +11674,27 @@ static int fixup_bpf_calls(struct bpf_verifier_env *env)
> > > >                       continue;
> > > >               }
> > > >
> > > > +             /* BPF_CMPXCHG always loads a value into R0, therefore always
> > > > +              * zero-extends. However some archs' equivalent instruction only
> > > > +              * does this load when the comparison is successful. So here we
> > > > +              * add a BPF_ZEXT_REG after every 32-bit CMPXCHG, so that such
> > > > +              * archs' JITs don't need to deal with the issue. Archs that
> > > > +              * don't face this issue may use insn_is_zext to detect and skip
> > > > +              * the added instruction.
> > > > +              */
> > > > +             if (insn->code == (BPF_STX | BPF_W | BPF_ATOMIC) && insn->imm == BPF_CMPXCHG) {
> > > > +                     struct bpf_insn zext_patch[2] = { *insn, BPF_ZEXT_REG(BPF_REG_0) };
> > > Then should this zext_patch only be done for "!bpf_jit_needs_zext()"
> > > such that the above change in opt_subreg_zext_lo32_rnd_hi32()
> > > becomes unnecessary?
> >
> > Yep that would work but I IMO it would be a more fragile expression of
> > the logic: instead of directly checking whether something was done
> > we'd be looking at a proxy for another part of the system's behaviour.
> > I don't think it would win us anything in terms of clarity either?
> hmmm... I find it quite confusing to read.
>
> While the current opt_subreg_zext_lo32_rnd_hi32() has
> already been doing the actual zext patching work based
> on the zext_dst marking,
> this patch does zext patch for cmpxchg before opt_subreg_zext_lo32_rnd_hi32()
> even the zext_dst has already been marked.
>
> Then later in opt_subreg_zext_lo32_rnd_hi32(), code is
> added to avoid doing the zext patch again for the
> "!bpf_jit_needs_zext()" case.
>
> If there is other cases later, then changes have to be made
> in both places, one does zext patch and then another to
> avoid double patch for the "!bpf_jit_needs_zext()" case.
>
> Why not only patch it when there is no other places doing it?
>
> It may be better to do the zext patch for cmpxchg in
> opt_subreg_zext_lo32_rnd_hi32() also.  Then all zext patch
> is done in one place.  Something like:
>
> static int opt_subreg_zext_lo32_rnd_hi32(struct bpf_verifier_env *env,
>                                         const union bpf_attr *attr)
> {
>
>         for (i = 0; i < len; i++) {
>                 /* ... */
>
>                 if (!bpf_jit_needs_zext() && !is_cmpxchg_insn(insn))
>                         continue;
>
>                 /* do zext patch */
>         }
> }
>
> Would that work?

Yep - this is so much simpler and clearer. Thanks! Sending another spin.