On Thu, 18 Feb 2021 at 00:12, KP Singh <kpsingh@xxxxxxxxxx> wrote:
>
> On Wed, Feb 17, 2021 at 7:30 PM Ilya Leoshkevich <iii@xxxxxxxxxxxxx> wrote:
> >
> > On Wed, 2021-02-17 at 09:28 +0000, Brendan Jackman wrote:
> > > As pointed out by Ilya and explained in the new comment, there's a
> > > discrepancy between x86 and BPF CMPXCHG semantics: BPF always loads
> > > the value from memory into r0, while x86 only does so when r0 and the
> > > value in memory are different. The same issue affects s390.
> > >
> > > At first this might sound like pure semantics, but it makes a real
> > > difference when the comparison is 32-bit, since the load will
> > > zero-extend r0/rax.
> > >
> > > The fix is to explicitly zero-extend rax after doing such a
> > > CMPXCHG. Since this problem affects multiple archs, this is done in
> > > the verifier by patching in a BPF_ZEXT_REG instruction after every
> > > 32-bit cmpxchg. Any archs that don't need such manual zero-extension
> > > can do a look-ahead with insn_is_zext to skip the unnecessary mov.
> > >
> > > Reported-by: Ilya Leoshkevich <iii@xxxxxxxxxxxxx>
> > > Fixes: 5ffa25502b5a ("bpf: Add instructions for atomic_[cmp]xchg")
> > > Signed-off-by: Brendan Jackman <jackmanb@xxxxxxxxxx>
> > > ---
> > >
> > > Differences v2->v3[1]:
> > > - Moved patching into fixup_bpf_calls (patch incoming to rename this
> > > function)
> > > - Added extra commentary on bpf_jit_needs_zext
> > > - Added check to avoid adding a pointless zext(r0) if there's
> > > already one there.
> > >
> > > Difference v1->v2[1]: Now solved centrally in the verifier instead of
> > > specifically for the x86 JIT. Thanks to Ilya and Daniel for the
> > > suggestions!
> > >
> > > [1] v2:
> > > https://lore.kernel.org/bpf/08669818-c99d-0d30-e1db-53160c063611@xxxxxxxxxxxxx/T/#t
> > > v1:
> > > https://lore.kernel.org/bpf/d7ebaefb-bfd6-a441-3ff2-2fdfe699b1d2@xxxxxxxxxxxxx/T/#t
> > >
> > > kernel/bpf/core.c | 4 +++
> > > kernel/bpf/verifier.c | 26
> > > +++++++++++++++++++
> > > .../selftests/bpf/verifier/atomic_cmpxchg.c | 25
> > > ++++++++++++++++++
> > > .../selftests/bpf/verifier/atomic_or.c | 26
> > > +++++++++++++++++++
> > > 4 files changed, 81 insertions(+)
> >
> > [...]
> >
> > > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> > > index 16ba43352a5f..a0d19be13558 100644
> > > --- a/kernel/bpf/verifier.c
> > > +++ b/kernel/bpf/verifier.c
> > > @@ -11662,6 +11662,32 @@ 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] = { [1] =
> > > BPF_ZEXT_REG(BPF_REG_0) };
> > > +
> > > + if (!memcmp(&insn[1], &zext_patch[1],
> > > sizeof(struct bpf_insn)))
> > > + /* Probably done by
> > > opt_subreg_zext_lo32_rnd_hi32. */
> > > + continue;
> > > +
> >
> > Isn't opt_subreg_zext_lo32_rnd_hi32() called after fixup_bpf_calls()?
>
> Indeed, this check should not be needed.
Ah yep, right. Do you folks think I should keep the optimisation (i.e.
move this memcmp into opt_subreg_zext_lo32_rnd_hi32)? It feels like a
bit of a toss-up to me.
