On Wed, Feb 24, 2021 at 03:16:18PM +0100, Ilya Leoshkevich wrote:
> On Tue, 2021-02-23 at 15:08 +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.
> >
> > There was actually already logic to patch in zero-extension insns
> > after 32-bit cmpxchgs, in opt_subreg_zext_lo32_rnd_hi32. To avoid
> > bloating the prog with unnecessary movs, we now explicitly check and
> > skip that logic for this case.
> >
> > Reported-by: Ilya Leoshkevich <iii@xxxxxxxxxxxxx>
> > Fixes: 5ffa25502b5a ("bpf: Add instructions for atomic_[cmp]xchg")
> > Signed-off-by: Brendan Jackman <jackmanb@xxxxxxxxxx>
> > ---
> >
> > Differences v3->v4[1]:
> > - Moved the optimization against pointless zext into the correct
> > place:
> > opt_subreg_zext_lo32_rnd_hi32 is called _after_ fixup_bpf_calls.
> >
> > 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] v3:
> > https://lore.kernel.org/bpf/08669818-c99d-0d30-e1db-53160c063611@xxxxxxxxxxxxx/T/#t
> > 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 | 33
> > +++++++++++++++++--
> > .../selftests/bpf/verifier/atomic_cmpxchg.c | 25 ++++++++++++++
> > .../selftests/bpf/verifier/atomic_or.c | 26 +++++++++++++++
> > 4 files changed, 86 insertions(+), 2 deletions(-)
>
> I think I managed to figure out what is wrong with
> adjust_insn_aux_data(): insn_has_def32() does not know about BPF_FETCH.
> I'll post a fix shortly; in the meantime, based on my debugging
> experience and on looking at the code for a while, I have a few
> comments regarding the patch.
Ah. good catch.
If adjust_insn_aux_data()/insn_has_def32() is fixed to set zext_dst
properly for BPF_FETCH, then that alone should be enough for s390?
