Re: [PATCH v2 bpf-next] bpf: Explicitly zero-extend R0 after 32-bit cmpxchg

[Brendan Jackman <jackmanb@google.com>]

On Tue, 16 Feb 2021 at 20:55, Ilya Leoshkevich <iii@linux.ibm.com> wrote:
>
> On Tue, 2021-02-16 at 14:19 +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@linux.ibm.com>
> > Fixes: 5ffa25502b5a ("bpf: Add instructions for atomic_[cmp]xchg")
> > Signed-off-by: Brendan Jackman <jackmanb@google.com>
> > ---
> >
> > Difference from v1[1]: Now solved centrally in the verifier instead
> > of
> >   specifically for the x86 JIT. Thanks to Ilya and Daniel for the
> > suggestions!
> >
> > [1]
> > https://lore.kernel.org/bpf/d7ebaefb-bfd6-a441-3ff2-2fdfe699b1d2@iogearbox.net/T/#t
> >
> >  kernel/bpf/verifier.c                         | 36
> > +++++++++++++++++++
> >  .../selftests/bpf/verifier/atomic_cmpxchg.c   | 25 +++++++++++++
> >  .../selftests/bpf/verifier/atomic_or.c        | 26 ++++++++++++++
> >  3 files changed, 87 insertions(+)
>
> I tried this with my s390 atomics patch, and it's working, thanks!
>
> I was thinking whether this could go through the existing zext_dst
> flag infrastructure, but it probably won't play too nicely with the
> x86_64 JIT, which doesn't override bpf_jit_needs_zext().

Ah right, I actually didn't understand what the
opt_subreg_zext_lo32_rnd_hi32 was doing until now so didn't consider
this.

But yeah I think cmpxchg is properly special here because the zext is
sometimes (e.g. on x86_64) needed even on architectures that don't
_generally_ need explicit zext.

I think I'll update some comments to reflect these learnings, thanks.

> Acked-by: Ilya Leoshkevich <iii@linux.ibm.com>
> Tested-by: Ilya Leoshkevich <iii@linux.ibm.com>
>
> [...]
>