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(-)
Unfortunately this still gives me 2 x `w0 = w0` on s390, but the
culprit seems to be not your patch, but rather that
adjust_insn_aux_data() is messing up zext_dst. I'll try to debug
further and come up with a fix.
