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

[Ilya Leoshkevich <iii@linux.ibm.com>]

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@linux.ibm.com>
> Fixes: 5ffa25502b5a ("bpf: Add instructions for atomic_[cmp]xchg")
> Signed-off-by: Brendan Jackman <jackmanb@google.com>
> ---
> 
> 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@iogearbox.net/T/#t
>     v2: 
> https://lore.kernel.org/bpf/08669818-c99d-0d30-e1db-53160c063611@iogearbox.net/T/#t
>     v1: 
> https://lore.kernel.org/bpf/d7ebaefb-bfd6-a441-3ff2-2fdfe699b1d2@iogearbox.net/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.