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

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

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().

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

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