R11 is invalid with LLVM 12 and later

[Paul Chaignon <paul@cilium.io>]

Hello,

While trying to use LLVM 12.0.0 in Cilium, we've noticed that it can
generate invalid BPF bytecode:

    $ clang --version
    Ubuntu clang version 12.0.0-++20210409092622+fa0971b87fb2-1~exp1~20210409193326.73
    Target: x86_64-pc-linux-gnu
    Thread model: posix
    InstalledDir: /usr/bin
    $ make -C bpf -j6 KERNEL=419
    $ llvm-objdump -D -section=2/20 bpf/bpf_lxc.o | grep -i r11
         171:   7b ba 18 ff 00 00 00 00 *(u64 *)(r10 - 232) = r11
         436:   79 ab 18 ff 00 00 00 00 r11 = *(u64 *)(r10 - 232)
         484:   bf 8b 00 00 00 00 00 00 r11 = r8

That bytecode is of course rejected by the verifier:

    171: (7b) *(u64 *)(r10 -232) = r11
    R11 is invalid

LLVM 12.0.1 and latest LLVM sources (e.g., commit 2b4a1d4b from today)
have the same issue. We've bisected it to LLVM commit 552c6c23
("PR44406: Follow behavior of array bound constant folding in more
recent versions of GCC."), but that could just be the commit where
the regression was exposed in Cilium's case.

--
Paul