On 8/9/21 8:12 AM, Paul Chaignon wrote:
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
Thanks for reporting. I can reproduce the problem and will take a look soon.
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
