Yonghong Song <yhs@xxxxxx> writes: > On 9/1/20 1:07 PM, Andrii Nakryiko wrote: >> On Mon, Aug 24, 2020 at 11:47 PM Yonghong Song <yhs@xxxxxx> wrote: >>> >>> bpf selftest test_progs/test_sk_assign failed with llvm 11 and llvm 12. >>> Compared to llvm 10, llvm 11 and 12 generates xor instruction which >> >> Does this mean that some perfectly working BPF programs will now fail >> to verify on older kernels, if compiled with llvm 11 or llvm 12? If > > Right. > >> yes, is there something that one can do to prevent Clang from using >> xor in such situations? > > The xor is generated by the combination of llvm simplifyCFG and > instrCombine phase. > > The following is a hack to prevent compiler from generating xor's. Wait, so this means that we can no longer tell people to just use the newest LLVM version - now we have to keep track of a minimum *and* maximum LLVM version for each kernel version? Could we maybe try to not *keep* making it harder for people to use BPF? :/ As for the patch, sure, make the verifier smarter, but I also feel like LLVM should be fixed to not suddenly emit such xor instructions... -Toke
