Yonghong Song wrote: > > > On 8/23/22 4:15 PM, John Fastabend wrote: > > Eduard Zingerman wrote: > >> Propagate nullness information for branches of register to register > >> equality compare instructions. The following rules are used: > >> - suppose register A maybe null > >> - suppose register B is not null > >> - for JNE A, B, ... - A is not null in the false branch > >> - for JEQ A, B, ... - A is not null in the true branch > >> > >> E.g. for program like below: > >> > >> r6 = skb->sk; > >> r7 = sk_fullsock(r6); > >> r0 = sk_fullsock(r6); > >> if (r0 == 0) return 0; (a) > >> if (r0 != r7) return 0; (b) > >> *r7->type; (c) > >> return 0; > >> > >> It is safe to dereference r7 at point (c), because of (a) and (b). > > > > I think the idea makes sense. Perhaps Yonhong can comment seeing he was active > > on the LLVM thread. I just scanned the LLVM side for now will take a look > > in more detail in a bit. > > The issue is discovered when making some changes in llvm compiler. > I think it is good to add support in verifier so in the future > if compiler generates such code patterns, user won't get > surprised verification failure. > I agree. Read the LLVM thread as well.
