On Wed, Oct 14, 2020 at 09:27:17PM -0700, John Fastabend wrote: > Alexei Starovoitov wrote: > > On Wed, Oct 14, 2020 at 09:04:23PM -0700, John Fastabend wrote: > > > Andrii Nakryiko wrote: > > > > On Wed, Oct 14, 2020 at 10:59 AM Alexei Starovoitov > > > > <alexei.starovoitov@xxxxxxxxx> wrote: > > > > > > > > > > From: Alexei Starovoitov <ast@xxxxxxxxxx> > > > > > > > > > > The 64-bit JEQ/JNE handling in reg_set_min_max() was clearing reg->id in either > > > > > true or false branch. In the case 'if (reg->id)' check was done on the other > > > > > branch the counter part register would have reg->id == 0 when called into > > > > > find_equal_scalars(). In such case the helper would incorrectly identify other > > > > > registers with id == 0 as equivalent and propagate the state incorrectly. > > > > > > One thought. It seems we should never have reg->id=0 in find_equal_scalars() > > > would it be worthwhile to add an additional check here? Something like, > > > > > > if (known_reg->id == 0) > > > return > > > > > > Or even a WARN_ON_ONCE() there? Not sold either way, but maybe worth thinking > > > about. > > > > That cannot happen anymore due to > > if (dst_reg->id && !WARN_ON_ONCE(dst_reg->id != other_branch_regs[insn->dst_reg].id)) > > check in the caller. > > I prefer not to repeat the same check twice. Also I really don't like defensive programming. > > if (known_reg->id == 0) > > return; > > is exactly that. > > If we had that already, as Andrii argued in the original thread, we would have > > never noticed this issue. <, >, <= ops would have worked, but == would be > > sort-of working. It would mark one branch instead of both, and sometimes > > neither of the branches. I'd rather have bugs like this one hurting and caught > > quickly instead of warm feeling of being safe and sailing into unknown. > > Agree. Although a WARN_ON_ONCE would have also been caught. Right. Such WARN_ON_ONCE would definitely have been nice either in the caller or in the callee. If I could have thought that id could be zero somehow here. In retrospect it makes sense that there is possibility that IDs of regs in this_branch and other_branch may diverge. Hence I'm adding the warn to check for this specific divergence.
