On Tue, Apr 28, 2020 at 02:46:27PM +0200, Peter Zijlstra wrote: > > I'm thinking something like this should fix it. Peter, does this look > > ok? > > Unfortunate. But also, I fear, insufficient. Specifically consider > things like: > > ALTERNATIVE "jmp 1f", > "alt... > "..." > "...insn", X86_FEAT_foo > 1: > > This results in something like: > > > .text .altinstr_replacement > e8 xx ... > 90 > 90 > ... > 90 > > Where all our normal single byte nops (0x90) are unreachable with > undefined CFI, but the alternative might have CFI, which is never > propagated. > > We ran into this with the validate_alternative stuff from Alexandre.
I don't get what you're saying. We decided not to allow CFI changes in alternatives. And how does this relate to my patch? > > @@ -773,12 +772,26 @@ static int handle_group_alt(struct objtool_file *file, > > struct instruction *last_orig_insn, *last_new_insn, *insn, *fake_jump = > > NULL; > > unsigned long dest_off; > > > > + /* > > + * For uaccess checking, propagate the STAC/CLAC from the alternative > > + * to the original insn to avoid paths where we see the STAC but then > > + * take the NOP instead of CLAC (and vice versa). > > + */ > > + if (!orig_insn->ignore_alts && orig_insn->type == INSN_NOP && > > + *new_insn && > > + ((*new_insn)->type == INSN_STAC || > > + (*new_insn)->type == INSN_CLAC)) > > + orig_insn->type = (*new_insn)->type; > > Also, this generates a mis-match between actual instruction text and > type. We now have a single byte instruction (0x90) with the type of a 3 > byte (SLAC/CLAC). Which currently isn't a problem, but I'm looking at > adding infrastructure for having objtool rewrite instructions. But it doesn't actually change the original instruction bytes, it just changes the decoding. Is that really going to be a problem? We do that in other places as well, and it helps simplify code flow. Also might I ask why you're going to be rewriting instructions? That sounds scary. > So rather than hacking around this issue, should we not make > create_orc() smarter? Maybe, though I don't see how that logic belongs in create_orc(). It might be tricky distinguishing between normal undefined and "undefined because of a skip_orig". Right now create_orc() is blissfully ignorant. -- Josh
