On Wed, Jan 10, 2018 at 12:26:25PM -0800, Linus Torvalds wrote: > On Wed, Jan 10, 2018 at 12:15 PM, Josh Poimboeuf <[email protected]> wrote: > > > > I think .altinstruction relocations *do* work if they're for the first > > instruction, and it's a jump or a call. > > Yes - for the alternative that is in-line - not in the "altinstruction" > section. > > Because then the alternative is in the right spot at link-time already. > > But the "altinstruction" section definitely should not have > relocations.
I misspoke, it's really .altinstr_replacement which has the replacement instructions. And it has a bunch of relocations: Relocation section [ 8] '.rela.altinstr_replacement' for section [ 7] '.altinstr_replacement' at offset 0x14439710 contains 355 entries: > I guess you could hack them up by hand by explicitly > trying to take the difference between the non-altinstruction and the > altinstruction into account, but it would be error-prone and fragile > as hell. apply_alternatives() already does that today. It actually seems pretty solid, except for the whole "only works on the first instruction" thing. > > I think Boris had a patch floating around to add an instruction decoder > > to alternatives, so you can do a call/jmp anywhere. > > .. and no, we're not doing that. Christ. > > People, we need to try to be *robust* here. That's doubly (triply!) > true of things like altinstructions where people - very much by design > - won't even *test* the alternatives very much, because very much by > design the altinstructions are only used on certain architectures or > in certain situations. > > And we almost certainly don't actuially _need_ relocations. But we > need to protect against the "oops, I didn't realize" issue, exactly > because testing won't actually catch the odd cases. If we need objtool to detect them, it's certainly possible. But maybe I missed the previous discussion -- what's the, um, alternative to relocations, when we have calls and jumps being patched in? > Because we don't want to be in the situation where some random poor > user hits it because they have an old CPU that no developer has, and > then the relocation will basically do completely random things. > > Imagine just how crazy that would be to debug. You'd be basically > executing insane code, and looking at the sources - or even the > binaries - it would _look_ completely sane. Well, I think we already made that deal with the devil when we added alternatives/paravirt/smp_locks/jump_labels/kprobes/ftrace/bpf, etc. -- Josh
