Messages in this thread |  | | Date | Sun, 7 Jan 2018 12:46:45 +0100 | From | Borislav Petkov <> | Subject | Re: [PATCH v3 01/13] x86/retpoline: Add initial retpoline support |
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On Sun, Jan 07, 2018 at 09:40:42AM +0000, David Woodhouse wrote: > Right, so it all tends to work out OK purely by virtue of the fact that > oldinstr and altinstr end up far enough apart in the image that they're > 5-byte jumps. Which isn't perfect but we've lived with worse.
Well, the reference point is important. And I don't think we've done more involved things than jumping back to something in .text proper. However, I think I know how to fix this so that arbitrary jump offsets would work but I need to talk to our gcc guys first.
If the jump is close enough for 2 bytes, then it should work as long as the offset to the target doesn't change.
The main thing recompute_jumps() does is turn 5-byte jumps - which gas creates because the jump target is in .text but the jump itself is in .altinstr_replacement - into 2-byte ones. Because when you copy the jump back into .text, the offset might fit in a signed byte all of a sudden.
There are still some nasties with forcing 5-byte jumps but I think I know how to fix those. Stay tuned...
> I'm relatively pleased that we've managed to eliminate this as a > dependency for inverting the X86_FEATURE_RETPOLINE logic though, by > following Linus' suggestion to just emit the thunk inline instead of > calling the same one as GCC. > > The other fun one for alternatives is in entry_64.S, where we really > need the return address of the call instruction to be *precisely* the > .Lentry_SYSCALL_64_after_fastpath_call label, so we have to eschew the > normal NOSPEC_CALL there:
So CALL, as the doc says, pushes the offset of the *next* insn onto the stack and branches to the target address.
So I'm thinking, as long as the next insn doesn't move and gcc doesn't pad anything, you're fine.
However, I suspect that I'm missing something else here and I guess I'll have more clue if I look at the whole thing. So can you point me to your current branch so that I can take a look at the code?
Thx.
-- Regards/Gruss, Boris.
SUSE Linux GmbH, GF: Felix Imendörffer, Jane Smithard, Graham Norton, HRB 21284 (AG Nürnberg) --
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