Messages in this thread | | | Date | Mon, 4 Jan 2016 22:02:28 +0100 | From | Borislav Petkov <> | Subject | Re: [PATCH v6 1/4] x86: Clean up extable entry format (and free up a bit) |
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On Mon, Jan 04, 2016 at 10:08:43AM -0800, Andy Lutomirski wrote: > All of that's correct, including the part where it's confusing. The > comments aren't the best. > > How about adding a comment like: > > ----- begin comment ----- > > The offset to the fixup is signed, and we're trying to use the high > bits for a different purpose. In C, we could just do: > > u32 class_and_offset = ((target - here) & 0x3fffffff) | class; > > Then, to decode it, we'd mask off the class and sign-extend to recover > the offset. > > In asm, we can't do that, because this all gets laundered through the > linker, and there's no relocation type that supports this chicanery. > Instead we cheat a bit. We first add a large number to the offset > (0x20000000). The result is still nominally signed, but now it's > always positive, and the two high bits are always clear. We can then > set high bits by ordinary addition or subtraction instead of using > bitwise operations. As far as the linker is concerned, all we're > doing is adding a large constant to the difference between here (".") > and the target, and that's a valid relocation type. > > In the C code, we just mask off the class bits and subtract 0x20000000 > to get the offset. > > ----- end comment -----
Yeah, that makes more sense, thanks.
That nasty "." current position thing stays in the way to do it cleanly. :-)
Anyway, ok, I see it now. It still feels a bit hacky to me. I probably would've added the third int to the exception table instead. It would've been much more straightforward and clean this way and I'd gladly pay the additional 6K growth.
-- Regards/Gruss, Boris.
ECO tip #101: Trim your mails when you reply.
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