Re: [PATCH v6 1/4] x86: Clean up extable entry format (and free up a bit)

[Borislav Petkov]

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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