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

[Andy Lutomirski <luto@amacapital.net>]

On Mon, Jan 4, 2016 at 1:02 PM, Borislav Petkov <bp@alien8.de> wrote:
> 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.

Josh will argue with you if he sees that :)

We could maybe come up with a way to compress the table and get that
space and more back, but maybe that should be a follow-up that someone
else can do if they're inspired.

--Andy

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