[PATCH 0/8] ARM: clean up PC-relative arithmetic

[Ard Biesheuvel]

On 4 August 2016 at 11:49, Russell King - ARM Linux
<linux at armlinux.org.uk> wrote:
> On Thu, Aug 04, 2016 at 09:17:04AM +0200, Ard Biesheuvel wrote:
>> On 3 August 2016 at 20:17, Russell King - ARM Linux
>> <linux at armlinux.org.uk> wrote:
>> > I don't buy that argument, sorry, and the argument is actually wrong.
>> > No, we're _not_ letting the linker do the calculations for us, we're
>> > letting the linker do _some_ of the calculation, but not all.
>> >
>> > What you're replacing the above with is stuff like (I guess, because
>> > I've no idea what this :pc_g0: notation is):
>> >
>> >         add     rX, pc, #(sym - . - 8) & 0xff
>> >         add     rX, rX, #(sym - . - 4) & 0xff00
>> >         add     rX, rX, #(sym - .) & 0xff0000
>> >
>> > which I think is a more complex (and less obvious) way to calculate it.
>> > It's also buggy when we end up with a relative offset greater than 16MB,
>> > which we have in multi-zImage kernels.
>> >
>>
>> Even if you think this is a more complex way to calculate it, at least
>> it is encapsulated in a single macro instead of having similar but not
>> identical open coded instances all over the place.
>
> ... and, it may come as a shocker, but I don't have a problem with
> that.
>
>> As for the range: the ldr/str variants have 28 bits of range (2x
>> scaled 8 bit immediate for the adds and a single unscaled 12 bit
>> immediate for the ldr/str). The adr variant has 26 bits (3x scaled
>> immediate counting from bit 2) range for word aligned symbols, which
>> gives us +/- 64 MB, which should be plenty. The only pathological
>> outlier is allyesconfig, but that uses Thumb2 anyway.
>
> Our existing code allows for a range of the full address space - the only
> thing it relies upon is that the literal data is placed within reach of
> the code - which it will be, because it's always placed near the code
> which is using it.
>
>> The relocations documented here
>> http://infocenter.arm.com/help/topic/com.arm.doc.ihi0044f/IHI0044F_aaelf.pdf
>
> Right, so it's an EABI thing, and I guess you haven't tested OABI
> builds, where I suspect these relocations aren't supported.
>

I suppose that's a fair point. But then, I'm only 40 so I am too young
to remember this OABI stuff anyway. Does it require GCC 2.95 from your
toolchain museum?