[PATCH] arm64: Add support for bigger u-boot when CONFIG_POSITION_INDEPENDENT=y

[Michal Simek]

Hi,

On 02. 09. 20 16:43, André Przywara wrote:
> On 02/09/2020 12:15, Michal Simek wrote:
> 
> Hi,
> 
>> From: "Edgar E. Iglesias" <edgar.iglesias at xilinx.com>
>>
>> When U-Boot binary exceeds 1MB with CONFIG_POSITION_INDEPENDENT=y
>> compilation error is shown:
>> /mnt/disk/u-boot/arch/arm/cpu/armv8/start.S:71:(.text+0x3c): relocation
>> truncated to fit: R_AARCH64_ADR_PREL_LO21 against symbol `__rel_dyn_end'
>> defined in .bss_start section in u-boot.
>>
>> It is caused by adr instruction which permits the calculation of any byte
>> address within +- 1MB of the current PC.
>> Because U-Boot is bigger then 1MB calculation is failing.
>>
>> The patch is using adrp/add instructions where adrp shifts a signed, 21-bit
>> immediate left by 12 bits (4k page), adds it to the value of the program
>> counter with the bottom 12 bits cleared to zero. Then add instruction
>> provides the lower 12 bits which is offset within 4k page.
>> These two instructions together compose full 32bit offset which should be
>> more then enough to cover the whole u-boot size.
>>
>> Signed-off-by: Edgar E. Iglesias <edgar.iglesias at xilinx.com>
>> Signed-off-by: Michal Simek <michal.simek at xilinx.com>
> 
> It's a bit scary that you need more than 1MB, but indeed what you do
> below is the canonical pattern to get the full range of PC relative
> addressing (this is used heavily in Trusted Firmware, for instance).

We have generic u-boot where we are enabling all features which you can
use and it is up to everybody to disable what they don't need. We are at
1MB limit that's why I spot this issue and trying to solve it.

> 
> The only thing to keep in mind is that this assumes that the load
> address of the binary is 4K aligned, so that the low 12 bits of the
> symbol stay the same. I wonder if we should enforce this somehow? But
> the load address is not controlled by the build process (the whole
> purpose of PIE), so that's not doable just in the build system?

That's more question to you if we can enforce is somehow. :-)


> Shall we at least document this? I guess typical load address are
> actually quite well aligned, so it might not be an issue in practice.

maybe note this in Kconfig help for POSITION_INDEPENDENT?
Any other location?

Thanks,
Michal