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

[André Przywara]

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).

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?

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.

Cheers,
Andre

> ---
> 
>  arch/arm/cpu/armv8/start.S | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/arm/cpu/armv8/start.S b/arch/arm/cpu/armv8/start.S
> index 002698b501c3..52cf1230c205 100644
> --- a/arch/arm/cpu/armv8/start.S
> +++ b/arch/arm/cpu/armv8/start.S
> @@ -67,8 +67,10 @@ pie_fixup:
>  	adr	x0, _start		/* x0 <- Runtime value of _start */
>  	ldr	x1, _TEXT_BASE		/* x1 <- Linked value of _start */
>  	sub	x9, x0, x1		/* x9 <- Run-vs-link offset */
> -	adr	x2, __rel_dyn_start	/* x2 <- Runtime &__rel_dyn_start */
> -	adr	x3, __rel_dyn_end	/* x3 <- Runtime &__rel_dyn_end */
> +	adrp    x2, __rel_dyn_start     /* x2 <- Runtime &__rel_dyn_start */
> +	add     x2, x2, #:lo12:__rel_dyn_start
> +	adrp    x3, __rel_dyn_end       /* x3 <- Runtime &__rel_dyn_end */
> +	add     x3, x3, #:lo12:__rel_dyn_end
>  pie_fix_loop:
>  	ldp	x0, x1, [x2], #16	/* (x0, x1) <- (Link location, fixup) */
>  	ldr	x4, [x2], #8		/* x4 <- addend */
>