[U-Boot] [PATCH 3/4] ARMv8: Allow dynamic early stack pointer

[Stephen Warren]

On 01/08/2018 03:11 PM, Simon Glass wrote:
> Hi Stephen,
> 
> On 8 January 2018 at 11:34, Stephen Warren <swarren at wwwdotorg.org> wrote:
>>
>> On 01/07/2018 08:40 PM, Simon Glass wrote:
>>>
>>> Hi Stephen,
>>>
>>> On 19 December 2017 at 18:30, Stephen Warren <swarren at wwwdotorg.org> wrote:
>>>>
>>>> From: Stephen Warren <swarren at nvidia.com>
>>>>
>>>> U-Boot typically uses a hard-coded value for the stack pointer before
>>>> relocation. Implement option SYS_INIT_SP_BSS_OFFSET to instead calculate
>>>> the initial SP at run-time. This is useful to avoid hard-coding addresses
>>>> into U-Boot, so that can be loaded and executed at arbitrary addresses and
>>>> thus avoid using arbitrary addresses at runtime. This option's value is
>>>> the offset added to &_bss_start in order to calculate the stack pointer.
>>>> This offset should be large enough so that the early malloc region, global
>>>> data (gd), and early stack usage do not overlap any appended DTB.
>>>
>>>
>>> I don't see why this is an offset from bss_start - shouldn't it be bss_end?
>>
>>
>> BSS can vary in size based on the set of config options enabled, code growth/shrinkage, etc. Thus, basing the initial SP address on bss_end would provide too much variability, and hence would be unsafe, whereas basing the initial SP address on bss_start always provides the exact same amount of available stack space (assuming an identical DTB for the comparison).
> 
> OK. I suppose we are no worse off anyway, since the DTB can vary in size.

I've sent a patch to check for overflow.