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

[Simon Glass]

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.

>
>> Also this seems error-prone since we don't know how large the DTB is.
>> Can we improve this, e.g. by:
>>
>> - using binman to provide the stack start value or offset
>
>
> I'd rather not involve binman in the code generation process. Packaging is fine, but I think the source code and makefiles should dictate everything that goes into the actual binary to keep things simple.
>
> I did originally think about having binman patch up the init SP address, but rejected it due to the complexity added by the extra step, and the fact that we'd be inventing a new tool (the new part of binman implemented for this feature) to do what we already have a perfectly good tool for already; the linker. I'm also looking to backport this change to an older branch of U-Boot that doesn't have binman, although I believe I'd have the same thought process either way.
>

There is actually a feature for this, or something similar:

http://git.denx.de/?p=u-boot.git;a=blob;f=tools/binman/README;h=08c3e56bdef8d3ee47ac2ec86411465a78dbf567;hb=HEAD#l476

>> - checking the DTB size and automatically using the address
>> immediately after it finishes (again I suppose binman could provide
>> that)
>
>
> Perhaps we can add an extra step in binman that validates the build; i.e. that (SYS_INIT_SP_BSS_OFFSET - size_of_dtb) > some_minimum_stack_size. That would be a useful error check, but prevent binman having to edit parts of the binary that were already created by the main build process. Note that for a given build, it should be completely deterministic whether DTB corruption occurs, and whether that corruption actually impacts U-Boot's operation, so any such check would be pretty much equivalent to just running U-Boot and seeing if it works. Admittedly the check might save some annoying debugging though, so would be a good idea.

Sounds good. It could even be done in Makefile - we have a few checks
that that at present. But if binman is a better place, that is fine.

Regards,
Simon