Re: [PATCH v5 07/21] x86, boot: Fix run_size calculation

[Kees Cook]

On Fri, Apr 15, 2016 at 1:31 AM, Ingo Molnar <mingo@xxxxxxxxxx> wrote:
>
> * Kees Cook <keescook@xxxxxxxxxxxx> wrote:
>
>> From: Yinghai Lu <yinghai@xxxxxxxxxx>
>>
>> Currently, the kernel run_size (size of code plus brk and bss) is
>> calculated via the shell script arch/x86/tools/calc_run_size.sh.
>> It gets the file offset and mem size of for the .bss and .brk sections in
>> vmlinux, and adds them as follows:
>
> 'of for'?
>
>> So, from this we can see that the existing run_size calculation is
>> 0x400000 too high.
>
> Btw., why is it too high? Were some sections discarded?

I'm not sure why it's that specific value, but the old method used the
section's ELF _file_ position, not the virtual address, and that
seemed to cause the problem.

>
>> [...] And, as it turns out, the correct run_size is
>> actually equal to VO_end - VO_text, which is certainly easier to calculate.
>> _end: 0xffffffff8205f000
>> _text:0xffffffff81000000
>>
>> 0xffffffff8205f000 - 0xffffffff81000000 = 0x105f000
>>
>> As a result, run_size is a simple constant, so we don't need to pass it
>> around; we already have voffset.h such things.
>
> (Typo.)
>
>> [...] We can share voffset.h
>> between misc.c and header.S instead of getting run_size in other ways.
>> This patch moves voffset.h creation code to boot/compressed/Makefile,
>> and switches misc.c to use the VO_end - VO_text calculation.
>
> Btw., 'run_size' is a pretty meaningless name, it doesn't really tell us the most
> important thing: that this is the complete size of the uncompressed kernel, the
> total physically continuous size we need to allocate to it so it can execute?
>
> So can we rename it to something more expressive, such as kernel_total_size or so?

You got it. Thanks again for digging through all this!

-Kees

-- 
Kees Cook
Chrome OS & Brillo Security