[U-Boot] [PATCH 2/2] arm64: booti: allow to place kernel image anywhere in physical memory

[Mark Rutland]

On Tue, Feb 28, 2017 at 12:15:09PM -0500, Tom Rini wrote:
> On Wed, Mar 01, 2017 at 02:03:58AM +0900, Masahiro Yamada wrote:
> > 2017-02-27 7:41 GMT+09:00 Tom Rini <trini at konsulko.com>:
> > > On Thu, Feb 23, 2017 at 10:31:17AM -0500, Tom Rini wrote:

> > > c) I'm not convinced your math above is correct.  images->ep is where we
> > > were put in memory.  This is what we should make sure is 2MiB aligned,
> > > and then add to it the text_offset.  And some quick testing with
> > > CONFIG_ARM64_RANDOMIZE_TEXT_OFFSET enabled Images :)
> > 
> > My intention is
> > 
> >   images->ep  =  (2MiB aligned base)  + text_offset
> > 
> > If this equation is met, the image is already placed at the bootable position.
> > We can skip the relocation.
> > 
> > Theoretically, we can not know the value of text_offset in advance
> > (especially for CONFIG_ARM64_RANDOMIZE_TEXT_OFFSET).
> > However, in practice, we know text_offset is 0x80000.

Per the arm64 Image header, no particular value  of text_offset should
be assumed. Please do not assume a particular value

It has always been the intent that bootloaders should read this, though
this evidently wasn't very clear (and the bootwrapper set a bad
example). I tried to clear this up with documentation updates (and the
addition of ARM64_RANDOMIZE_TEXT_OFFSET).

> > If we put the image at 2MiB aligned base, the relocation would
> > always happen.
> 
> Correct.  But I honestly don't know if non-randomized text offset is the
> common case people will optimize for or randomized for added security will be
> the more common case.  

FWIW, the randomized text_offset is a bootloader debugging/testing
feature, and there's no security aspect to it.

It was added [1] as an additional to hint to bootloader authors that
they must respect the text_offset field.

Thanks,
Mark.

[1] https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=da57a369d3bc5cd61db90f7e9555840381db9b09