[PATCH 2/2] ARM: remove the 4x expansion presumption while decompressing the kernel

On Wed, Feb 16, 2011 at 6:48 PM, Nicolas Pitre [off-list ref] wrote:

On Wed, 16 Feb 2011, Grant Likely wrote:

quoted

Patch looks good to me, but on a related note, I'm looking for a way
to also find the end of .bss.

When John looks at adding support for appending an initrd and/or dtb
to the zimage, it will be important to make sure the start of the
initrd/dtb does not overlap the ram the kernel is expecting to be
empty and available. ?I was thinking about simply grepping System.map
for __end and doing some sed magic to extract the offset from the
beginning of the kernel.

I'd use the 'size' command instead:

$ arm-linux-size vmlinux
? text ? ?data ? ? bss ? ? dec ? ? hex filename
3707759 ?135316 ?137808 3980883 ?3cbe53 vmlinux

quoted

Do you think that is sufficient to protect appended data images?

Starting with my compressed/head.S rework, you'd have to:

1) Detect if there is some valuable data past _edata, and if so then
? increase the cached _edata value in r6 accordingly. ?This way, if
? zImage needs to relocate itself then that will include that data.

2) Modify the conditions and parameters for a zImage relocation to
? ensure the end of the kernel .bss is below the start of your data.

3) Modify the relocation of .bss entries in the GOT table so .bss is
? effectively moved after that appended data.

4) Tell the kernel about the data via extra ATAG/DTB entries.

Except for (4), this can be done with only a few assembly instructions.
And even in the ATAG case there is a good example in
arch/arm/boot/bootp/init.S for (4), but that could as well be done in C
instead, like the initial patch that John Bonesio did.

Cool, that matches my thinking.  We'll start working on this.

g.