[RFC] bootwrapper: prevent fdt from being overwritten by kernel

Paul Mackerras paulus at samba.org
Mon Oct 30 15:16:57 EST 2006

Mark A. Greer writes:

> If the fdt that's wrapped in a zImage isn't edited, it will still be
> sitting in the zImage when the kernel boots
> (i.e., at 0x400000 + <some offset into the zImage>).
> That's okay until you get a large enough kernel (e.g., initramfs)
> which will overwrite the fdt when the kernel is relocated to 0.

Oops :)

> To handle that, I changed the sandpoint.c:platform_init() to base the
> simple_alloc heap at the greater of either the end of the zImage or the end
> of the kernel once its decompressed & relocated to 0.  It then copies
> the dtb to an area allocated by simple_alloc so I know its safe from
> being overwritten.

That sounds reasonable.

> In addition, I moved the vmlinux elf header decompress code from
> prep_kernel() to its own routine and called it before platform_init.
> I then pass the elf header ptr into platform_init.  That way,
> platform_init can figure out where the end of the kernel will
> be and put the dtb beyond it.  The OF code can now change
> claim_base in platform_init instead of needing the platform_ops.image_hdr
> hook.

Hmmm.  What if platform_init needs to e.g. turn on caches or do other
things that need to be done before decompressing?  I think it's
important to provide some way for the platform code to do things
really early on.


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