[RFC/PATCH] of: Mark property::value as const

On Mon, Feb 13, 2017 at 8:50 PM, Stephen Boyd [off-list ref] wrote:

The 'blob' we pass into populate_properties() is marked as const,
but we cast that const away when we assign the result of
fdt_getprop_by_offset() to pp->value. Let's mark value as const
instead, so that code can't mistakenly write to the value of the
property that we've so far advertised as const.

Unfortunately, this exposes a problem with the fdt resolver code,
where we overwrite the value member of properties of phandles to
update them with their final value. Add a comment for now to
indicate where we're potentially writing over const data.

You can see the problem here by loading an overlay dtb into
the kernel via the request firmware helper method (not direct
loading) and then passing that tree to the resolver on an arm64
device. In this case, the firmware data is vmapped with KERNEL_PAGE_RO
and the code crashes when attempting to write to the blob to update
the phandle properties.

This problem exists with or without this patch, right?

Signed-off-by: Stephen Boyd <redacted>
---

I was thinking perhaps it would work to store another __be32 variant
of the phandle in each device node, but then we still have a problem
with properties that have phandles inside them at some offset that we
need to update. I guess the only real solution is to deep copy the
property in that case and then save around some info to free the
duplicated property later on?

Couldn't we just copy the overlay and free the firmware blob in this
case? That seems easier than trying to selectively copy things. We
keep the base FDT to pass to userspace and we could need that for
overlays, but not ones we load from userspace.

Rob