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

On 02/13/17 18:50, Stephen Boyd 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.

The resolver should not be over writing anything in the FDT.  I'll
look at what is going on there.

The FDT we expose to user space should be the FDT we booted with,
not something later modified.

-Frank

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.

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?

 drivers/of/base.c     |  2 +-
 drivers/of/fdt.c      | 12 ++++++------
 drivers/of/resolver.c |  3 +++
 include/linux/of.h    |  2 +-
 4 files changed, 11 insertions(+), 8 deletions(-)

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