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

[Frank Rowand]

On 02/23/17 11:54, Frank Rowand wrote:
> 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.

OK, now that I have looked at the resolver, I see that you meant that
the resolver code is modifying the _overlay_ FDT, not the FDT that
was used at initial boot.  So not what I thought you meant.

I'll reply separately to the original patch email with a more complete
response.

-Frank

> 
> 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 <stephen.boyd at linaro.org>
>> ---
>>
>> 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(-)
> 
> < snip >
>