Re: [PATCH] arm: Use kernel mm when updating section permissions

From: Laura Abbott
Date: Fri Nov 06 2015 - 18:42:05 EST

On 11/06/2015 12:46 PM, Russell King - ARM Linux wrote:
On Fri, Nov 06, 2015 at 10:44:32AM -0800, Laura Abbott wrote:
with my test patch. I think setting both current->active_mm and &init_mm
is sufficient. Maybe explicitly setting swapper_pg_dir would be cleaner?

Please, stop thinking like this. If you're trying to change the kernel
section mappings after threads have been spawned, you need to change
them for _all_ threads, which means you need to change them for every
page table that's in existence at that time - you can't do just one
table and hope everyone updates, it doesn't work like that.

That's a bad assumption assumption on my part based on what I was
observing. At the time of mark_rodata_ro, the only threads present
are kernel threads which aren't going to have task->mm. Only the
running thread is going to have active_mm. None of those are init_mm.
To be complete we need:

- Update every task->mm for every thread in every process
- Update current->active_mm
- Update &init_mm explicitly

All this would need to be done under stop_machine as well. Does that cover
everything or am I still off?

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