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SubjectRe: ptrace and pfn mappings
On Tue, Oct 10, 2006 at 11:16:27AM +1000, Benjamin Herrenschmidt wrote:
> And the last of my "issues" here:
>
> get_user_pages() can't handle pfn mappings, thus access_process_vm()
> can't, and thus ptrace can't. When they were limited to dodgy /dev/mem
> things, it was probably ok. But with more drivers needing that, like the
> DRM, sound drivers, and now with SPU problem state registers and local
> store mapped that way, it's becoming a real issues to be unable to
> access any of those mappings from gdb.
>
> The "easy" way out I can see, but it may have all sort of bad side
> effects I haven't thought about at this point, is to switch the mm in
> access_process_vm (at least if it's hitting such a VMA).

Switch the mm and do a copy_from_user? (rather than the GUP).
Sounds pretty ugly :P

Can you do a get_user_pfns, and do a copy_from_user on the pfn
addresses? In other words, is the memory / mmio at the end of a
given address the same from the perspective of any process? It
is for physical memory of course, which is why get_user_pages
works...

> That means that the ptracing process will temporarily be running in the
> kernel using a task->active_mm different from task->mm which might have
> funny side effects due to assumptions that this won't happen here or
> there, though I don't see any fundamental reasons why it couldn't be
> made to work.
>
> That do you guys think ? Any better idea ? The problem with mappings
> like what SPUfs or the DRM want is that they can change (be remapped
> between HW and backup memory, as described in previous emails), thus we
> don't want to get struct pages even if available and peek at them as
> they might not be valid anymore, same with PFNs (we could imagine
> ioremap'ing those PFN's but that would be racy too). The only way that
> is guaranteed not to be racy is to do exactly what a user do, that is do
> user accesses via the target process vm itself....

What if you hold your per-object lock over the operation? (I guess
it would have to nest *inside* mmap_sem, but that should be OK).
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