Re: ptrace and pfn mappings

[Christoph Hellwig <hch@infradead.org>]

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).

Switching the mm is definitly no acceptable.  Too many things could
break when violating the existing assumptions.

> 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....

I think the best idea is to add a new ->access method to the vm_operations
that's called by access_process_vm() when it exists and VM_IO or VM_PFNMAP
are set.   ->access would take the required object locks and copy out the
data manually.  This should work both for spufs and drm.
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