Re: double fault -> PAGE_KERNEL flagged memory

[Tom Zanussi <zanussi at us dot ibm dot com>]

Frank Ch. Eigler writes:
 > 
 > zanussi wrote:
 > 
 > > [...]  What would cause a double fault would be if the vmalloc_fault
 > > tried logging before the page table was updated, which would cause
 > > the same vmalloc fault.
 > 
 > Then this is analogous to the problem of calling printk from within an
 > inconveniently placed kprobe.  What can we do to eliminate this
 > vulnerability?  Can we somehow arrange to "fault in" all probe-related
 > kernel-space vmalloc areas into new process' address spaces, so we don't
 > encounter this unintentional and undesirable reentrancy?
 > 

I'll think about it, but it doesn't sound like fun.  It sounds like it
might be one of those cases where you only allow a tapset to
instrument a certain area, in this case a page fault tapset to
instrument the page fault path.  I can't remember, how is the
possibility of a printk() in a problematic function currently handled
in systemtap?

Tom