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SubjectRe: [PATCH v7 3.2-rc2 4/30] uprobes: Define hooks for mmap/munmap.
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On Tue, 2011-11-29 at 12:48 +0100, Peter Zijlstra wrote:
> There's 2 main cases,
> A) vma_adjust() vs unregister_uprobe() and
> B) mmap() vs unregister_uprobe().
>
> The result of A should be -1 reference in total, since we're removing
> the one probe.

This might not be correct for A[23], please double check.

> The result of B should be 0 since we're removing the
> probe and we shouldn't be installing new ones.
>
> A1)
> vma_adjust()
> munmap_uprobe()
> unregister_uprobe()
> mmap_uprobe()
> delete_uprobe()
>
>
> munmap will to -1, mmap will do +1, __unregister_uprobe() which is
> serialized against vma_adjust() will do -1 on either the old or new vma,
> resulting in a grand total of: -1+1-1=-1, OK
>
> A2) breakpoint is in old, not in new, again two cases:
>
> A2a) __unregister_uprobe() sees old
>
> munmap -1, __unregister_uprobe -1, mmap 0: -2 FAIL
>
> A2b) __unregister_uprobe() sees new
>
> munmap -1, __unregister_uprobe 0, mmap 0: -1 OK
>
> A3) breakpoint is in new, not in old, again two cases:
>
> A3a) __unregister_uprobe() sees old
>
> munmap 0, __unregister_uprobe 0, mmap: 1: 1 FAIL
>
> A3b) __unregister_uprobe() seed new
>
> munmap 0, __unregister_uprobe -1, mmap: 1: 0 FAIL

There's more cases, I forgot the details of how the prio_tree stuff
works, so please consider if its possible to also have:

__unregister_uprobe() will observe neither old nor new

This could happen if we first munmap, __unregister_uprobe() will iterate
past where mmap() will insert the new vma, mmap will insert the new vma,
and __unregister_uprobe() will now not observe it.

and

__unregister_uprobe() will observe both old _and_ new

This latter could happen by favourably interleaving the prio_tree
iteration with the munmap and mmap operations, so that we first observe
the old vma, do the munmap, do the mmap, and then have the
find_next_vma_info() thing find the new vma.

> B1)
> unregister_uprobe()
> mmap()
> mmap_uprobe()
> __unregister_uprobe()
> delete_uprobe()
>
> mmap +1, __unregister_uprobe() -1: 0 OK
>
> B2)
> unregister_uprobe()
> mmap()
> __unregister_uprobe()
> mmap_uprobe()
> delete_uprobe()
>
> mmap +1, __unregister_uprobe() 0: +1 FAIL


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