* Peter Zijlstra [off-list ref] [2011-01-26 16:45:56]:

On Wed, 2011-01-26 at 21:00 +0530, Srikar Dronamraju wrote:

quoted

* Peter Zijlstra [off-list ref] [2011-01-26 11:11:48]:

quoted

On Wed, 2011-01-26 at 13:25 +0530, Srikar Dronamraju wrote:

quoted

quoted

quoted

+
+               list_add(&mm->uprobes_list, &tmp_list);
+               mm->uprobes_vaddr = vma->vm_start + offset;
+       }
+       spin_unlock(&mapping->i_mmap_lock);

Both this and unregister are racy, what is to say:
 - the vma didn't get removed from the mm
 - no new matching vma got added

register_uprobe, unregister_uprobe, uprobe_mmap are all synchronized by
uprobes_mutex. So I dont see one unregister_uprobe getting thro when
another register_uprobe is working with a vma.

If I am missing something elementary, please explain a bit more.

afaict you're not holding the mmap_sem, so userspace can simply unmap
the vma.

When we do the actual insert/remove of the breakpoint we hold the
mmap_sem. During the actual insertion/removal, if the vma for the
specific inode is not found, we just come out without doing the
actual insertion/deletion.

Right, but then install_uprobe() should:

 - lookup the vma relating to the address you stored,

We already do this thro get_user_pages in write_opcode().

 - validate that the vma is indeed a map of the right inode

We can add a check in write_opcode( we need to pass the inode to
write_opcode).

 - validate that the offset of the probe corresponds with the stored
address

I am not clear on this. We would have derived the address from the
offset. So is that we check for
 (vaddr == vma->vm_start + uprobe->offset)

Otherwise you can race with unmap/map and end up installing the probe in
a random location.

Also, I think the whole thing goes funny if someone maps the same text
twice ;-)

I am not sure if we can map the same text twice. If something like
this is possible then we would have 2 addresses for each function.
So how does the linker know which address to jump to out of the 2 or
multiple matching addresses. What would be the usecases for same
text being mapped multiple times and both being executable?

-- 
Thanks and Regards
Srikar

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