Re: [RFC] c/r: prctl: Add ability to set new mm_struct::exe_file v3

[Cyrill Gorcunov <gorcunov@openvz.org>]

On Fri, Mar 09, 2012 at 02:02:44PM -0800, Matt Helsley wrote:
...
> 
> Sorry about the other email -- hadn't full caught up on this thread.

No problem.

> This is even better, yes.
>

Well, in final version I switched back to

+	if (mm->num_exe_file_vmas)
+		return -EBUSY;

simply because it's more flexible than mm->exe_file.

With mm->exe_file this prctl option become a one-shot
only, and while at moment our user-space tool can perfectly
live with that I thought that there is no strict need to
limit the option this way from the very beginning.

> Of course I'd prefer it if there was a way to keep num_exe_file_vmas
> correct and not special-case c/r. The first approximation of a solution

It remains correct actually. There is no way to map new VM_EXECUTABLE
from user-space after we've unmapped previous ones, so num_exe_file_vmas
will remain 0.

> might be to increment the count whenever a new mmap filp == mm->exe_file
> and decrement on unmap. I think there are a bunch of details needed to
> make that work but my feeling is it's do-able. Have you investigated this
> already and rejected it for some reason (did I miss that discussion
> somehow?)?

As far as I understand overall num_exe_file_vmas concept -- we track
a number of VM_EXECUTABLE with it, so setting new exe_file should not
change num_exe_file_vmas I think.

	Cyrill