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

[Matt Helsley <matthltc@us.ibm.com>]

On Sat, Mar 10, 2012 at 02:39:32AM +0400, Cyrill Gorcunov wrote:
> 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.

As far as backward compatibility, isn't it better to lift that restriction
later rather than add it? I think the latter would very likely "break"
things whereas the former would not.

I also prefer that restriction because it establishes a bound on how
frequently the symlink can change. Keeping it a one-shot deal makes the
values that show up in tools like top more reliable for admins.

> 
> > 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.

True, it's literally correct. However the whole reason for having it
is to turn the mm->exe_file reference into a sort of weak reference
which happens to coincide with counting the number of VM_EXECUTABLE vmas
until you do c/r (really just the restart side of c/r).

Cheers,
	-Matt