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

[Cyrill Gorcunov <gorcunov@openvz.org>]

On Thu, Mar 01, 2012 at 08:41:20PM +0100, Oleg Nesterov wrote:
...
> 
> Still can't understand. I think you need:
> 
> 	file = fget(fd);
> 	if (!file)
> 		return -EBADF;
> 
> 	down_write(&mm->mmap_sem);
> 	if (mm->num_exe_file_vmas) {
> 		fput(mm->exe_file);
> 		mm->exe_file = file;
> 		file = NULL;
> 	}
> 	up_write(&mm->mmap_sem);
> 
> 	if (!file)
> 		return 0;
> 
> 	fput(file);
> 	return -ESOMETHING;
> 
> and that is all.

This breaks overall logic of num_exe_file_vmas.
What the point to have it at all then? I mean,
if there several executable sections in elf file,
once loader finish its work we will have
num_exe_file_vmas more than 1.

Then the process calls for prctl and replaces
own exe_file (I'm talking about possible scenario
since for our own tool we know that there will be
only one .text section and we're more-less safe
in replacing own exe_file, but this interface
will be available for everyone who has c/r config
entry turned on, so I'm trying to find which
negative impact this feature might have, call me
paranoid), so once process has replaced own exe_file
to something else the code which depends on
num_exe_file_vmas become broken.

May not we have a scenario when removed_exe_file_vma
is be called somewhere else later, once this prctl
finished its work? That's what I fear of.

	Cyrill