Re: [PATCH] prctl: remove one-shot limitation for changing exe link

[Cyrill Gorcunov]

On Sat, Jul 30, 2016 at 12:31:40PM -0500, Eric W. Biederman wrote:
...
> 
> It is necessary to look at the ordinary situation.  Without
> prctl_set_mm_exe /proc/[pid]/exe can be counted on as a record
> of which executable was last passed to execve.

True.

> Furthermore the state of a process can be counted on to be a state
> reachable from calling execve on /proc/[pid]/exe.

Absolutely not. The state is valid until kernel jumped back to userspace
and give control to an interpretator.

> Which means to preserve those expectations prctl_set_mm_exe_file should
> in practice just be a nicer less cumbersome interface to things you can
> already achieve with execve.
> 
> Justifying removale of the one-short nature for prctl_set_mm_exe_file
> is as straight forward as noting that a process can call execve on
> any executable file.
> 
> However when I compare the invariants that execve has on a file (such as
> the executable being mmaped) I see some noticable disparities between
> what prctl_set_mm_exe_file allows and what execve allows.  With
> prctl_set_mm_exe being less strict.
> 
> So what I am requesting is very simple.  That the checks in
> prctl_set_mm_exe_file be tightened up to more closely approach what
> execve requires.  Thus preserving the value of the /proc/[pid]/exe for
> the applications that want to use the exe link.
> 
> Once the checks in prctl_set_mm_exe_file are tightened up please feel
> free to remove the one shot test.

Thanks a huge for the detailed explanation, but i don't agree here because
assuming that state of a process reachable from calling execve on
/proc/[pid]/exe is not always true.