Re: [PATCH 1/3] mm: get_cmdline use arg_lock instead of mmap_sem

[Cyrill Gorcunov]

On Tue, Apr 30, 2019 at 12:56:10PM +0200, Michal Koutný wrote:
> On Tue, Apr 30, 2019 at 01:45:17PM +0300, Cyrill Gorcunov <gorcunov@xxxxxxxxx> wrote:
> > It setups these parameters unconditionally. I need to revisit
> > this moment. Technically (if only I'm not missing something
> > obvious) we might have a race here with prctl setting up new
> > params, but this should be harmless since most of them (except
> > stack setup) are purely informative data.
>
> FTR, when I reviewed that usage, I noticed it was missing the
> synchronization. My understanding was that the mm_struct isn't yet
> shared at this moment. I can see some of the operations take place after
> flush_old_exec (where current->mm = mm_struct), so potentially it is
> shared since then. OTOH, I guess there aren't concurrent parties that
> could access the field at this stage of exec.

Just revisited this code -- we're either executing prctl, either execve.
Since both operates with current task we're safe.