Re: kmod: provide wrappers for kmod_concurrent inc/dec

[Jessica Yu]

+++ Luis R. Rodriguez [16/12/16 09:05 +0100]:
> On Thu, Dec 15, 2016 at 01:46:25PM +0100, Petr Mladek wrote:
> > On Thu 2016-12-08 22:08:59, Luis R. Rodriguez wrote:
> > > On Thu, Dec 08, 2016 at 12:29:42PM -0800, Kees Cook wrote:
> > > > On Thu, Dec 8, 2016 at 11:48 AM, Luis R. Rodriguez <mcgrof@xxxxxxxxxx> wrote:
> > > > > kmod_concurrent is used as an atomic counter for enabling
> > > > > the allowed limit of modprobe calls, provide wrappers for it
> > > > > to enable this to be expanded on more easily. This will be done
> > > > > later.
> > > > >
> > > > > Signed-off-by: Luis R. Rodriguez <mcgrof@xxxxxxxxxx>
> > > > > ---
> > > > >  kernel/kmod.c | 27 +++++++++++++++++++++------
> > > > >  1 file changed, 21 insertions(+), 6 deletions(-)
> > > > >
> > > > > diff --git a/kernel/kmod.c b/kernel/kmod.c
> > > > > index cb6f7ca7b8a5..049d7eabda38 100644
> > > > > --- a/kernel/kmod.c
> > > > > +++ b/kernel/kmod.c
> > > > > @@ -108,6 +111,20 @@ static int call_modprobe(char *module_name, int wait)
> > > > >         return -ENOMEM;
> > > > >  }
> > > > >
> > > > > +static int kmod_umh_threads_get(void)
> > > > > +{
> > > > > +       atomic_inc(&kmod_concurrent);
> >
> > This approach might actually cause false failures. If we
> > are on the limit and more processes do this increment
> > in parallel, it makes the number bigger that it should be.
>
> This approach is *exactly* what the existing code does :P
> I just provided wrappers. I agree with the old approach though,
> reason is it acts as a lock in for the bump. 

I think what Petr meant was that we could run into false failures when multiple
atomic increments happen between the first increment and the subsequent
atomic_read.

Say max_modprobes is 64 -

      atomic_inc(&kmod_concurrent); // thread 1: kmod_concurrent is 63
           atomic_inc(&kmod_concurrent); // thread 2: kmod_concurrent is 64
                atomic_inc(&kmod_concurrent); // thread 3: kmod_concurrent is 65
      if (atomic_read(&kmod_concurrent) < max_modprobes) // if all threads read 65 here, then all will error out
              return 0;                                  // when the first two should have succeeded (false failures)
      atomic_dec(&kmod_concurrent);
      return -ENOMEM;

But yeah, I think this issue was already in the existing kmod code..

Jessica