Re: [RFC PATCH 01/11] counters: Introduce counter and counter_atomic

Greg KH
2020-09-23T19:34:32Z

On Wed, Sep 23, 2020 at 12:04:08PM -0700, Kees Cook wrote:
> On Tue, Sep 22, 2020 at 07:43:30PM -0600, Shuah Khan wrote:
> > Introduce Simple atomic and non-atomic counters.
> >
> > There are a number of atomic_t usages in the kernel where atomic_t api
> > is used strictly for counting and not for managing object lifetime. In
> > some cases, atomic_t might not even be needed.
>
> Thank you for working on a counter API! I'm glad to see work here,
> though I have some pretty significant changes to request; see below...
>
> >
> > The purpose of these counters is twofold: 1. clearly differentiate
> > atomic_t counters from atomic_t usages that guard object lifetimes,
> > hence prone to overflow and underflow errors. It allows tools that scan
> > for underflow and overflow on atomic_t usages to detect overflow and
> > underflows to scan just the cases that are prone to errors. 2. provides
> > non-atomic counters for cases where atomic isn't necessary.
> >
> > Simple atomic and non-atomic counters api provides interfaces for simple
> > atomic and non-atomic counters that just count, and don't guard resource
> > lifetimes. Counters will wrap around to 0 when it overflows and should
> > not be used to guard resource lifetimes, device usage and open counts
> > that control state changes, and pm states.
> >
> > Using counter_atomic to guard lifetimes could lead to use-after free
> > when it overflows and undefined behavior when used to manage state
> > changes and device usage/open states.
> >
> > Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>
>
> I would really like these APIs to be _impossible_ to use for object
> lifetime management. To that end, I would like to have all of the
> *_return() functions removed. It should be strictly init, inc, dec,
> read.
>
> > +There are a number of atomic_t usages in the kernel where atomic_t api
> > +is used strictly for counting and not for managing object lifetime. In
> > +some cases, atomic_t might not even be needed.
>
> Why even force the distinction? I think all the counters should be
> atomic and then there is no chance they will get accidentally used in
> places where someone *thinks* it's safe to use a non-atomic. So,
> "_atomic" can be removed from the name and the non-atomic implementation
> can get removed. Anyone already using non-atomic counters is just using
> "int" and "long" anyway. Let's please only create APIs that are always
> safe to use, and provide some benefit over a native time.

For "statistics", why take the extra overhead for an atomic variable
just to be able to show to a debugging file the number of USB packets
have been sent through the system (a current use of an atomic variable
for some odd reason...)

And really, a "int" should be pretty safe to write from multiple places,
you aren't going to get "tearing" on any processors that run Linux,
worst case you get a stale value when reading them.

So I would argue that the default for a counter be just an int, not
atomic, as odds are, most atomics are not really needed for this type of
thing at all.

thanks,

greg k-h