linux-kernel - RE: [PATCH 14/15] futex: convert futex_pi_state.refcount to refcount

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Date:   Fri, 1 Sep 2017 11:05:33 +0000
From:   "Reshetova, Elena" <elena.reshetova@...el.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
        "viro@...iv.linux.org.uk" <viro@...iv.linux.org.uk>,
        "tj@...nel.org" <tj@...nel.org>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "hannes@...xchg.org" <hannes@...xchg.org>,
        "lizefan@...wei.com" <lizefan@...wei.com>,
        "acme@...nel.org" <acme@...nel.org>,
        "alexander.shishkin@...ux.intel.com" 
        <alexander.shishkin@...ux.intel.com>,
        "eparis@...hat.com" <eparis@...hat.com>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "arnd@...db.de" <arnd@...db.de>,
        "luto@...nel.org" <luto@...nel.org>,
        "keescook@...omium.org" <keescook@...omium.org>,
        "dvhart@...radead.org" <dvhart@...radead.org>,
        "ebiederm@...ssion.com" <ebiederm@...ssion.com>
Subject: RE: [PATCH 14/15] futex: convert futex_pi_state.refcount to
 refcount_t

 > On Fri, 1 Sep 2017, Peter Zijlstra wrote:
> > > On Fri, Sep 01, 2017 at 09:39:50AM +0200, Thomas Gleixner wrote:
> > > > On Wed, 30 Aug 2017, Elena Reshetova wrote:
> > > > > atomic_t variables are currently used to implement reference
> > > > > counters with the following properties:
> > > > >  - counter is initialized to 1 using atomic_set()
> > > > >  - a resource is freed upon counter reaching zero
> > > > >  - once counter reaches zero, its further
> > > > >    increments aren't allowed
> > > > >  - counter schema uses basic atomic operations
> > > > >    (set, inc, inc_not_zero, dec_and_test, etc.)
> > > > >
> > > > > Such atomic variables should be converted to a newly provided
> > > > > refcount_t type and API that prevents accidental counter overflows
> > > > > and underflows. This is important since overflows and underflows
> > > > > can lead to use-after-free situation and be exploitable.
> > > > >
> > > > > The variable futex_pi_state.refcount is used as pure
> > > > > reference counter. Convert it to refcount_t and fix up
> > > > > the operations.
> > > > >
> > > > > Suggested-by: Kees Cook <keescook@...omium.org>
> > > > > Reviewed-by: David Windsor <dwindsor@...il.com>
> > > > > Reviewed-by: Hans Liljestrand <ishkamiel@...il.com>
> > > > > Signed-off-by: Elena Reshetova <elena.reshetova@...el.com>
> > > >
> > > > Reviewed-by: Thomas Gleixner <tglx@...utronix.de>
> > >
> > > So the thing to be careful with for things like futex and some of the
> > > other core kernel code is the memory ordering.
> > >
> > > atomic_dec_and_test() provides a full smp_mb() before and after,
> > > refcount_dec_and_test() only provides release semantics.
> > >
> > > This is typically sufficient, and I would argue that if we rely on more
> > > than that, there _should_ be a comment, however reality isn't always as
> > > nice.
> > >
> > > That said, I think this conversion is OK, pi_state->refcount isn't
> > > relied upon to provide additional memory ordering above and beyond what
> > > refcounting requires.
> >
> > So the changelogs should reflect that. The current one suggests that this
> > is a one to one replacement for atomic_t just with the extra sanity checks
> > added.
> 
> I will update the commit texts accordingly and resend the whole series since
> this should be then mentioned in every commit to make sure it is not missed.

Actually on the second thought: does the above memory ordering differences
really apply when  we have ARCH_HAS_REFCOUNT? To me it looks like the way
how it is currently implemented for x86 is the same way as it is for atomic cases.


> 
> Best Regards,
> Elena.
> 
> >
> > Thanks,
> >
> > 	tglx