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Date: Thu, 17 Nov 2016 09:34:58 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Kees Cook <keescook@chromium.org>
Cc: Greg KH <gregkh@linuxfoundation.org>,
        Will Deacon <will.deacon@arm.com>,
        "Reshetova, Elena" <elena.reshetova@intel.com>,
        Arnd Bergmann <arnd@arndb.de>, Thomas Gleixner <tglx@linutronix.de>,
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        Alexei Starovoitov <alexei.starovoitov@gmail.com>,
        Daniel Borkmann <daniel@iogearbox.net>
Subject: Re: [RFC][PATCH 2/7] kref: Add kref_read()
Message-ID: <20161117083458.GZ3142@twins.programming.kicks-ass.net>
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On Wed, Nov 16, 2016 at 10:58:38AM -0800, Kees Cook wrote:
> On Wed, Nov 16, 2016 at 2:09 AM, Peter Zijlstra <peterz@infradead.org> wrote:
> > On Tue, Nov 15, 2016 at 12:53:35PM -0800, Kees Cook wrote:
> >>
> >> What should we do about things like this (bpf_prog_put() and callbacks
> >> from kernel/bpf/syscall.c):
> >>
> >>
> >> static void bpf_prog_uncharge_memlock(struct bpf_prog *prog)
> >> {
> >>         struct user_struct *user = prog->aux->user;
> >>
> >>         atomic_long_sub(prog->pages, &user->locked_vm);
> >>         free_uid(user);
> >> }
> >>
> >> static void __bpf_prog_put_rcu(struct rcu_head *rcu)
> >> {
> >>         struct bpf_prog_aux *aux = container_of(rcu, struct bpf_prog_aux, rcu);
> >>
> >>         free_used_maps(aux);
> >>         bpf_prog_uncharge_memlock(aux->prog);
> >>         bpf_prog_free(aux->prog);
> >> }
> >>
> >> void bpf_prog_put(struct bpf_prog *prog)
> >> {
> >>         if (atomic_dec_and_test(&prog->aux->refcnt))
> >>                 call_rcu(&prog->aux->rcu, __bpf_prog_put_rcu);
> >> }
> >>
> >>
> >> Not only do we want to protect prog->aux->refcnt, but I think we want
> >> to protect user->locked_vm too ... I don't think it's sane for
> >> user->locked_vm to be a stats_t ?
> >
> > Why would you want to mess with locked_vm? You seem of the opinion that
> > everything atomic_t is broken, this isn't the case.
> 
> What I mean to say is that while the refcnt here should clearly be
> converted to kref or refcount_t, it looks like locked_vm should become
> a new stats_t. However, it seems weird for locked_vm to ever wrap
> either...

No, its not a statistic. Also, I'm far from convinced stats_t is an
actually useful thing to have.

refcount_t brought special semantics that clearly are different from
regular atomic_t, stats_t would not, so why would it need to exist.

Not to mention that you seem over eager to apply it, which doesn't
inspire confidence.