Date: Thu, 17 Nov 2016 17:34:09 +0100 (CET)
From: Thomas Gleixner <tglx@linutronix.de>
To: Alexei Starovoitov <alexei.starovoitov@gmail.com>
cc: Peter Zijlstra <peterz@infradead.org>,
        Kees Cook <keescook@chromium.org>,
        Greg KH <gregkh@linuxfoundation.org>,
        Will Deacon <will.deacon@arm.com>,
        "Reshetova, Elena" <elena.reshetova@intel.com>,
        Arnd Bergmann <arnd@arndb.de>, Ingo Molnar <mingo@kernel.org>,
        "H. Peter Anvin" <hpa@zytor.com>, David Windsor <dave@progbits.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>
Subject: Re: [RFC][PATCH 2/7] kref: Add kref_read()
In-Reply-To: <20161117161937.GA46515@ast-mbp.thefacebook.com>
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On Thu, 17 Nov 2016, Alexei Starovoitov wrote:
> On Thu, Nov 17, 2016 at 09:53:42AM +0100, Peter Zijlstra wrote:
> > On Wed, Nov 16, 2016 at 12:08:52PM -0800, Alexei Starovoitov wrote:
> > 
> > > I prefer to avoid 'fixing' things that are not broken.
> > > Note, prog->aux->refcnt already has explicit checks for overflow.
> > > locked_vm is used for resource accounting and not refcnt,
> > > so I don't see issues there either.
> > 
> > The idea is to use something along the lines of:
> > 
> >   http://lkml.kernel.org/r/20161115104608.GH3142@twins.programming.kicks-ass.net
> > 
> > for all refcounts in the kernel.
> 
> I understand the idea. I'm advocating to fix refcnts
> explicitly the way we did in bpf land instead of leaking memory,
> making processes unkillable and so on.
> If refcnt can be bounds checked, it should be done that way, since
> it's a clean error path without odd side effects.
> Therefore I'm against unconditionally applying refcount to all atomics.
> 
> > Also note that your:
> > 
> > struct bpf_prog *bpf_prog_add(struct bpf_prog *prog, int i)
> > {
> >         if (atomic_add_return(i, &prog->aux->refcnt) > BPF_MAX_REFCNT) {
> >                 atomic_sub(i, &prog->aux->refcnt);
> >                 return ERR_PTR(-EBUSY);
> >         }
> >         return prog;
> > }
> > 
> > is actually broken in the face of an actual overflow. Suppose @i is big
> > enough to wrap refcnt into negative space.
> 
> 'i' is not controlled by user. It's a number of nic hw queues
> and BPF_MAX_REFCNT is 32k, so above is always safe.
> 
> > Also, the current sentiment is to strongly discourage add/sub operations
> > for refcounts.
> 
> I agree with this reasoning as well, but it's not hard and fast rule.
> If we know we can do 'add' safely, we should.

In principle yes. OTOH, history shows that developers have a pretty bad
judgement what is safe and not. They rather copy code from random places,
modify it in creative ways and be done with it.

Thanks,

	tglx