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Date: Thu, 17 Nov 2016 09:53:42 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Alexei Starovoitov <alexei.starovoitov@gmail.com>
Cc: 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>, Thomas Gleixner <tglx@linutronix.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()
Message-ID: <20161117085342.GB3142@twins.programming.kicks-ass.net>
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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.

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.

Also, the current sentiment is to strongly discourage add/sub operations
for refcounts.