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Date: Fri, 16 Dec 2016 12:10:21 +0000
From: "Reshetova, Elena" <elena.reshetova@...el.com>
To: Peter Zijlstra <peterz@...radead.org>, Liljestrand Hans
<ishkamiel@...il.com>
CC: "kernel-hardening@...ts.openwall.com"
<kernel-hardening@...ts.openwall.com>, Greg KH <gregkh@...uxfoundation.org>,
Kees Cook <keescook@...omium.org>, "will.deacon@....com"
<will.deacon@....com>, Boqun Feng <boqun.feng@...il.com>, David Windsor
<dwindsor@...il.com>, "aik@...abs.ru" <aik@...abs.ru>,
"david@...son.dropbear.id.au" <david@...son.dropbear.id.au>
Subject: RE: Conversion from atomic_t to refcount_t: summary of issues
> On Fri, Dec 02, 2016 at 05:44:34PM +0200, Liljestrand Hans wrote:
> >
> > Then there's at least include/net/ip_vs.h that does unchecked decs and
> > instead has this dedicated free function that checks for negative values
> > (so with unsigned refcount it is broken anyway, guess we could do a
> > conditional dec with a _read, but then its no longer atomic):
> >
> > http://lxr.free-electrons.com/source/include/net/ip_vs.h#L1424
> >
> > static inline void ip_vs_dest_put_and_free(struct ip_vs_dest *dest)
> > {
> > if (atomic_dec_return(&dest->refcnt) < 0)
> > kfree(dest);
> > }
>
> This looks like one that uses -1 to free, so doing a +1 on the entire
> scheme would restore 'sanity', but that's fairly thick code and I
> couldn't say for sure.
>
> > Then there's cases that check for the first increment, like here (maybe
> > something like inc_and_one could allow these without too much leeway?):
> >
> > http://lxr.free-electrons.com/source/drivers/tty/serial/zs.c#L764
> >
> > irq_guard = atomic_add_return(1, &scc->irq_guard);
> > if (irq_guard == 1) {
> >
> > http://lxr.free-
> electrons.com/source/drivers/usb/gadget/function/f_fs.c#L1497
> >
> > if (atomic_add_return(1, &ffs->opened) == 1 &&
> > ffs->state == FFS_DEACTIVATED) {
> >
> >
> > And finally some cases with other uses/values:
> >
> > http://lxr.free-
> electrons.com/source/drivers/staging/lustre/lustre/ptlrpc/client.c#L3081
> >
> > if (atomic_inc_return(&req->rq_refcount) == 2)
>
> Greg already went through these, they're not proper refcounts.
>
>
> > http://lxr.free-electrons.com/source/kernel/bpf/syscall.c#L231
> >
> > if (atomic_inc_return(&map->refcnt) > BPF_MAX_REFCNT) {
>
> I think this one already got discussed, its a custom refcount limit
> scheme (with holes in).
>
> All in all I'm not inclined to add {add,sub.inc,dec}_return() to
> refcount, as previously stated, they don't make sense.
Is it ok to add at least refcount_inc_if_zero() ?
We already have refcount_dec_if_one(), reffcount_dec_not_one() and refcount_inc_not_zero(), so this one is the only missing one and would greatly help in couple of cases.
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