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Message-ID: <20161207135241.GI3107@twins.programming.kicks-ass.net>
Date: Wed, 7 Dec 2016 14:52:41 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Liljestrand Hans <ishkamiel@...il.com>
Cc: "Reshetova, Elena" <elena.reshetova@...el.com>,
"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.
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