Date: Mon, 28 Nov 2016 14:12:19 +0000 From: "Reshetova, Elena" <elena.reshetova@...el.com> To: Peter Zijlstra <peterz@...radead.org> 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>, Hans Liljestrand <ishkamiel@...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 > > Also, refcount_add() seems to be needed in number of places since it > > looks like refcounts in some cases are increased by two or by some > > constant. Luckily we haven't seen a need a sub(). > > There is sub_and_test() usage in for example memcontrol.c. Right (sub_and_test is one pattern I haven't processed yet, but I am *hoping* that there are not that many). Also now I run into atomic_sub(len -1, &sk->sk_wmem_alloc) in net/core/sock.c > > > The following functions are also needed quite commonly: > > > refcount_inc_return() > > refcount_dec_return() > > What for? They don't typicaly make sense for refcounting? Other than the > trivial pattern of dec_return() == 0, which is already well covered. Well, I guess you have to ask developers what for. They do verify the return to be equal to different numbers or values... Sometimes it is ">0", sometimes "==1", sometimes "!=-1". These all seem to be border cases and checked in some scenarios. > > I also saw one use of this from net/ipv4/udp.c: > > if (!sk || !atomic_inc_not_zero_hint(&sk->sk_refcnt, 2)) > > Yes, that one is quite unfortunate, we can trivially support that > ofcourse, but it does make a bit of a mess of things. > > > Lastly as I mentioned previously, almost half of invocations of dec() > > in the code is plain atomic_dec() without any if statements and any > > checks on what happens as a result of dec(). Peter previously > > suggested to turn them into WARN_ON(refcount_dec_and_test()), but > > looking in the code, it is not really clear what would this help to > > achieve? > > Well, it clearly marks where refcounting goes bad and we leak crap. A > regular decrement should _never_ hit 0. > > > It is clear that in that places the caller explicitly > > doesn't care about how the dec() goes and what is the end result.... > > No, the typical usage would be you _know_ it will not hit 0. Any other > usage is broken and bad. Ok, makes sense.
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