Date: Tue, 19 Jan 2021 10:05:03 +0530 From: Kaiwan N Billimoria <kaiwan@...wantech.com> To: Alexey Gladkov <gladkov.alexey@...il.com> Cc: Linus Torvalds <torvalds@...ux-foundation.org>, LKML <linux-kernel@...r.kernel.org>, io-uring <io-uring@...r.kernel.org>, Kernel Hardening <kernel-hardening@...ts.openwall.com>, Linux Containers <containers@...ts.linux-foundation.org>, Linux-MM <linux-mm@...ck.org>, Andrew Morton <akpm@...ux-foundation.org>, Christian Brauner <christian.brauner@...ntu.com>, "Eric W . Biederman" <ebiederm@...ssion.com>, Jann Horn <jannh@...gle.com>, Jens Axboe <axboe@...nel.dk>, Kees Cook <keescook@...omium.org>, Oleg Nesterov <oleg@...hat.com> Subject: Re: [RFC PATCH v3 1/8] Use refcount_t for ucounts reference counting (Sorry for the gmail client) My 0.2, HTH: a) AFAIK, refcount_inc() (and similar friends) don't return any value b) they're designed to just WARN() if they saturate or if you're attempting to increment the value 0 (as it's possibly a UAF bug) c) refcount_inc_checked() is documented as "Similar to atomic_inc(), but will saturate at UINT_MAX and WARN" d) we should avoid using the __foo() when foo() 's present as far as is sanely possible... So is one expected to just fix things when they break? - as signalled by the WARN firing? -- Regards, kaiwan. On Tue, Jan 19, 2021 at 2:26 AM Alexey Gladkov <gladkov.alexey@...il.com> wrote: > > On Mon, Jan 18, 2021 at 12:34:29PM -0800, Linus Torvalds wrote: > > On Mon, Jan 18, 2021 at 11:46 AM Alexey Gladkov > > <gladkov.alexey@...il.com> wrote: > > > > > > Sorry about that. I thought that this code is not needed when switching > > > from int to refcount_t. I was wrong. > > > > Well, you _may_ be right. I personally didn't check how the return > > value is used. > > > > I only reacted to "it certainly _may_ be used, and there is absolutely > > no comment anywhere about why it wouldn't matter". > > I have not found examples where checked the overflow after calling > refcount_inc/refcount_add. > > For example in kernel/fork.c:2298 : > > current->signal->nr_threads++; > atomic_inc(¤t->signal->live); > refcount_inc(¤t->signal->sigcnt); > > $ semind search signal_struct.sigcnt > def include/linux/sched/signal.h:83 refcount_t sigcnt; > m-- kernel/fork.c:723 put_signal_struct if (refcount_dec_and_test(&sig->sigcnt)) > m-- kernel/fork.c:1571 copy_signal refcount_set(&sig->sigcnt, 1); > m-- kernel/fork.c:2298 copy_process refcount_inc(¤t->signal->sigcnt); > > It seems to me that the only way is to use __refcount_inc and then compare > the old value with REFCOUNT_MAX > > Since I have not seen examples of such checks, I thought that this is > acceptable. Sorry once again. I have not tried to hide these changes. > > -- > Rgrds, legion > >
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