Date: Tue, 19 Jan 2021 19:57:36 -0600 From: ebiederm@...ssion.com (Eric W. Biederman) 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>, 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 Alexey Gladkov <gladkov.alexey@...il.com> writes: > 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. The current ucount code does check for overflow and fails the increment in every case. So arguably it will be a regression and inferior error handling behavior if the code switches to the ``better'' refcount_t data structure. I originally didn't use refcount_t because silently saturating and not bothering to handle the error makes me uncomfortable. Not having to acquire the ucounts_lock every time seems nice. Perhaps the path forward would be to start with stupid/correct code that always takes the ucounts_lock for every increment of ucounts->count, that is later replaced with something more optimal. Not impacting performance in the non-namespace cases and having good performance in the other cases is a fundamental requirement of merging code like this. Eric
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