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From: Linus Torvalds <torvalds@linux-foundation.org>
To: Marco Elver <elver@google.com>
Cc: Peter Zijlstra <peterz@infradead.org>,
	Will Deacon <will@kernel.org>, Boqun Feng <boqun.feng@gmail.com>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	"the arch/x86 maintainers" <x86@kernel.org>,
	Mark Rutland <mark.rutland@arm.com>,
	Kees Cook <keescook@chromium.org>,
	Christoph Hellwig <hch@infradead.org>,
	Jens Axboe <axboe@kernel.dk>
Subject: Re: [PATCH v2 8/9] atomic,x86: Alternative atomic_*_overflow() scheme
Date: Mon, 13 Dec 2021 10:24:45 -0800	[thread overview]
Message-ID: <CAHk-=wicxUkovFa7D0YyhSweMVbL47VDimr-nytxr93SbgV9oQ@mail.gmail.com> (raw)
In-Reply-To: <YbeOjq20FCdzcK1Q@elver.google.com>

On Mon, Dec 13, 2021 at 10:19 AM Marco Elver <elver@google.com> wrote:
>
> I'm still genuinely worried about this:
>
> >       2. Yet another potentially larger issue is if some code
> >          kmalloc()s some structs containing refcount_t, and relies on
> >          GFP_ZERO (kzalloc()) to initialize their data assuming that a
> >          freshly initialized refcount_t contains 0.
>
> Even with everything properly wrapped up in atomic_ref_t, it's not going
> to prevent mis-initialization via kzalloc() and friends.

I agree that it's an issue, but it's not a new issue. We've had the
exact same thing with a lot of other core data structures.

And a ref-count of zero isn't valid _anyway_. When you allocate a
structure, a zero ref-count by definition is wrong. You need to set
the ref-count to the user that allocated it.

So I don't actually think the "implicit zero" is an issue in practice,
because it would be wrong in the first place. Code that relies on
kzmalloc() to initialize a refcount cannot work right.

(And by "cannot" I obviously mean "can, if you do wrong things" - it's
not like it's *impossible* to do an "atomic_inc_ref()" to change a 0
refcount to a 1, but it's both wrong *AND* actively stupid, since an
allocation does not need to set the refcount atomically).

             Linus

  reply	other threads:[~2021-12-13 18:25 UTC|newest]

Thread overview: 34+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-12-10 16:16 [PATCH v2 0/9] refcount: Improve code-gen Peter Zijlstra
2021-12-10 16:16 ` [PATCH v2 1/9] atomic: Prepare scripts for macro ops Peter Zijlstra
2021-12-10 17:27   ` Mark Rutland
2021-12-10 17:43   ` Marco Elver
2021-12-10 16:16 ` [PATCH v2 2/9] atomic: Add xchg.tbl Peter Zijlstra
2021-12-13  9:50   ` Mark Rutland
2021-12-10 16:16 ` [PATCH v2 3/9] atomic: Introduce atomic_{inc,dec,dec_and_test}_overflow() Peter Zijlstra
2021-12-13 10:06   ` Mark Rutland
2021-12-13 10:57     ` Peter Zijlstra
2021-12-13 10:59     ` Peter Zijlstra
2021-12-13 11:09       ` Mark Rutland
2021-12-10 16:16 ` [PATCH v2 4/9] refcount: Use atomic_*_overflow() Peter Zijlstra
2021-12-13 10:35   ` Mark Rutland
2021-12-10 16:16 ` [PATCH v2 5/9] atomic,x86: Implement atomic_dec_and_test_overflow() Peter Zijlstra
2021-12-13 11:04   ` Mark Rutland
2021-12-10 16:16 ` [PATCH v2 6/9] refcount: Fix refcount_dec_not_one() Peter Zijlstra
2021-12-10 16:16 ` [PATCH v2 7/9] refcount: Prepare for atomic_*_overflow() offsets Peter Zijlstra
2021-12-10 16:16 ` [PATCH v2 8/9] atomic,x86: Alternative atomic_*_overflow() scheme Peter Zijlstra
2021-12-10 16:53   ` Linus Torvalds
2021-12-10 17:27     ` Linus Torvalds
2021-12-17  3:38     ` Herbert Xu
2021-12-13 16:43   ` Peter Zijlstra
2021-12-13 17:29     ` Marco Elver
2021-12-13 18:11     ` Linus Torvalds
2021-12-13 18:18       ` Marco Elver
2021-12-13 18:24         ` Linus Torvalds [this message]
2021-12-13 19:35           ` Marco Elver
2021-12-13 18:21       ` Linus Torvalds
2021-12-10 16:16 ` [PATCH v2 9/9] refcount: Optimize __refcount_add_not_zero(.i=1) Peter Zijlstra
2021-12-10 19:37 ` [PATCH v2 0/9] refcount: Improve code-gen Peter Zijlstra
2021-12-13 12:15 ` [PATCH v2 10/9] atomic: Document the atomic_{}_overflow() functions Peter Zijlstra
2021-12-13 12:20 ` [PATCH v2 0/9] refcount: Improve code-gen Peter Zijlstra
2021-12-13 14:42   ` Marco Elver
2021-12-13 16:11     ` Peter Zijlstra

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