Messages in this thread | | | From | Linus Torvalds <> | Date | Mon, 13 Dec 2021 10:24:45 -0800 | Subject | Re: [PATCH v2 8/9] atomic,x86: Alternative atomic_*_overflow() scheme |
| |
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
| |