linux-rng/include/linux/refcount.h, branch linus/master

refcount: introduce __refcount_{add|inc}_not_zero_limited_acquire

2025-03-17T05:06:19Z

Introduce functions to increase refcount but with a top limit above which they will fail to increase (the limit is inclusive). Setting the limit to INT_MAX indicates no limit. Link: https://lkml.kernel.org/r/20250213224655.1680278-12-surenb@google.com Signed-off-by: Suren Baghdasaryan Tested-by: Shivank Garg Link: https://lkml.kernel.org/r/5e19ec93-8307-47c2-bb13-3ddf7150624e@amd.com Cc: Christian Brauner Cc: David Hildenbrand Cc: David Howells Cc: Davidlohr Bueso Cc: Hugh Dickins Cc: Jann Horn Cc: Johannes Weiner Cc: Jonathan Corbet Cc: Klara Modin Cc: Liam R. Howlett Cc: Lokesh Gidra Cc: Lorenzo Stoakes Cc: Mateusz Guzik Cc: Matthew Wilcox Cc: Mel Gorman Cc: Michal Hocko Cc: Minchan Kim Cc: Oleg Nesterov Cc: Pasha Tatashin Cc: "Paul E . McKenney" Cc: Peter Xu Cc: Peter Zijlstra (Intel) Cc: Shakeel Butt Cc: Sourav Panda Cc: Vlastimil Babka Cc: Wei Yang Cc: Will Deacon Cc: Heiko Carstens Cc: Stephen Rothwell Signed-off-by: Andrew Morton

refcount: provide ops for cases when object's memory can be reused

2025-03-17T05:06:19Z

For speculative lookups where a successful inc_not_zero() pins the object, but where we still need to double check if the object acquired is indeed the one we set out to acquire (identity check), needs this validation to happen *after* the increment. Similarly, when a new object is initialized and its memory might have been previously occupied by another object, all stores to initialize the object should happen *before* refcount initialization. Notably SLAB_TYPESAFE_BY_RCU is one such an example when this ordering is required for reference counting. Add refcount_{add|inc}_not_zero_acquire() to guarantee the proper ordering between acquiring a reference count on an object and performing the identity check for that object. Add refcount_set_release() to guarantee proper ordering between stores initializing object attributes and the store initializing the refcount. refcount_set_release() should be done after all other object attributes are initialized. Once refcount_set_release() is called, the object should be considered visible to other tasks even if it was not yet added into an object collection normally used to discover it. This is because other tasks might have discovered the object previously occupying the same memory and after memory reuse they can succeed in taking refcount for the new object and start using it. Object reuse example to consider: consumer: obj = lookup(collection, key); if (!refcount_inc_not_zero_acquire(&obj->ref)) return; if (READ_ONCE(obj->key) != key) { /* identity check */ put_ref(obj); return; } use(obj->value); producer: remove(collection, obj->key); if (!refcount_dec_and_test(&obj->ref)) return; obj->key = KEY_INVALID; free(obj); obj = malloc(); /* obj is reused */ obj->key = new_key; obj->value = new_value; refcount_set_release(obj->ref, 1); add(collection, new_key, obj); refcount_{add|inc}_not_zero_acquire() is required to prevent the following reordering when refcount_inc_not_zero() is used instead: consumer: obj = lookup(collection, key); if (READ_ONCE(obj->key) != key) { /* reordered identity check */ put_ref(obj); return; } producer: remove(collection, obj->key); if (!refcount_dec_and_test(&obj->ref)) return; obj->key = KEY_INVALID; free(obj); obj = malloc(); /* obj is reused */ obj->key = new_key; obj->value = new_value; refcount_set_release(obj->ref, 1); add(collection, new_key, obj); if (!refcount_inc_not_zero(&obj->ref)) return; use(obj->value); /* USING WRONG OBJECT */ refcount_set_release() is required to prevent the following reordering when refcount_set() is used instead: consumer: obj = lookup(collection, key); producer: remove(collection, obj->key); if (!refcount_dec_and_test(&obj->ref)) return; obj->key = KEY_INVALID; free(obj); obj = malloc(); /* obj is reused */ obj->key = new_key; /* new_key == old_key */ refcount_set(obj->ref, 1); if (!refcount_inc_not_zero_acquire(&obj->ref)) return; if (READ_ONCE(obj->key) != key) { /* pass since new_key == old_key */ put_ref(obj); return; } use(obj->value); /* USING STALE obj->value */ obj->value = new_value; /* reordered store */ add(collection, key, obj); [surenb@google.com: fix title underlines in refcount-vs-atomic.rst] Link: https://lkml.kernel.org/r/20250217161645.3137927-1-surenb@google.com Link: https://lkml.kernel.org/r/20250213224655.1680278-11-surenb@google.com Signed-off-by: Suren Baghdasaryan Acked-by: Vlastimil Babka [slab] Tested-by: Shivank Garg Link: https://lkml.kernel.org/r/5e19ec93-8307-47c2-bb13-3ddf7150624e@amd.com Cc: Peter Zijlstra Cc: Will Deacon Cc: Paul E. McKenney Cc: Christian Brauner Cc: David Hildenbrand Cc: David Howells Cc: Davidlohr Bueso Cc: Hugh Dickins Cc: Jann Horn Cc: Johannes Weiner Cc: Jonathan Corbet Cc: Klara Modin Cc: Liam R. Howlett Cc: Lokesh Gidra Cc: Lorenzo Stoakes Cc: Mateusz Guzik Cc: Matthew Wilcox Cc: Mel Gorman Cc: Michal Hocko Cc: Minchan Kim Cc: Oleg Nesterov Cc: Pasha Tatashin Cc: Peter Xu Cc: Shakeel Butt Cc: Sourav Panda Cc: Wei Yang Cc: Heiko Carstens Cc: Stephen Rothwell Signed-off-by: Andrew Morton

refcount: Report UAF for refcount_sub_and_test(0) when counter==0

2024-08-05T21:34:23Z

When a reference counter is at zero and refcount_sub_and_test() is invoked to subtract zero, the function accepts this request without any warning and returns true. This behavior does not seem ideal because the counter being already at zero indicates a use-after-free. Furthermore, returning true by refcount_sub_and_test() in this case potentially results in a double-free done by its caller. Modify the underlying function __refcount_sub_and_test() to warn about this case as a use-after-free and have it return false to avoid the potential double-free. Signed-off-by: Petr Pavlu Acked-by: Peter Zijlstra (Intel) Link: https://lore.kernel.org/r/20240717130023.5675-1-petr.pavlu@suse.com Signed-off-by: Kees Cook

refcount: Annotated intentional signed integer wrap-around

2024-02-29T21:38:02Z

Mark the various refcount_t functions with __signed_wrap, as we depend on the wrapping behavior to detect the overflow and perform saturation. Silences warnings seen with the LKDTM REFCOUNT_* tests: UBSAN: signed-integer-overflow in ../include/linux/refcount.h:189:11 2147483647 + 1 cannot be represented in type 'int' Reviewed-by: Miguel Ojeda Link: https://lore.kernel.org/r/20240221051634.work.287-kees@kernel.org Signed-off-by: Kees Cook

refcount: Split out refcount_types.h

2023-12-21T00:26:31Z

More trimming of sched.h dependencies. Signed-off-by: Kent Overstreet

sparse: introduce conditional lock acquire function attribute

2022-07-03T18:32:22Z

The kernel tends to try to avoid conditional locking semantics because it makes it harder to think about and statically check locking rules, but we do have a few fundamental locking primitives that take locks conditionally - most obviously the 'trylock' functions. That has always been a problem for 'sparse' checking for locking imbalance, and we've had a special '__cond_lock()' macro that we've used to let sparse know how the locking works: # define __cond_lock(x,c) ((c) ? ({ __acquire(x); 1; }) : 0) so that you can then use this to tell sparse that (for example) the spinlock trylock macro ends up acquiring the lock when it succeeds, but not when it fails: #define raw_spin_trylock(lock) __cond_lock(lock, _raw_spin_trylock(lock)) and then sparse can follow along the locking rules when you have code like if (!spin_trylock(&dentry->d_lock)) return LRU_SKIP; .. sparse sees that the lock is held here.. spin_unlock(&dentry->d_lock); and sparse ends up happy about the lock contexts. However, this '__cond_lock()' use does result in very ugly header files, and requires you to basically wrap the real function with that macro that uses '__cond_lock'. Which has made PeterZ NAK things that try to fix sparse warnings over the years [1]. To solve this, there is now a very experimental patch to sparse that basically does the exact same thing as '__cond_lock()' did, but using a function attribute instead. That seems to make PeterZ happy [2]. Note that this does not replace existing use of '__cond_lock()', but only exposes the new proposed attribute and uses it for the previously unannotated 'refcount_dec_and_lock()' family of functions. For existing sparse installations, this will make no difference (a negative output context was ignored), but if you have the experimental sparse patch it will make sparse now understand code that uses those functions, the same way '__cond_lock()' makes sparse understand the very similar 'atomic_dec_and_lock()' uses that have the old '__cond_lock()' annotations. Note that in some cases this will silence existing context imbalance warnings. But in other cases it may end up exposing new sparse warnings for code that sparse just didn't see the locking for at all before. This is a trial, in other words. I'd expect that if it ends up being successful, and new sparse releases end up having this new attribute, we'll migrate the old-style '__cond_lock()' users to use the new-style '__cond_acquires' function attribute. The actual experimental sparse patch was posted in [3]. Link: https://lore.kernel.org/all/20130930134434.GC12926@twins.programming.kicks-ass.net/ [1] Link: https://lore.kernel.org/all/Yr60tWxN4P568x3W@worktop.programming.kicks-ass.net/ [2] Link: https://lore.kernel.org/all/CAHk-=wjZfO9hGqJ2_hGQG3U_XzSh9_XaXze=HgPdvJbgrvASfA@mail.gmail.com/ [3] Acked-by: Peter Zijlstra Cc: Alexander Aring Cc: Luc Van Oostenryck Signed-off-by: Linus Torvalds

refcount: Fix a kernel-doc markup

2020-12-03T10:20:52Z

The kernel-doc markup is wrong: it is asking the tool to document struct refcount_struct, instead of documenting typedef refcount_t. Signed-off-by: Mauro Carvalho Chehab Signed-off-by: Peter Zijlstra (Intel) Acked-by: Kees Cook Link: https://lkml.kernel.org/r/afb9bb1e675bf5f72a34a55d780779d7d5916b4c.1606823973.git.mchehab+huawei@kernel.org

locking/refcount: move kernel-doc markups to the proper place

2020-10-28T17:41:15Z

Changeset a435b9a14356 ("locking/refcount: Provide __refcount API to obtain the old value") added a set of functions starting with __ that have a new parameter, adding a series of new warnings: $ ./scripts/kernel-doc -none include/linux/refcount.h include/linux/refcount.h:169: warning: Function parameter or member 'oldp' not described in '__refcount_add_not_zero' include/linux/refcount.h:208: warning: Function parameter or member 'oldp' not described in '__refcount_add' include/linux/refcount.h:239: warning: Function parameter or member 'oldp' not described in '__refcount_inc_not_zero' include/linux/refcount.h:261: warning: Function parameter or member 'oldp' not described in '__refcount_inc' include/linux/refcount.h:291: warning: Function parameter or member 'oldp' not described in '__refcount_sub_and_test' include/linux/refcount.h:327: warning: Function parameter or member 'oldp' not described in '__refcount_dec_and_test' include/linux/refcount.h:347: warning: Function parameter or member 'oldp' not described in '__refcount_dec' The issue is that the kernel-doc markups are now misplaced, as they should be added just before the functions. So, move the kernel-doc markups to the proper places, in order to drop the warnings. It should be noticed that git show produces a crappy output, for this patch without "--patience" flag. Fixes: a435b9a14356 ("locking/refcount: Provide __refcount API to obtain the old value") Signed-off-by: Mauro Carvalho Chehab Link: https://lore.kernel.org/r/7985c31d1ace591bc5e1faa05c367f1295b78afd.1603791716.git.mchehab+huawei@kernel.org Signed-off-by: Jonathan Corbet

locking/refcount: Provide __refcount API to obtain the old value

2020-08-26T10:42:02Z

David requested means to obtain the old/previous value from the refcount API for tracing purposes. Duplicate (most of) the API as __refcount*() with an additional 'int *' argument into which, if !NULL, the old value will be stored. Requested-by: David Howells Signed-off-by: Peter Zijlstra (Intel) Reviewed-by: Kees Cook Link: https://lkml.kernel.org/r/20200729111120.GA2638@hirez.programming.kicks-ass.net

locking/refcount: Document interaction with PID_MAX_LIMIT

2020-04-08T10:05:07Z

Document the circumstances under which refcount_t's saturation mechanism works deterministically. Acked-by: Kees Cook Acked-by: Will Deacon Signed-off-by: Jann Horn Signed-off-by: Peter Zijlstra (Intel) Signed-off-by: Ingo Molnar Link: https://lkml.kernel.org/r/20200303105427.260620-1-jannh@google.com