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Subject[PATCH 03/16] sched: convert signal_struct.sigcnt to refcount_t
atomic_t variables are currently used to implement reference
counters with the following properties:
- counter is initialized to 1 using atomic_set()
- a resource is freed upon counter reaching zero
- once counter reaches zero, its further
increments aren't allowed
- counter schema uses basic atomic operations
(set, inc, inc_not_zero, dec_and_test, etc.)

Such atomic variables should be converted to a newly provided
refcount_t type and API that prevents accidental counter overflows
and underflows. This is important since overflows and underflows
can lead to use-after-free situation and be exploitable.

The variable signal_struct.sigcnt is used as pure reference counter.
Convert it to refcount_t and fix up the operations.

**Important note for maintainers:

Some functions from refcount_t API defined in lib/refcount.c
have different memory ordering guarantees than their atomic
The full comparison can be seen in and it is hopefully soon
in state to be merged to the documentation tree.
Normally the differences should not matter since refcount_t provides
enough guarantees to satisfy the refcounting use cases, but in
some rare cases it might matter.
Please double check that you don't have some undocumented
memory guarantees for this variable usage.

For the signal_struct.sigcnt it might make a difference
in following places:
- put_signal_struct(): decrement in refcount_dec_and_test() only
provides RELEASE ordering and control dependency on success
vs. fully ordered atomic counterpart

Suggested-by: Kees Cook <>
Reviewed-by: David Windsor <>
Reviewed-by: Hans Liljestrand <>
Signed-off-by: Elena Reshetova <>
include/linux/sched/signal.h | 2 +-
kernel/fork.c | 6 +++---
2 files changed, 4 insertions(+), 4 deletions(-)

diff --git a/include/linux/sched/signal.h b/include/linux/sched/signal.h
index 4a0e2d8..14e3a0c 100644
--- a/include/linux/sched/signal.h
+++ b/include/linux/sched/signal.h
@@ -78,7 +78,7 @@ struct thread_group_cputimer {
* the locking of signal_struct.
struct signal_struct {
- atomic_t sigcnt;
+ refcount_t sigcnt;
atomic_t live;
int nr_threads;
struct list_head thread_head;
diff --git a/kernel/fork.c b/kernel/fork.c
index be451af..a65ec7d 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -642,7 +642,7 @@ static inline void free_signal_struct(struct signal_struct *sig)

static inline void put_signal_struct(struct signal_struct *sig)
- if (atomic_dec_and_test(&sig->sigcnt))
+ if (refcount_dec_and_test(&sig->sigcnt))

@@ -1443,7 +1443,7 @@ static int copy_signal(unsigned long clone_flags, struct task_struct *tsk)

sig->nr_threads = 1;
atomic_set(&sig->live, 1);
- atomic_set(&sig->sigcnt, 1);
+ refcount_set(&sig->sigcnt, 1);

/* list_add(thread_node, thread_head) without INIT_LIST_HEAD() */
sig->thread_head = (struct list_head)LIST_HEAD_INIT(tsk->thread_node);
@@ -1952,7 +1952,7 @@ static __latent_entropy struct task_struct *copy_process(
} else {
- atomic_inc(&current->signal->sigcnt);
+ refcount_inc(&current->signal->sigcnt);
 \ /
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