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Subject[PATCH for 5.3 2/3] rseq: Fix: Unregister rseq for CLONE_SETTLS
It has been reported by Google that rseq is not behaving properly
with respect to clone when CLONE_VM is used without CLONE_THREAD.
It keeps the prior thread's rseq TLS registered when the TLS of the
thread has moved, so the kernel deals with the wrong TLS.

The approach of clearing the per task-struct rseq registration
on clone with CLONE_THREAD flag is incomplete. It does not cover
the use-case of clone with CLONE_VM set, but without CLONE_THREAD.

Looking more closely at each of the clone flags:


It appears that the flag we really want to track is CLONE_SETTLS, which
moves the location of the TLS for the child, making the rseq
registration point to the wrong TLS.

Suggested-by: "H . Peter Anvin" <>
Signed-off-by: Mathieu Desnoyers <>
Cc: Thomas Gleixner <>
Cc: Peter Zijlstra (Intel) <>
Cc: "Paul E. McKenney" <>
Cc: Boqun Feng <>
Cc: "H . Peter Anvin" <>
Cc: Paul Turner <>
Cc: Dmitry Vyukov <>
Cc: <>
include/linux/sched.h | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 9f51932bd543..76bf55b5cccf 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1919,11 +1919,11 @@ static inline void rseq_migrate(struct task_struct *t)

* If parent process has a registered restartable sequences area, the
- * child inherits. Only applies when forking a process, not a thread.
+ * child inherits. Unregister rseq for a clone with CLONE_SETTLS set.
static inline void rseq_fork(struct task_struct *t, unsigned long clone_flags)
- if (clone_flags & CLONE_THREAD) {
+ if (clone_flags & CLONE_SETTLS) {
t->rseq = NULL;
t->rseq_sig = 0;
t->rseq_event_mask = 0;
 \ /
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