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: - CLONE_THREAD, - CLONE_VM, - CLONE_SETTLS. It appears that the flag we really want to track is CLONE_SETTLS, which moves the location of the TLS for the child, which makes the rseq registration point to the wrong TLS. Suggested-by: "H . Peter Anvin" <hpa@xxxxxxxxx> Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx> Cc: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx> Cc: "Paul E. McKenney" <paulmck@xxxxxxxxxxxxx> Cc: Boqun Feng <boqun.feng@xxxxxxxxx> Cc: "H . Peter Anvin" <hpa@xxxxxxxxx> Cc: Paul Turner <pjt@xxxxxxxxxx> Cc: Dmitry Vyukov <dvyukov@xxxxxxxxxx> Cc: linux-api@xxxxxxxxxxxxxxx --- 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..deb4154dbf11 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_TLS set. */ static inline void rseq_fork(struct task_struct *t, unsigned long clone_flags) { - if (clone_flags & CLONE_THREAD) { + if (clone_flags & CLONE_TLS) { t->rseq = NULL; t->rseq_sig = 0; t->rseq_event_mask = 0; -- 2.17.1