rseq: Deprecate RSEQ_CS_FLAG_NO_RESTART_ON_* flags

The pretty much unused RSEQ_CS_FLAG_NO_RESTART_ON_* flags introduce
complexity in rseq, and are subtly buggy [1]. Solving those issues
requires introducing additional complexity in the rseq implementation
for each supported architecture.

Considering that it complexifies the rseq ABI, I am proposing that we
deprecate those flags. [2]

So far there appears to be consensus from maintainers of user-space
projects impacted by this feature that its removal would be a welcome
simplification. [3]

The deprecation approach proposed here is to issue WARN_ON_ONCE() when
encountering those flags and kill the offending process with sigsegv.
This should allow us to quickly identify whether anyone yells at us for
removing this.

Link: https://lore.kernel.org/lkml/20220618182515.95831-1-minhquangbui99@gmail.com/ [1]
Link: https://lore.kernel.org/lkml/258546133.12151.1655739550814.JavaMail.zimbra@efficios.com/ [2]
Link: https://lore.kernel.org/lkml/87pmj1enjh.fsf@email.froward.int.ebiederm.org/ [3]
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/lkml/20220622194617.1155957-1-mathieu.desnoyers@efficios.com

kernel/rseq.c

@@ -18,8 +18,9 @@
 #define CREATE_TRACE_POINTS
 #include <trace/events/rseq.h>
 
-#define RSEQ_CS_PREEMPT_MIGRATE_FLAGS (RSEQ_CS_FLAG_NO_RESTART_ON_MIGRATE | \
-				       RSEQ_CS_FLAG_NO_RESTART_ON_PREEMPT)
+#define RSEQ_CS_NO_RESTART_FLAGS (RSEQ_CS_FLAG_NO_RESTART_ON_PREEMPT | \
+				  RSEQ_CS_FLAG_NO_RESTART_ON_SIGNAL | \
+				  RSEQ_CS_FLAG_NO_RESTART_ON_MIGRATE)
 
 /*
  *
@@ -175,23 +176,15 @@ static int rseq_need_restart(struct task_struct *t, u32 cs_flags)
 	u32 flags, event_mask;
 	int ret;
 
+	if (WARN_ON_ONCE(cs_flags & RSEQ_CS_NO_RESTART_FLAGS))
+		return -EINVAL;
+
 	/* Get thread flags. */
 	ret = get_user(flags, &t->rseq->flags);
 	if (ret)
 		return ret;
 
-	/* Take critical section flags into account. */
-	flags |= cs_flags;
-
-	/*
-	 * Restart on signal can only be inhibited when restart on
-	 * preempt and restart on migrate are inhibited too. Otherwise,
-	 * a preempted signal handler could fail to restart the prior
-	 * execution context on sigreturn.
-	 */
-	if (unlikely((flags & RSEQ_CS_FLAG_NO_RESTART_ON_SIGNAL) &&
-		     (flags & RSEQ_CS_PREEMPT_MIGRATE_FLAGS) !=
-		     RSEQ_CS_PREEMPT_MIGRATE_FLAGS))
+	if (WARN_ON_ONCE(flags & RSEQ_CS_NO_RESTART_FLAGS))
 		return -EINVAL;
 
 	/*
@@ -203,7 +196,7 @@ static int rseq_need_restart(struct task_struct *t, u32 cs_flags)
 	t->rseq_event_mask = 0;
 	preempt_enable();
 
-	return !!(event_mask & ~flags);
+	return !!event_mask;
 }
 
 static int clear_rseq_cs(struct task_struct *t)