rseq: Use pr_warn_once() when deprecated/unknown ABI flags are encountered

These commits use WARN_ON_ONCE() and kill the offending processes when
deprecated and unknown flags are encountered:

commit c17a6ff93213 ("rseq: Kill process when unknown flags are encountered in ABI structures")
commit 0190e4198e47 ("rseq: Deprecate RSEQ_CS_FLAG_NO_RESTART_ON_* flags")

The WARN_ON_ONCE() triggered by userspace input prevents use of
Syzkaller to fuzz the rseq system call.

Replace this WARN_ON_ONCE() by pr_warn_once() messages which contain
actually useful information.

Reported-by: Mark Rutland <[e-mail]>
Signed-off-by: Mathieu Desnoyers <[e-mail]>
Signed-off-by: Peter Zijlstra (Intel) <[e-mail]>
Acked-by: Mark Rutland <[e-mail]>
Acked-by: Paul E. McKenney <[e-mail]>
Link: https://lkml.kernel.org/r/[e-mail]

kernel/rseq.c

···
 	return 0;
 }
 
+static bool rseq_warn_flags(const char *str, u32 flags)
+{
+	u32 test_flags;
+
+	if (!flags)
+		return false;
+	test_flags = flags & RSEQ_CS_NO_RESTART_FLAGS;
+	if (test_flags)
+		pr_warn_once("Deprecated flags (%u) in %s ABI structure", test_flags, str);
+	test_flags = flags & ~RSEQ_CS_NO_RESTART_FLAGS;
+	if (test_flags)
+		pr_warn_once("Unknown flags (%u) in %s ABI structure", test_flags, str);
+	return true;
+}
+
 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) || cs_flags)
+	if (rseq_warn_flags("rseq_cs", cs_flags))
 		return -EINVAL;
 
 	/* Get thread flags. */
···
 	if (ret)
 		return ret;
 
-	if (WARN_ON_ONCE(flags & RSEQ_CS_NO_RESTART_FLAGS) || flags)
+	if (rseq_warn_flags("rseq", flags))
 		return -EINVAL;
 
 	/*