Add support to rh_waived cmdline boot parameter

JIRA: https://issues.redhat.com/browse/RHEL-122981

Conflicts: Minor context difference due to RHEL-only commit
           c7db2ac ("kernel: Add redhat code")

This patch is a backport of the following RHEL-only commit:
commit 13d53ad70d726bfd8ffcea9c4891c5eaa6254dfb
Author: Ricardo Robaina <rrobaina@redhat.com>
Date:   Wed May 22 10:20:47 2024 -0300

    Add support to rh_waived cmdline boot parameter

    JIRA: https://issues.redhat.com/browse/RHEL-26170

    Upstream Status: RHEL only

    This commit adds support to a standard mechanism to hide waived features
    behind a common cmdline parameter in RHEL, called rh_waived.

    Signed-off-by: Ricardo Robaina <rrobaina@redhat.com>

Signed-off-by: Ricardo Robaina <rrobaina@redhat.com>

Author: rprobaina

Documentation/admin-guide/kernel-parameters.txt

@@ -5734,6 +5734,17 @@
 	rhash_entries=	[KNL,NET]
 			Set number of hash buckets for route cache
 
+	rh_waived=
+			Enable waived features in RHEL.
+
+			Waived features are disabled by default in RHEL, this parameter
+			provides support to enable such features, as needed.
+
+			Format: <feat-1>,<feat-2>...<feat-n>
+
+			Use 'rh_waived' to enable all waived features listed at
+			Documentation/admin-guide/rh-waived-features.rst
+
 	ring3mwait=disable
 			[KNL] Disable ring 3 MONITOR/MWAIT feature on supported
 			CPUs.

Documentation/admin-guide/rh-waived-features.rst

@@ -0,0 +1,21 @@
+.. _rh_waived_features:
+
+=======================
+Red Hat Waived Features
+=======================
+
+Red Hat waived features are features considered unmaintained, insecure, rudimentary, or
+deprecated and are shipped in RHEL only for customer convenience. These features are disabled
+by default but can be enabled on demand via the ``rh_waived`` kernel boot parameter. To allow
+a set of waived features, append ``rh_waived=<feature name>,...,<feature name>`` to the kernel
+cmdline. Appending only ``rh_waived`` (with no arguments) will enable all waived features
+listed below.
+
+The waived features listed in the next session follow the pattern below:
+
+- feature name
+        feature description
+
+List of Red Hat Waived Features
+===============================
+

include/linux/rh_waived.h

@@ -0,0 +1,19 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * include/linux/rh_waived.h
+ *
+ * rh_waived cmdline parameter interface.
+ *
+ * Copyright (C) 2024, Red Hat, Inc.  Ricardo Robaina <rrobaina@redhat.com>
+ */
+#ifndef _RH_WAIVED_H
+#define _RH_WAIVED_H
+
+enum rh_waived_feat {
+	/* RH_WAIVED_FEAT_ITEMS must always be the last item in the enum */
+	RH_WAIVED_FEAT_ITEMS,
+};
+
+bool is_rh_waived(enum rh_waived_feat feat);
+
+#endif /* _RH_WAIVED_H */

kernel/Makefile

@@ -12,7 +12,7 @@ obj-y     = fork.o exec_domain.o panic.o \
 	    notifier.o ksysfs.o cred.o reboot.o \
 	    async.o range.o smpboot.o ucount.o regset.o ksyms_common.o
 
-obj-$(CONFIG_RHEL_DIFFERENCES) += rh_messages.o rh_flags.o rh_shadowman.o
+obj-$(CONFIG_RHEL_DIFFERENCES) += rh_messages.o rh_flags.o rh_waived.o rh_shadowman.o
 obj-$(CONFIG_USERMODE_DRIVER) += usermode_driver.o
 obj-$(CONFIG_MULTIUSER) += groups.o
 

kernel/rh_waived.c

@@ -0,0 +1,104 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * kernel/rh_waived.c
+ *
+ * rh_waived cmdline parameter support.
+ *
+ * Copyright (C) 2024, Red Hat, Inc.  Ricardo Robaina <rrobaina@redhat.com>
+ */
+#include <linux/types.h>
+#include <linux/init.h>
+#include <linux/printk.h>
+#include <linux/string.h>
+#include <linux/panic.h>
+#include <linux/module.h>
+#include <linux/kernel.h>
+#include <linux/rh_flags.h>
+#include <linux/rh_waived.h>
+
+/*
+ * RH_INSERT_WAIVED_FEAT
+ *   This macro is intended to be used to insert items into the
+ *   rh_waived_feat_list array. It expects to get an item from
+ *   enum rh_waived_feat as its first argument, and a string
+ *   holding the feature name as its second argument.
+ *
+ *   The feature name is also utilized as the token for the
+ *   boot parameter parser.
+ *
+ *   Example usage:
+ *   struct rh_waived_feat_item foo[RH_WAIVED_FEAT_ITEMS] = {
+ *   	RH_INSERT_WAIVED_FEAT(FOO_FEAT, "foo_feat_short_str"),
+ *   };
+ */
+#define RH_INSERT_WAIVED_FEAT(enum_item, name) \
+	[(enum_item)] = {.feat_name = (name), .enabled = 0,}
+
+/* Indicates if the rh_flag 'rh_waived' should be added. */
+bool __initdata add_rh_flag = false;
+
+struct rh_waived_feat_item {
+	char *feat_name;
+	unsigned int enabled;
+};
+
+/* Always use the marco RH_INSERT_WAIVED_FEAT to insert items to this array. */
+struct rh_waived_feat_item rh_waived_feat_list[RH_WAIVED_FEAT_ITEMS] = {
+};
+
+/*
+ * is_rh_waived() - Checks if a given waived feature has been enabled.
+ *
+ * @feat: waived feature.
+ */
+__inline__ bool is_rh_waived(enum rh_waived_feat feat)
+{
+	return !!rh_waived_feat_list[feat].enabled;
+}
+EXPORT_SYMBOL(is_rh_waived);
+
+static int __init rh_waived_setup(char *s)
+{
+	int i;
+	char *token;
+
+	pr_info(KERN_CONT "rh_waived: ");
+
+	if (!s) {
+		for (i = 0; i < RH_WAIVED_FEAT_ITEMS; i++) {
+			rh_waived_feat_list[i].enabled = 1;
+			pr_info(KERN_CONT "%s%s", rh_waived_feat_list[i].feat_name,
+				i < RH_WAIVED_FEAT_ITEMS - 1 ? " " : "\n");
+		}
+	}
+
+	while ((token = strsep(&s, ",")) != NULL) {
+		for (i = 0; i < RH_WAIVED_FEAT_ITEMS; i++) {
+			if (!strcmp(token, rh_waived_feat_list[i].feat_name)) {
+				rh_waived_feat_list[i].enabled = 1;
+				pr_info(KERN_CONT "%s%s", rh_waived_feat_list[i].feat_name,
+					i < RH_WAIVED_FEAT_ITEMS - 1 ? " " : "\n");
+			}
+		}
+	}
+
+	add_rh_flag = true;
+
+	return 0;
+}
+early_param("rh_waived", rh_waived_setup);
+
+/*
+ * rh_flags is initialized at subsys_initcall, calling rh_add_flag()
+ * from rh_waived_setup() would result in a can't boot situation.
+ * Deffering the inclusion 'rh_waived' rh_flag to late_initcall to
+ * avoid this issue.
+ */
+static int __init __add_rh_flag(void)
+{
+	if (add_rh_flag)
+		rh_add_flag("rh_waived");
+
+	return 0;
+}
+late_initcall(__add_rh_flag);