SYSFS: Allow boot time switching between deprecated and modern sysfs layout

I have some systems which need legacy sysfs due to old tools that are
making assumptions that a directory can never be a symlink to another
directory, and it's a big hazzle to compile separate kernels for them.

This patch turns CONFIG_SYSFS_DEPRECATED into a run time option
that can be switched on/off the kernel command line. This way
the same binary can be used in both cases with just a option
on the command line.

The old CONFIG_SYSFS_DEPRECATED_V2 option is still there to set
the default. I kept the weird name to not break existing
config files.

Also the compat code can be still completely disabled by undefining
CONFIG_SYSFS_DEPRECATED_SWITCH -- just the optimizer takes
care of this now instead of lots of ifdefs. This makes the code
look nicer.

v2: This is an updated version on top of Kay's patch to only
handle the block devices. I tested it on my old systems
and that seems to work.

Cc: axboe@kernel.dk
Signed-off-by: Andi Kleen <ak@linux.intel.com>
Cc: Kay Sievers <kay.sievers@vrfy.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

Documentation/kernel-parameters.txt

@@ -2365,6 +2365,15 @@ and is between 256 and 4096 characters. It is defined in the file
 
 	switches=	[HW,M68k]
 
+	sysfs.deprecated=0|1 [KNL]
+			Enable/disable old style sysfs layout for old udev
+			on older distributions. When this option is enabled
+			very new udev will not work anymore. When this option
+			is disabled (or CONFIG_SYSFS_DEPRECATED not compiled)
+			in older udev will not work anymore.
+			Default depends on CONFIG_SYSFS_DEPRECATED_V2 set in
+			the kernel configuration.
+
 	sysrq_always_enabled
 			[KNL]
 			Ignore sysrq setting - this boot parameter will

block/genhd.c

@@ -22,9 +22,7 @@
 #include "blk.h"
 
 static DEFINE_MUTEX(block_class_lock);
-#ifndef CONFIG_SYSFS_DEPRECATED
 struct kobject *block_depr;
-#endif
 
 /* for extended dynamic devt allocation, currently only one major is used */
 #define MAX_EXT_DEVT		(1 << MINORBITS)
@@ -803,10 +801,9 @@ static int __init genhd_device_init(void)
 
 	register_blkdev(BLOCK_EXT_MAJOR, "blkext");
 
-#ifndef CONFIG_SYSFS_DEPRECATED
 	/* create top-level block dir */
+	if (!sysfs_deprecated)
 		block_depr = kobject_create_and_add("block", NULL);
-#endif
 	return 0;
 }
 

drivers/base/class.c

@@ -184,9 +184,9 @@ int __class_register(struct class *cls, struct lock_class_key *key)
 	if (!cls->dev_kobj)
 		cls->dev_kobj = sysfs_dev_char_kobj;
 
-#if defined(CONFIG_SYSFS_DEPRECATED) && defined(CONFIG_BLOCK)
+#if defined(CONFIG_BLOCK)
 	/* let the block class directory show up in the root of sysfs */
-	if (cls != &block_class)
+	if (!sysfs_deprecated || cls != &block_class)
 		cp->class_subsys.kobj.kset = class_kset;
 #else
 	cp->class_subsys.kobj.kset = class_kset;

drivers/base/core.c

@@ -26,6 +26,19 @@
 #include "base.h"
 #include "power/power.h"
 
+#ifdef CONFIG_SYSFS_DEPRECATED
+#ifdef CONFIG_SYSFS_DEPRECATED_V2
+long sysfs_deprecated = 1;
+#else
+long sysfs_deprecated = 0;
+#endif
+static __init int sysfs_deprecated_setup(char *arg)
+{
+	return strict_strtol(arg, 10, &sysfs_deprecated);
+}
+early_param("sysfs.deprecated", sysfs_deprecated_setup);
+#endif
+
 int (*platform_notify)(struct device *dev) = NULL;
 int (*platform_notify_remove)(struct device *dev) = NULL;
 static struct kobject *dev_kobj;
@@ -617,14 +630,13 @@ static struct kobject *get_device_parent(struct device *dev,
 		struct kobject *parent_kobj;
 		struct kobject *k;
 
-#ifdef CONFIG_SYSFS_DEPRECATED
 		/* block disks show up in /sys/block */
-		if (dev->class == &block_class) {
+		if (sysfs_deprecated && dev->class == &block_class) {
 			if (parent && parent->class == &block_class)
 				return &parent->kobj;
 			return &block_class.p->class_subsys.kobj;
 		}
-#endif
+
 		/*
 		 * If we have no parent, we live in "virtual".
 		 * Class-devices with a non class-device as parent, live
@@ -707,11 +719,9 @@ static int device_add_class_symlinks(struct device *dev)
 			goto out_subsys;
 	}
 
-#ifdef CONFIG_SYSFS_DEPRECATED
 	/* /sys/block has directories and does not need symlinks */
-	if (dev->class == &block_class)
+	if (sysfs_deprecated && dev->class == &block_class)
 		return 0;
-#endif
 
 	/* link in the class directory pointing to the device */
 	error = sysfs_create_link(&dev->class->p->class_subsys.kobj,
@@ -738,10 +748,8 @@ static void device_remove_class_symlinks(struct device *dev)
 	if (dev->parent && device_is_not_partition(dev))
 		sysfs_remove_link(&dev->kobj, "device");
 	sysfs_remove_link(&dev->kobj, "subsystem");
-#ifdef CONFIG_SYSFS_DEPRECATED
-	if (dev->class == &block_class)
+	if (sysfs_deprecated && dev->class == &block_class)
 		return;
-#endif
 	sysfs_delete_link(&dev->class->p->class_subsys.kobj, &dev->kobj, dev_name(dev));
 }
 

fs/partitions/check.c

@@ -513,14 +513,14 @@ void register_disk(struct gendisk *disk)
 
 	if (device_add(ddev))
 		return;
-#ifndef CONFIG_SYSFS_DEPRECATED
+	if (!sysfs_deprecated) {
 		err = sysfs_create_link(block_depr, &ddev->kobj,
 					kobject_name(&ddev->kobj));
 		if (err) {
 			device_del(ddev);
 			return;
 		}
-#endif
+	}
 	disk->part0.holder_dir = kobject_create_and_add("holders", &ddev->kobj);
 	disk->slave_dir = kobject_create_and_add("slaves", &ddev->kobj);
 
@@ -737,8 +737,7 @@ void del_gendisk(struct gendisk *disk)
 	kobject_put(disk->part0.holder_dir);
 	kobject_put(disk->slave_dir);
 	disk->driverfs_dev = NULL;
-#ifndef CONFIG_SYSFS_DEPRECATED
+	if (!sysfs_deprecated)
 		sysfs_remove_link(block_depr, dev_name(disk_to_dev(disk)));
-#endif
 	device_del(disk_to_dev(disk));
 }

include/linux/device.h

@@ -751,4 +751,11 @@ do {						     \
 	MODULE_ALIAS("char-major-" __stringify(major) "-" __stringify(minor))
 #define MODULE_ALIAS_CHARDEV_MAJOR(major) \
 	MODULE_ALIAS("char-major-" __stringify(major) "-*")
+
+#ifdef CONFIG_SYSFS_DEPRECATED
+extern long sysfs_deprecated;
+#else
+#define sysfs_deprecated 0
+#endif
+
 #endif /* _DEVICE_H_ */

init/Kconfig

@@ -660,8 +660,12 @@ config SYSFS_DEPRECATED
 	depends on SYSFS
 	default n
 	help
-	  This option switches the layout of the "block" class devices, to not
-	  show up in /sys/class/block/, but only in /sys/block/.
+	  This option adds code that switches the layout of the "block" class
+	  devices, to not show up in /sys/class/block/, but only in
+	  /sys/block/.
+
+	  This switch is only active when the sysfs.deprecated=1 boot option is
+	  passed or the SYSFS_DEPRECATED_V2 option is set.
 
 	  This option allows new kernels to run on old distributions and tools,
 	  which might get confused by /sys/class/block/. Since 2007/2008 all
@@ -672,8 +676,22 @@ config SYSFS_DEPRECATED
 	  option enabled.
 
 	  Only if you are using a new kernel on an old distribution, you might
-	  need to say Y here. Never say Y, if the original kernel, that came
-	  with your distribution, has not set this option.
+	  need to say Y here.
+
+config SYSFS_DEPRECATED_V2
+	bool "enabled deprecated sysfs features by default"
+	default n
+	depends on SYSFS
+	depends on SYSFS_DEPRECATED
+	help
+	  Enable deprecated sysfs by default.
+
+	  See the CONFIG_SYSFS_DEPRECATED option for more details about this
+	  option.
+
+	  Only if you are using a new kernel on an old distribution, you might
+	  need to say Y here. Even then, odds are you would not need it
+	  enabled, you can always pass the boot option if absolutely necessary.
 
 config RELAY
 	bool "Kernel->user space relay support (formerly relayfs)"