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>

7 files changed, 68 insertions, 30 deletions

diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
index 3d854c9ae53a..67fa3fdc128f 100644
--- a/Documentation/kernel-parameters.txt
+++ b/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
diff --git a/block/genhd.c b/block/genhd.c
index 59a2db6fecef..4e28a840bde0 100644
--- a/block/genhd.c
+++ b/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 */
-        block_depr = kobject_create_and_add("block", NULL);
-#endif
+        if (!sysfs_deprecated)
+                block_depr = kobject_create_and_add("block", NULL);
         return 0;
 }
 
diff --git a/drivers/base/class.c b/drivers/base/class.c
index 1078969889fd..9c63a5687d69 100644
--- a/drivers/base/class.c
+++ b/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;
diff --git a/drivers/base/core.c b/drivers/base/core.c
index 6cf9069f3150..f7f906f0a2f2 100644
--- a/drivers/base/core.c
+++ b/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));
 }
 
diff --git a/fs/partitions/check.c b/fs/partitions/check.c
index 79fbf3f390f0..137bf9787853 100644
--- a/fs/partitions/check.c
+++ b/fs/partitions/check.c
@@ -513,14 +513,14 @@ void register_disk(struct gendisk *disk)
 
         if (device_add(ddev))
                 return;
-#ifndef CONFIG_SYSFS_DEPRECATED
-        err = sysfs_create_link(block_depr, &ddev->kobj,
-                                kobject_name(&ddev->kobj));
-        if (err) {
-                device_del(ddev);
-                return;
+        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
-        sysfs_remove_link(block_depr, dev_name(disk_to_dev(disk)));
-#endif
+        if (!sysfs_deprecated)
+                sysfs_remove_link(block_depr, dev_name(disk_to_dev(disk)));
         device_del(disk_to_dev(disk));
 }
diff --git a/include/linux/device.h b/include/linux/device.h
index 516fecacf27b..dd4895313468 100644
--- a/include/linux/device.h
+++ b/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_ */
diff --git a/init/Kconfig b/init/Kconfig
index 137609f33ebc..d742b6fca8d2 100644
--- a/init/Kconfig
+++ b/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)"