mdev: create devices from /sys/dev

Currently some new devices that have a bus but no class will
be missed by mdev coldplug device creation after boot. This
happens because mdev recursively searches /sys/class which will
by definition only find class devices.

Some important devices such as iio and gpiochip does not have
a class. But users will need them.

This switches from using /sys/class as the place to look for
devices to create to using /sys/dev where all char and block
devices are listed.

The subsystem lookup code that provide the G.subsystem
environment variable is changed from using the directory
name of the class device to instead dereference the
"subsystem" symlink for the device, and look at the last
element of the path of the symlink for the subsystem, which
will work with class devices and bus devices alike. (The new
bus-only devices only symlink to the /sys/bus/* hierarchy.)

We delete the legacy kernel v2.6.2x /sys/block device path
code as part of this change. It's too old to be kept alive.

Tested on kernel v4.6-rc2 with a bunch of devices, including
some IIO and gpiochip devices.

With a print inserted before make_device() the log looks
like so:

Create device from "/sys/dev/char/1:1", subsystem "mem"
Create device from "/sys/dev/char/1:2", subsystem "mem"
Create device from "/sys/dev/char/1:3", subsystem "mem"
Create device from "/sys/dev/char/1:5", subsystem "mem"
(...)
Create device from "/sys/dev/block/179:56", subsystem "block"
Create device from "/sys/dev/block/179:64", subsystem "block"

function                                             old     new   delta
mdev_main                                           1388    1346     -42
dirAction                                            134      14    -120
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 0/2 up/down: 0/-162)           Total: -162 bytes

Cc: Isaac Dunham <ibid.ag@gmail.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Jonathan Cameron <jic23@cam.ac.uk>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

1 files changed, 41 insertions, 47 deletions

diff --git a/util-linux/mdev.c b/util-linux/mdev.c
index 37514eb54..a59115dd4 100644
--- a/util-linux/mdev.c
+++ b/util-linux/mdev.c
@@ -543,8 +543,7 @@ static char *build_alias(char *alias, const char *device_name)
 
 /* mknod in /dev based on a path like "/sys/block/hda/hda1"
  * NB1: path parameter needs to have SCRATCH_SIZE scratch bytes
- * after NUL, but we promise to not mangle (IOW: to restore NUL if needed)
- * path string.
+ * after NUL, but we promise to not mangle it (IOW: to restore NUL if needed).
  * NB2: "mdev -s" may call us many times, do not leak memory/fds!
  *
  * device_name = $DEVNAME (may be NULL)
@@ -810,41 +809,39 @@ static void make_device(char *device_name, char *path, int operation)
 	} /* for (;;) */
 }
 
-/* File callback for /sys/ traversal */
+/* File callback for /sys/ traversal.
+ * We act only on "/sys/.../dev" (pseudo)file
+ */
 static int FAST_FUNC fileAction(const char *fileName,
 		struct stat *statbuf UNUSED_PARAM,
 		void *userData,
 		int depth UNUSED_PARAM)
 {
 	size_t len = strlen(fileName) - 4; /* can't underflow */
-	char *scratch = userData;
-
-	/* len check is for paranoid reasons */
-	if (strcmp(fileName + len, "/dev") != 0 || len >= PATH_MAX)
-		return FALSE;
-
-	strcpy(scratch, fileName);
-	scratch[len] = '\0';
-	make_device(/*DEVNAME:*/ NULL, scratch, OP_add);
-
-	return TRUE;
-}
-
-/* Directory callback for /sys/ traversal */
-static int FAST_FUNC dirAction(const char *fileName UNUSED_PARAM,
-		struct stat *statbuf UNUSED_PARAM,
-		void *userData UNUSED_PARAM,
-		int depth)
-{
-	/* Extract device subsystem -- the name of the directory
-	 * under /sys/class/ */
-	if (1 == depth) {
+	char *path = userData;	/* char array[PATH_MAX + SCRATCH_SIZE] */
+	char subsys[PATH_MAX];
+	int res;
+
+	/* Is it a ".../dev" file? (len check is for paranoid reasons) */
+	if (strcmp(fileName + len, "/dev") != 0 || len >= PATH_MAX - 32)
+		return FALSE; /* not .../dev */
+
+	strcpy(path, fileName);
+	path[len] = '\0';
+
+	/* Read ".../subsystem" symlink in the same directory where ".../dev" is */
+	strcpy(subsys, path);
+	strcpy(subsys + len, "/subsystem");
+	res = readlink(subsys, subsys, sizeof(subsys)-1);
+	if (res > 0) {
+		subsys[res] = '\0';
 		free(G.subsystem);
 		if (G.subsys_env) {
 			bb_unsetenv_and_free(G.subsys_env);
 			G.subsys_env = NULL;
 		}
-		G.subsystem = strrchr(fileName, '/');
+		/* Set G.subsystem and $SUBSYSTEM from symlink's last component */
+		G.subsystem = strrchr(subsys, '/');
 		if (G.subsystem) {
 			G.subsystem = xstrdup(G.subsystem + 1);
 			G.subsys_env = xasprintf("%s=%s", "SUBSYSTEM", G.subsystem);
@@ -852,6 +849,17 @@ static int FAST_FUNC dirAction(const char *fileName UNUSED_PARAM,
 		}
 	}
 
+	make_device(/*DEVNAME:*/ NULL, path, OP_add);
+
+	return TRUE;
+}
+
+/* Directory callback for /sys/ traversal */
+static int FAST_FUNC dirAction(const char *fileName UNUSED_PARAM,
+		struct stat *statbuf UNUSED_PARAM,
+		void *userData UNUSED_PARAM,
+		int depth)
+{
 	return (depth >= MAX_SYSFS_DEPTH ? SKIP : TRUE);
 }
 
@@ -872,8 +880,9 @@ static void load_firmware(const char *firmware, const char *sysfs_path)
 	int firmware_fd, loading_fd;
 
 	/* check for /lib/firmware/$FIRMWARE */
-	xchdir("/lib/firmware");
-	firmware_fd = open(firmware, O_RDONLY); /* can fail */
+	firmware_fd = -1;
+	if (chdir("/lib/firmware") == 0)
+		firmware_fd = open(firmware, O_RDONLY); /* can fail */
 
 	/* check for /sys/$DEVPATH/loading ... give 30 seconds to appear */
 	xchdir(sysfs_path);
@@ -1065,25 +1074,10 @@ int mdev_main(int argc UNUSED_PARAM, char **argv)
 
 		putenv((char*)"ACTION=add");
 
-		/* ACTION_FOLLOWLINKS is needed since in newer kernels
-		 * /sys/block/loop* (for example) are symlinks to dirs,
-		 * not real directories.
-		 * (kernel's CONFIG_SYSFS_DEPRECATED makes them real dirs,
-		 * but we can't enforce that on users)
-		 */
-		if (access("/sys/class/block", F_OK) != 0) {
-			/* Scan obsolete /sys/block only if /sys/class/block
-			 * doesn't exist. Otherwise we'll have dupes.
-			 * Also, do not complain if it doesn't exist.
-			 * Some people configure kernel to have no blockdevs.
-			 */
-			recursive_action("/sys/block",
-				ACTION_RECURSE | ACTION_FOLLOWLINKS | ACTION_QUIET,
-				fileAction, dirAction, temp, 0);
-		}
-		recursive_action("/sys/class",
-			ACTION_RECURSE | ACTION_FOLLOWLINKS,
-			fileAction, dirAction, temp, 0);
+		/* Create all devices from /sys/dev hierarchy */
+		recursive_action("/sys/dev",
+				 ACTION_RECURSE | ACTION_FOLLOWLINKS,
+				 fileAction, dirAction, temp, 0);
 	} else {
 		char *fw;
 		char *seq;