[PATCH] Simplify proc/devices and fix early termination regression

Repair /proc/devices early-termination regression.

2.6.16 broke /proc/devices.  An application often gets an
EOF before the end of data is reached, if that application
uses a series of short read(2)s to access the data.  I have
used read buffers of varying sizes with varying degrees
of unsuccess (larger sizes get further into the data than
smaller sizes, following a simple pattern).  It appears
that the only safe way to get the data is to use a single
read buffer larger than all the data in /proc/devices.

The following example demonstates the problem:

    # dd if=/proc/devices bs=1
    Character devices:
      1 mem
    27+0 records in
    27+0 records out

This patch is a backport of the fix recently accepted to
Linus's tree:

    commit 68eef3b4791572ecb70249c7fb145bb3742dd899
    [PATCH] Simplify proc/devices and fix early termination regression

It replaces the complex, state-machine algorithm introduced
in 2.6.16 with a simple algorithm, modeled on the implementation
of /proc/interrupts.

[akpm@osdl.org: cleanups, simplifications]

Signed-off-by: Joe Korty <joe.korty@ccur.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

2 files changed, 47 insertions, 203 deletions

diff --git a/fs/char_dev.c b/fs/char_dev.c
index 21195c48163..4e163afc168 100644
--- a/fs/char_dev.c
+++ b/fs/char_dev.c
@@ -15,6 +15,7 @@
 #include <linux/module.h>
 #include <linux/smp_lock.h>
 #include <linux/devfs_fs_kernel.h>
+#include <linux/seq_file.h>
 
 #include <linux/kobject.h>
 #include <linux/kobj_map.h>
@@ -26,8 +27,6 @@
 
 static struct kobj_map *cdev_map;
 
-#define MAX_PROBE_HASH 255	/* random */
-
 static DECLARE_MUTEX(chrdevs_lock);
 
 static struct char_device_struct {
@@ -38,93 +37,29 @@ static struct char_device_struct {
 	char name[64];
 	struct file_operations *fops;
 	struct cdev *cdev;		/* will die */
-} *chrdevs[MAX_PROBE_HASH];
+} *chrdevs[CHRDEV_MAJOR_HASH_SIZE];
 
 /* index in the above */
 static inline int major_to_index(int major)
 {
-	return major % MAX_PROBE_HASH;
-}
-
-struct chrdev_info {
-	int index;
-	struct char_device_struct *cd;
-};
-
-void *get_next_chrdev(void *dev)
-{
-	struct chrdev_info *info;
-
-	if (dev == NULL) {
-		info = kmalloc(sizeof(*info), GFP_KERNEL);
-		if (!info)
-			goto out;
-		info->index=0;
-		info->cd = chrdevs[info->index];
-		if (info->cd)
-			goto out;
-	} else {
-		info = dev;
-	}
-
-	while (info->index < ARRAY_SIZE(chrdevs)) {
-		if (info->cd)
-			info->cd = info->cd->next;
-		if (info->cd)
-			goto out;
-		/*
-		 * No devices on this chain, move to the next
-		 */
-		info->index++;
-		info->cd = (info->index < ARRAY_SIZE(chrdevs)) ?
-			chrdevs[info->index] : NULL;
-		if (info->cd)
-			goto out;
-	}
-
-out:
-	return info;
-}
-
-void *acquire_chrdev_list(void)
-{
-	down(&chrdevs_lock);
-	return get_next_chrdev(NULL);
-}
-
-void release_chrdev_list(void *dev)
-{
-	up(&chrdevs_lock);
-	kfree(dev);
+	return major % CHRDEV_MAJOR_HASH_SIZE;
 }
 
+#ifdef CONFIG_PROC_FS
 
-int count_chrdev_list(void)
+void chrdev_show(struct seq_file *f, off_t offset)
 {
 	struct char_device_struct *cd;
-	int i, count;
-
-	count = 0;
 
-	for (i = 0; i < ARRAY_SIZE(chrdevs) ; i++) {
-		for (cd = chrdevs[i]; cd; cd = cd->next)
-			count++;
+	if (offset < CHRDEV_MAJOR_HASH_SIZE) {
+		down(&chrdevs_lock);
+		for (cd = chrdevs[offset]; cd; cd = cd->next)
+			seq_printf(f, "%3d %s\n", cd->major, cd->name);
+		up(&chrdevs_lock);
 	}
-
-	return count;
 }
 
-int get_chrdev_info(void *dev, int *major, char **name)
-{
-	struct chrdev_info *info = dev;
-
-	if (info->cd == NULL)
-		return 1;
-
-	*major = info->cd->major;
-	*name = info->cd->name;
-	return 0;
-}
+#endif /* CONFIG_PROC_FS */
 
 /*
  * Register a single major with a specified minor range.
diff --git a/fs/proc/proc_misc.c b/fs/proc/proc_misc.c
index 826c131994c..0c783121364 100644
--- a/fs/proc/proc_misc.c
+++ b/fs/proc/proc_misc.c
@@ -249,144 +249,60 @@ static int cpuinfo_open(struct inode *inode, struct file *file)
 	return seq_open(file, &cpuinfo_op);
 }
 
-enum devinfo_states {
-	CHR_HDR,
-	CHR_LIST,
-	BLK_HDR,
-	BLK_LIST,
-	DEVINFO_DONE
-};
-
-struct devinfo_state {
-	void *chrdev;
-	void *blkdev;
-	unsigned int num_records;
-	unsigned int cur_record;
-	enum devinfo_states state;
+static struct file_operations proc_cpuinfo_operations = {
+	.open		= cpuinfo_open,
+	.read		= seq_read,
+	.llseek		= seq_lseek,
+	.release	= seq_release,
 };
 
-static void *devinfo_start(struct seq_file *f, loff_t *pos)
+static int devinfo_show(struct seq_file *f, void *v)
 {
-	struct devinfo_state *info = f->private;
+	int i = *(loff_t *) v;
 
-	if (*pos) {
-		if ((info) && (*pos <= info->num_records))
-			return info;
-		return NULL;
+	if (i < CHRDEV_MAJOR_HASH_SIZE) {
+		if (i == 0)
+			seq_printf(f, "Character devices:\n");
+		chrdev_show(f, i);
+	} else {
+		i -= CHRDEV_MAJOR_HASH_SIZE;
+		if (i == 0)
+			seq_printf(f, "\nBlock devices:\n");
+		blkdev_show(f, i);
 	}
-	info = kmalloc(sizeof(*info), GFP_KERNEL);
-	f->private = info;
-	info->chrdev = acquire_chrdev_list();
-	info->blkdev = acquire_blkdev_list();
-	info->state = CHR_HDR;
-	info->num_records = count_chrdev_list();
-	info->num_records += count_blkdev_list();
-	info->num_records += 2; /* Character and Block headers */
-	*pos = 1;
-	info->cur_record = *pos;
-	return info;
+	return 0;
 }
 
-static void *devinfo_next(struct seq_file *f, void *v, loff_t *pos)
+static void *devinfo_start(struct seq_file *f, loff_t *pos)
 {
-	int idummy;
-	char *ndummy;
-	struct devinfo_state *info = f->private;
-
-	switch (info->state) {
-		case CHR_HDR:
-			info->state = CHR_LIST;
-			(*pos)++;
-			/*fallthrough*/
-		case CHR_LIST:
-			if (get_chrdev_info(info->chrdev,&idummy,&ndummy)) {
-				/*
-				 * The character dev list is complete
-				 */
-				info->state = BLK_HDR;
-			} else {
-				info->chrdev = get_next_chrdev(info->chrdev);
-			}
-			(*pos)++;
-			break;
-		case BLK_HDR:
-			info->state = BLK_LIST;
-			(*pos)++;
-			/*fallthrough*/
-		case BLK_LIST:
-			if (get_blkdev_info(info->blkdev,&idummy,&ndummy)) {
-				/*
-				 * The block dev list is complete
-				 */
-				info->state = DEVINFO_DONE;
-			} else {
-				info->blkdev = get_next_blkdev(info->blkdev);
-			}
-			(*pos)++;
-			break;
-		case DEVINFO_DONE:
-			(*pos)++;
-			info->cur_record = *pos;
-			info = NULL;
-			break;
-		default:
-			break;
-	}
-	if (info)
-		info->cur_record = *pos;
-	return info;
+	if (*pos < (BLKDEV_MAJOR_HASH_SIZE + CHRDEV_MAJOR_HASH_SIZE))
+		return pos;
+	return NULL;
 }
 
-static void devinfo_stop(struct seq_file *f, void *v)
+static void *devinfo_next(struct seq_file *f, void *v, loff_t *pos)
 {
-	struct devinfo_state *info = f->private;
-
-	if (info) {
-		release_chrdev_list(info->chrdev);
-		release_blkdev_list(info->blkdev);
-		f->private = NULL;
-		kfree(info);
-	}
+	(*pos)++;
+	if (*pos >= (BLKDEV_MAJOR_HASH_SIZE + CHRDEV_MAJOR_HASH_SIZE))
+		return NULL;
+	return pos;
 }
 
-static int devinfo_show(struct seq_file *f, void *arg)
-{
-	int major;
-	char *name;
-	struct devinfo_state *info = f->private;
-
-	switch(info->state) {
-		case CHR_HDR:
-			seq_printf(f,"Character devices:\n");
-			/* fallthrough */
-		case CHR_LIST:
-			if (!get_chrdev_info(info->chrdev,&major,&name))
-				seq_printf(f,"%3d %s\n",major,name);
-			break;
-		case BLK_HDR:
-			seq_printf(f,"\nBlock devices:\n");
-			/* fallthrough */
-		case BLK_LIST:
-			if (!get_blkdev_info(info->blkdev,&major,&name))
-				seq_printf(f,"%3d %s\n",major,name);
-			break;
-		default:
-			break;
-	}
-
-	return 0;
+static void devinfo_stop(struct seq_file *f, void *v)
+{
+	/* Nothing to do */
 }
 
-static  struct seq_operations devinfo_op = {
-	.start  = devinfo_start,
-	.next   = devinfo_next,
-	.stop   = devinfo_stop,
-	.show   = devinfo_show,
+static struct seq_operations devinfo_ops = {
+	.start = devinfo_start,
+	.next  = devinfo_next,
+	.stop  = devinfo_stop,
+	.show  = devinfo_show
 };
 
-static int devinfo_open(struct inode *inode, struct file *file)
+static int devinfo_open(struct inode *inode, struct file *filp)
 {
-	return seq_open(file, &devinfo_op);
+	return seq_open(filp, &devinfo_ops);
 }
 
 static struct file_operations proc_devinfo_operations = {
@@ -396,13 +312,6 @@ static struct file_operations proc_devinfo_operations = {
 	.release	= seq_release,
 };
 
-static struct file_operations proc_cpuinfo_operations = {
-	.open		= cpuinfo_open,
-	.read		= seq_read,
-	.llseek		= seq_lseek,
-	.release	= seq_release,
-};
-
 extern struct seq_operations vmstat_op;
 static int vmstat_open(struct inode *inode, struct file *file)
 {