[PATCH 1/1] cgroups: introduce cft->read_seq()

["Serge E. Hallyn" <serue@us.ibm.com>]

Hi Paul,

the following (against 2.6.25-rc8-mm1) is a first attempt
at a simple seq_file usage in cgroups.  Comments much
appreciated.

thanks,
-serge

>From bd0977a5819dc43866fff325ae1e2726e747e2f4 Mon Sep 17 00:00:00 2001
From: Serge E. Hallyn <serue@us.ibm.com>
Date: Wed, 2 Apr 2008 17:54:21 -0700
Subject: [PATCH 1/1] cgroups: introduce cft->read_seq()

Introduce a read_seq() helper in cftype, which uses
seq_file to print out lists.  Use it in the devices
cgroup.  Also split devices.allow into two files, so
now devices.deny and devices.allow are the ones to
use to manipulate the whitelist, while devices.list
outputs the cgroup's current whitelist.

Signed-off-by: Serge E. Hallyn <serue@us.ibm.com>
---
 include/linux/cgroup.h   |    6 ++++
 kernel/cgroup.c          |   22 ++++++++++++-
 security/device_cgroup.c |   74 ++++++++++++++-------------------------------
 3 files changed, 49 insertions(+), 53 deletions(-)

diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index 2d1d151..feb83dd 100644
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -227,6 +227,12 @@ struct cftype {
 	 */
 	int (*read_map) (struct cgroup *cont, struct cftype *cft,
 			 struct cgroup_map_cb *cb);
+	/*
+	 * read_seq() is used for outputting a simple sequence
+	 * using seqfile.
+	 */
+	int (*read_seq) (struct cgroup *cont, struct cftype *cft,
+			 struct seq_file *m);
 
 	ssize_t (*write) (struct cgroup *cgrp, struct cftype *cft,
 			  struct file *file,
diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 85f31ad..61dc509 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -1518,7 +1518,7 @@ static int cgroup_map_add(struct cgroup_map_cb *cb, const char *key, u64 value)
 	return seq_printf(sf, "%s %llu\n", key, (unsigned long long)value);
 }
 
-static int cgroup_seqfile_show(struct seq_file *m, void *arg)
+static int cgroup_map_seqfile_show(struct seq_file *m, void *arg)
 {
 	struct cgroup_seqfile_state *state = m->private;
 	struct cftype *cft = state->cft;
@@ -1529,6 +1529,13 @@ static int cgroup_seqfile_show(struct seq_file *m, void *arg)
 	return cft->read_map(state->cgroup, cft, &cb);
 }
 
+static int cgroup_simple_seqfile_show(struct seq_file *m, void *arg)
+{
+	struct cgroup_seqfile_state *state = m->private;
+	struct cftype *cft = state->cft;
+	return cft->read_seq(state->cgroup, cft, m);
+}
+
 int cgroup_seqfile_release(struct inode *inode, struct file *file)
 {
 	struct seq_file *seq = file->private_data;
@@ -1562,7 +1569,18 @@ static int cgroup_file_open(struct inode *inode, struct file *file)
 		state->cft = cft;
 		state->cgroup = __d_cgrp(file->f_dentry->d_parent);
 		file->f_op = &cgroup_seqfile_operations;
-		err = single_open(file, cgroup_seqfile_show, state);
+		err = single_open(file, cgroup_map_seqfile_show, state);
+		if (err < 0)
+			kfree(state);
+	} else if (cft->read_seq) {
+		struct cgroup_seqfile_state *state =
+			kzalloc(sizeof(*state), GFP_USER);
+		if (!state)
+			return -ENOMEM;
+		state->cft = cft;
+		state->cgroup = __d_cgrp(file->f_dentry->d_parent);
+		file->f_op = &cgroup_seqfile_operations;
+		err = single_open(file, cgroup_simple_seqfile_show, state);
 		if (err < 0)
 			kfree(state);
 	} else if (cft->open)
diff --git a/security/device_cgroup.c b/security/device_cgroup.c
index 4237b19..68a247c 100644
--- a/security/device_cgroup.c
+++ b/security/device_cgroup.c
@@ -9,6 +9,7 @@
 #include <linux/ctype.h>
 #include <linux/list.h>
 #include <linux/uaccess.h>
+#include <linux/seq_file.h>
 
 #define ACC_MKNOD 1
 #define ACC_READ  2
@@ -201,11 +202,15 @@ static void devcgroup_destroy(struct cgroup_subsys *ss,
 
 #define DEVCG_ALLOW 1
 #define DEVCG_DENY 2
+#define DEVCG_LIST 3
+
+#define MAJMINLEN 10
+#define ACCLEN 4
 
 static void set_access(char *acc, short access)
 {
 	int idx = 0;
-	memset(acc, 0, 4);
+	memset(acc, 0, ACCLEN);
 	if (access & ACC_READ)
 		acc[idx++] = 'r';
 	if (access & ACC_WRITE)
@@ -225,70 +230,33 @@ static char type_to_char(short type)
 	return 'X';
 }
 
-static void set_majmin(char *str, int len, unsigned m)
+static void set_majmin(char *str, unsigned m)
 {
-	memset(str, 0, len);
+	memset(str, 0, MAJMINLEN);
 	if (m == ~0)
 		sprintf(str, "*");
 	else
-		snprintf(str, len, "%d", m);
+		snprintf(str, MAJMINLEN, "%d", m);
 }
 
-static char *print_whitelist(struct dev_cgroup *devcgroup, int *len)
+static int devcgroup_seq_read(struct cgroup *cgroup, struct cftype *cft,
+				struct seq_file *m)
 {
-	char *buf, *s, acc[4];
+	struct dev_cgroup *devcgroup = cgroup_to_devcgroup(cgroup);
 	struct dev_whitelist_item *wh;
-	int ret;
-	int count = 0;
-	char maj[10], min[10];
-
-	buf = kmalloc(4096, GFP_KERNEL);
-	if (!buf)
-		return ERR_PTR(-ENOMEM);
-	s = buf;
-	*s = '\0';
-	*len = 0;
+	char maj[MAJMINLEN], min[MAJMINLEN], acc[ACCLEN];
 
 	spin_lock(&devcgroup->lock);
 	list_for_each_entry(wh, &devcgroup->whitelist, list) {
 		set_access(acc, wh->access);
-		set_majmin(maj, 10, wh->major);
-		set_majmin(min, 10, wh->minor);
-		ret = snprintf(s, 4095-(s-buf), "%c %s:%s %s\n",
-			type_to_char(wh->type), maj, min, acc);
-		if (s+ret >= buf+4095) {
-			kfree(buf);
-			buf = ERR_PTR(-ENOMEM);
-			break;
-		}
-		s += ret;
-		*len += ret;
-		count++;
+		set_majmin(maj, wh->major);
+		set_majmin(min, wh->minor);
+		seq_printf(m, "%c %s:%s %s\n", type_to_char(wh->type),
+			   maj, min, acc);
 	}
 	spin_unlock(&devcgroup->lock);
 
-	return buf;
-}
-
-static ssize_t devcgroup_access_read(struct cgroup *cgroup,
-			struct cftype *cft, struct file *file,
-			char __user *userbuf, size_t nbytes, loff_t *ppos)
-{
-	struct dev_cgroup *devcgroup = cgroup_to_devcgroup(cgroup);
-	int filetype = cft->private;
-	char *buffer;
-	int uninitialized_var(len);
-	int retval;
-
-	if (filetype != DEVCG_ALLOW)
-		return -EINVAL;
-	buffer = print_whitelist(devcgroup, &len);
-	if (IS_ERR(buffer))
-		return PTR_ERR(buffer);
-
-	retval = simple_read_from_buffer(userbuf, nbytes, ppos, buffer, len);
-	kfree(buffer);
-	return retval;
+	return 0;
 }
 
 /*
@@ -501,7 +469,6 @@ out1:
 static struct cftype dev_cgroup_files[] = {
 	{
 		.name = "allow",
-		.read = devcgroup_access_read,
 		.write  = devcgroup_access_write,
 		.private = DEVCG_ALLOW,
 	},
@@ -510,6 +477,11 @@ static struct cftype dev_cgroup_files[] = {
 		.write = devcgroup_access_write,
 		.private = DEVCG_DENY,
 	},
+	{
+		.name = "list",
+		.read_seq = devcgroup_seq_read,
+		.private = DEVCG_LIST,
+	},
 };
 
 static int devcgroup_populate(struct cgroup_subsys *ss,
-- 
1.5.3.6