proc: implement proc_single_file_operations

Currently many /proc/pid files use a crufty precursor to the current seq_file
api, and they don't have direct access to the pid_namespace or the pid of for
which they are displaying data.

So implement proc_single_file_operations to make the seq_file routines easy to
use, and to give access to the full state of the pid of we are displaying data
for.
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
Cc: Oleg Nesterov <oleg@tv-sign.ru>
Cc: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/proc/base.c

@@ -121,6 +121,10 @@ struct pid_entry {
 	NOD(NAME, (S_IFREG|(MODE)), 			\
 		NULL, &proc_info_file_operations,	\
 		{ .proc_read = &proc_##OTYPE } )
+#define ONE(NAME, MODE, OTYPE)				\
+	NOD(NAME, (S_IFREG|(MODE)), 			\
+		NULL, &proc_single_file_operations,	\
+		{ .proc_show = &proc_##OTYPE } )
 
 int maps_protect;
 EXPORT_SYMBOL(maps_protect);
@@ -658,6 +662,45 @@ static const struct file_operations proc_info_file_operations = {
 	.read		= proc_info_read,
 };
 
+static int proc_single_show(struct seq_file *m, void *v)
+{
+	struct inode *inode = m->private;
+	struct pid_namespace *ns;
+	struct pid *pid;
+	struct task_struct *task;
+	int ret;
+
+	ns = inode->i_sb->s_fs_info;
+	pid = proc_pid(inode);
+	task = get_pid_task(pid, PIDTYPE_PID);
+	if (!task)
+		return -ESRCH;
+
+	ret = PROC_I(inode)->op.proc_show(m, ns, pid, task);
+
+	put_task_struct(task);
+	return ret;
+}
+
+static int proc_single_open(struct inode *inode, struct file *filp)
+{
+	int ret;
+	ret = single_open(filp, proc_single_show, NULL);
+	if (!ret) {
+		struct seq_file *m = filp->private_data;
+
+		m->private = inode;
+	}
+	return ret;
+}
+
+static const struct file_operations proc_single_file_operations = {
+	.open		= proc_single_open,
+	.read		= seq_read,
+	.llseek		= seq_lseek,
+	.release	= single_release,
+};
+
 static int mem_open(struct inode* inode, struct file* file)
 {
 	file->private_data = (void*)((long)current->self_exec_id);

include/linux/proc_fs.h

@@ -262,6 +262,9 @@ extern void kclist_add(struct kcore_list *, void *, size_t);
 union proc_op {
 	int (*proc_get_link)(struct inode *, struct dentry **, struct vfsmount **);
 	int (*proc_read)(struct task_struct *task, char *page);
+	int (*proc_show)(struct seq_file *m,
+		struct pid_namespace *ns, struct pid *pid,
+		struct task_struct *task);
 };
 
 struct proc_inode {