[PATCH] Fix invisible threads problem

When the main thread of a thread group has done pthread_exit() and died,
the other threads are still happily running, but will not be visible
under /proc because their leader is no longer accessible.

This fixes the access control so that we can see the sub-threads again.
Signed-off-by: Sripathi Kodi <sripathik@in.ibm.com>
Acked-by: Al Viro <viro@ftp.linux.org.uk>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

fs/proc/base.c

@@ -340,6 +340,52 @@ static int proc_root_link(struct inode *inode, struct dentry **dentry, struct vf
 	return result;
 }
 
+
+/* Same as proc_root_link, but this addionally tries to get fs from other
+ * threads in the group */
+static int proc_task_root_link(struct inode *inode, struct dentry **dentry, struct vfsmount **mnt)
+{
+	struct fs_struct *fs;
+	int result = -ENOENT;
+	struct task_struct *leader = proc_task(inode);
+
+	task_lock(leader);
+	fs = leader->fs;
+	if (fs) {
+		atomic_inc(&fs->count);
+		task_unlock(leader);
+	} else {
+		/* Try to get fs from other threads */
+		task_unlock(leader);
+		struct task_struct *task = leader;
+		read_lock(&tasklist_lock);
+		if (pid_alive(task)) {
+			while ((task = next_thread(task)) != leader) {
+				task_lock(task);
+				fs = task->fs;
+				if (fs) {
+					atomic_inc(&fs->count);
+					task_unlock(task);
+					break;
+				}
+				task_unlock(task);
+			}
+		}
+		read_unlock(&tasklist_lock);
+	}
+
+	if (fs) {
+		read_lock(&fs->lock);
+		*mnt = mntget(fs->rootmnt);
+		*dentry = dget(fs->root);
+		read_unlock(&fs->lock);
+		result = 0;
+		put_fs_struct(fs);
+	}
+	return result;
+}
+
+
 #define MAY_PTRACE(task) \
 	(task == current || \
 	(task->parent == current && \
@@ -471,14 +517,14 @@ static int proc_oom_score(struct task_struct *task, char *buffer)
 
 /* permission checks */
 
-static int proc_check_root(struct inode *inode)
+/* If the process being read is separated by chroot from the reading process,
+ * don't let the reader access the threads.
+ */
+static int proc_check_chroot(struct dentry *root, struct vfsmount *vfsmnt)
 {
-	struct dentry *de, *base, *root;
-	struct vfsmount *our_vfsmnt, *vfsmnt, *mnt;
+	struct dentry *de, *base;
+	struct vfsmount *our_vfsmnt, *mnt;
 	int res = 0;
-
-	if (proc_root_link(inode, &root, &vfsmnt)) /* Ewww... */
-		return -ENOENT;
 	read_lock(&current->fs->lock);
 	our_vfsmnt = mntget(current->fs->rootmnt);
 	base = dget(current->fs->root);
@@ -511,6 +557,16 @@ static int proc_check_root(struct inode *inode)
 	goto exit;
 }
 
+static int proc_check_root(struct inode *inode)
+{
+	struct dentry *root;
+	struct vfsmount *vfsmnt;
+
+	if (proc_root_link(inode, &root, &vfsmnt)) /* Ewww... */
+		return -ENOENT;
+	return proc_check_chroot(root, vfsmnt);
+}
+
 static int proc_permission(struct inode *inode, int mask, struct nameidata *nd)
 {
 	if (generic_permission(inode, mask, NULL) != 0)
@@ -518,6 +574,20 @@ static int proc_permission(struct inode *inode, int mask, struct nameidata *nd)
 	return proc_check_root(inode);
 }
 
+static int proc_task_permission(struct inode *inode, int mask, struct nameidata *nd)
+{
+	struct dentry *root;
+	struct vfsmount *vfsmnt;
+
+	if (generic_permission(inode, mask, NULL) != 0)
+		return -EACCES;
+
+	if (proc_task_root_link(inode, &root, &vfsmnt))
+		return -ENOENT;
+
+	return proc_check_chroot(root, vfsmnt);
+}
+
 extern struct seq_operations proc_pid_maps_op;
 static int maps_open(struct inode *inode, struct file *file)
 {
@@ -1419,7 +1489,7 @@ static struct inode_operations proc_fd_inode_operations = {
 
 static struct inode_operations proc_task_inode_operations = {
 	.lookup		= proc_task_lookup,
-	.permission	= proc_permission,
+	.permission	= proc_task_permission,
 };
 
 #ifdef CONFIG_SECURITY