proc: Implement /proc/thread-self to point at the directory of the current thread

/proc/thread-self is derived from /proc/self.  /proc/thread-self
points to the directory in proc containing information about the
current thread.

This funtionality has been missing for a long time, and is tricky to
implement in userspace as gettid() is not exported by glibc.  More
importantly this allows fixing defects in /proc/mounts and /proc/net
where in a threaded application today they wind up being empty files
when only the initial pthread has exited, causing problems for other
threads.

Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>

1 files changed, 85 insertions, 0 deletions

diff --git a/fs/proc/thread_self.c b/fs/proc/thread_self.c
new file mode 100644
index 000000000000..59075b509df3
--- /dev/null
+++ b/fs/proc/thread_self.c
@@ -0,0 +1,85 @@
+#include <linux/sched.h>
+#include <linux/namei.h>
+#include <linux/slab.h>
+#include <linux/pid_namespace.h>
+#include "internal.h"
+
+/*
+ * /proc/thread_self:
+ */
+static int proc_thread_self_readlink(struct dentry *dentry, char __user *buffer,
+			      int buflen)
+{
+	struct pid_namespace *ns = dentry->d_sb->s_fs_info;
+	pid_t tgid = task_tgid_nr_ns(current, ns);
+	pid_t pid = task_pid_nr_ns(current, ns);
+	char tmp[PROC_NUMBUF + 6 + PROC_NUMBUF];
+	if (!pid)
+		return -ENOENT;
+	sprintf(tmp, "%d/task/%d", tgid, pid);
+	return readlink_copy(buffer, buflen, tmp);
+}
+
+static void *proc_thread_self_follow_link(struct dentry *dentry, struct nameidata *nd)
+{
+	struct pid_namespace *ns = dentry->d_sb->s_fs_info;
+	pid_t tgid = task_tgid_nr_ns(current, ns);
+	pid_t pid = task_pid_nr_ns(current, ns);
+	char *name = ERR_PTR(-ENOENT);
+	if (pid) {
+		name = kmalloc(PROC_NUMBUF + 6 + PROC_NUMBUF, GFP_KERNEL);
+		if (!name)
+			name = ERR_PTR(-ENOMEM);
+		else
+			sprintf(name, "%d/task/%d", tgid, pid);
+	}
+	nd_set_link(nd, name);
+	return NULL;
+}
+
+static const struct inode_operations proc_thread_self_inode_operations = {
+	.readlink	= proc_thread_self_readlink,
+	.follow_link	= proc_thread_self_follow_link,
+	.put_link	= kfree_put_link,
+};
+
+static unsigned thread_self_inum;
+
+int proc_setup_thread_self(struct super_block *s)
+{
+	struct inode *root_inode = s->s_root->d_inode;
+	struct pid_namespace *ns = s->s_fs_info;
+	struct dentry *thread_self;
+
+	mutex_lock(&root_inode->i_mutex);
+	thread_self = d_alloc_name(s->s_root, "thread-self");
+	if (thread_self) {
+		struct inode *inode = new_inode_pseudo(s);
+		if (inode) {
+			inode->i_ino = thread_self_inum;
+			inode->i_mtime = inode->i_atime = inode->i_ctime = CURRENT_TIME;
+			inode->i_mode = S_IFLNK | S_IRWXUGO;
+			inode->i_uid = GLOBAL_ROOT_UID;
+			inode->i_gid = GLOBAL_ROOT_GID;
+			inode->i_op = &proc_thread_self_inode_operations;
+			d_add(thread_self, inode);
+		} else {
+			dput(thread_self);
+			thread_self = ERR_PTR(-ENOMEM);
+		}
+	} else {
+		thread_self = ERR_PTR(-ENOMEM);
+	}
+	mutex_unlock(&root_inode->i_mutex);
+	if (IS_ERR(thread_self)) {
+		pr_err("proc_fill_super: can't allocate /proc/thread_self\n");
+		return PTR_ERR(thread_self);
+	}
+	ns->proc_thread_self = thread_self;
+	return 0;
+}
+
+void __init proc_thread_self_init(void)
+{
+	proc_alloc_inum(&thread_self_inum);
+}