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>

6 files changed, 111 insertions, 6 deletions

diff --git a/fs/proc/Makefile b/fs/proc/Makefile
index 239493ec718e..7151ea428041 100644
--- a/fs/proc/Makefile
+++ b/fs/proc/Makefile
@@ -23,6 +23,7 @@ proc-y	+= version.o
 proc-y  += softirqs.o
 proc-y  += namespaces.o
 proc-y  += self.o
+proc-y  += thread_self.o
 proc-$(CONFIG_PROC_SYSCTL)      += proc_sysctl.o
 proc-$(CONFIG_NET)              += proc_net.o
 proc-$(CONFIG_PROC_KCORE)       += kcore.o
diff --git a/fs/proc/base.c b/fs/proc/base.c
index ed34e405c6b9..0131156ce7c9 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -2847,7 +2847,7 @@ retry:
         return iter;
 }
 
-#define TGID_OFFSET (FIRST_PROCESS_ENTRY + 1)
+#define TGID_OFFSET (FIRST_PROCESS_ENTRY + 2)
 
 /* for the /proc/ directory itself, after non-process stuff has been done */
 int proc_pid_readdir(struct file *file, struct dir_context *ctx)
@@ -2859,14 +2859,19 @@ int proc_pid_readdir(struct file *file, struct dir_context *ctx)
         if (pos >= PID_MAX_LIMIT + TGID_OFFSET)
                 return 0;
 
-        if (pos == TGID_OFFSET - 1) {
+        if (pos == TGID_OFFSET - 2) {
                 struct inode *inode = ns->proc_self->d_inode;
                 if (!dir_emit(ctx, "self", 4, inode->i_ino, DT_LNK))
                         return 0;
-                iter.tgid = 0;
-        } else {
-                iter.tgid = pos - TGID_OFFSET;
+                ctx->pos = pos = pos + 1;
+        }
+        if (pos == TGID_OFFSET - 1) {
+                struct inode *inode = ns->proc_thread_self->d_inode;
+                if (!dir_emit(ctx, "thread-self", 11, inode->i_ino, DT_LNK))
+                        return 0;
+                ctx->pos = pos = pos + 1;
         }
+        iter.tgid = pos - TGID_OFFSET;
         iter.task = NULL;
         for (iter = next_tgid(ns, iter);
              iter.task;
diff --git a/fs/proc/inode.c b/fs/proc/inode.c
index 0adbc02d60e3..333080d7a671 100644
--- a/fs/proc/inode.c
+++ b/fs/proc/inode.c
@@ -442,6 +442,7 @@ struct inode *proc_get_inode(struct super_block *sb, struct proc_dir_entry *de)
 int proc_fill_super(struct super_block *s)
 {
         struct inode *root_inode;
+        int ret;
 
         s->s_flags |= MS_NODIRATIME | MS_NOSUID | MS_NOEXEC;
         s->s_blocksize = 1024;
@@ -463,5 +464,9 @@ int proc_fill_super(struct super_block *s)
                 return -ENOMEM;
         }
 
-        return proc_setup_self(s);
+        ret = proc_setup_self(s);
+        if (ret) {
+                return ret;
+        }
+        return proc_setup_thread_self(s);
 }
diff --git a/fs/proc/internal.h b/fs/proc/internal.h
index 3ab6d14e71c5..ee04619173b2 100644
--- a/fs/proc/internal.h
+++ b/fs/proc/internal.h
@@ -234,6 +234,12 @@ static inline int proc_net_init(void) { return 0; }
 extern int proc_setup_self(struct super_block *);
 
 /*
+ * proc_thread_self.c
+ */
+extern int proc_setup_thread_self(struct super_block *);
+extern void proc_thread_self_init(void);
+
+/*
  * proc_sysctl.c
  */
 #ifdef CONFIG_PROC_SYSCTL
diff --git a/fs/proc/root.c b/fs/proc/root.c
index 5dbadecb234d..48f1c03bc7ed 100644
--- a/fs/proc/root.c
+++ b/fs/proc/root.c
@@ -149,6 +149,8 @@ static void proc_kill_sb(struct super_block *sb)
         ns = (struct pid_namespace *)sb->s_fs_info;
         if (ns->proc_self)
                 dput(ns->proc_self);
+        if (ns->proc_thread_self)
+                dput(ns->proc_thread_self);
         kill_anon_super(sb);
         put_pid_ns(ns);
 }
@@ -170,6 +172,7 @@ void __init proc_root_init(void)
                 return;
 
         proc_self_init();
+        proc_thread_self_init();
         proc_symlink("mounts", NULL, "self/mounts");
 
         proc_net_init();
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);
+}