[PATCH] pid: make session_of_pgrp use struct pid instead of pid_t

To properly implement a pid namespace I need to deal exclusively in terms of
struct pid, because pid_t values become ambiguous.

To this end session_of_pgrp is transformed to take and return a struct pid
pointer.  To avoid the need to worry about reference counting I now require my
caller to hold the appropriate locks.  Leaving callers repsonsible for
increasing the reference count if they need access to the result outside of
the locks.

Since session_of_pgrp currently only has one caller and that caller simply
uses only test the result for equality with another process group, the locking
change means I don't actually have to acquire the tasklist_lock at all.

tiocspgrp is also modified to take and release the lock.  The logic there is a
little more complicated but nothing I won't need when I convert pgrp of a tty
to a struct pid pointer.

Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
Cc: Oleg Nesterov <oleg@tv-sign.ru>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

3 files changed, 26 insertions, 17 deletions

diff --git a/drivers/char/tty_io.c b/drivers/char/tty_io.c
index abfe24d28c51..95f3596189cf 100644
--- a/drivers/char/tty_io.c
+++ b/drivers/char/tty_io.c
@@ -2990,7 +2990,8 @@ static int tiocgpgrp(struct tty_struct *tty, struct tty_struct *real_tty, pid_t
 
 static int tiocspgrp(struct tty_struct *tty, struct tty_struct *real_tty, pid_t __user *p)
 {
-        pid_t pgrp;
+        struct pid *pgrp;
+        pid_t pgrp_nr;
         int retval = tty_check_change(real_tty);
 
         if (retval == -EIO)
@@ -3001,14 +3002,23 @@ static int tiocspgrp(struct tty_struct *tty, struct tty_struct *real_tty, pid_t
             (current->signal->tty != real_tty) ||
             (real_tty->session != process_session(current)))
                 return -ENOTTY;
-        if (get_user(pgrp, p))
+        if (get_user(pgrp_nr, p))
                 return -EFAULT;
-        if (pgrp < 0)
+        if (pgrp_nr < 0)
                 return -EINVAL;
-        if (session_of_pgrp(pgrp) != process_session(current))
-                return -EPERM;
-        real_tty->pgrp = pgrp;
-        return 0;
+        rcu_read_lock();
+        pgrp = find_pid(pgrp_nr);
+        retval = -ESRCH;
+        if (!pgrp)
+                goto out_unlock;
+        retval = -EPERM;
+        if (session_of_pgrp(pgrp) != task_session(current))
+                goto out_unlock;
+        retval = 0;
+        real_tty->pgrp = pgrp_nr;
+out_unlock:
+        rcu_read_unlock();
+        return retval;
 }
 
 /**
diff --git a/include/linux/kernel.h b/include/linux/kernel.h
index 3531764318f2..9ddf25c21538 100644
--- a/include/linux/kernel.h
+++ b/include/linux/kernel.h
@@ -134,7 +134,8 @@ extern unsigned long long memparse(char *ptr, char **retptr);
 extern int core_kernel_text(unsigned long addr);
 extern int __kernel_text_address(unsigned long addr);
 extern int kernel_text_address(unsigned long addr);
-extern int session_of_pgrp(int pgrp);
+struct pid;
+extern struct pid *session_of_pgrp(struct pid *pgrp);
 
 extern void dump_thread(struct pt_regs *regs, struct user *dump);
 
diff --git a/kernel/exit.c b/kernel/exit.c
index 14f17033f563..3ac6a7a6f857 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -185,21 +185,19 @@ repeat:
  * This checks not only the pgrp, but falls back on the pid if no
  * satisfactory pgrp is found. I dunno - gdb doesn't work correctly
  * without this...
+ *
+ * The caller must hold rcu lock or the tasklist lock.
  */
-int session_of_pgrp(int pgrp)
+struct pid *session_of_pgrp(struct pid *pgrp)
 {
         struct task_struct *p;
-        int sid = 0;
-
-        read_lock(&tasklist_lock);
+        struct pid *sid = NULL;
 
-        p = find_task_by_pid_type(PIDTYPE_PGID, pgrp);
+        p = pid_task(pgrp, PIDTYPE_PGID);
         if (p == NULL)
-                p = find_task_by_pid(pgrp);
+                p = pid_task(pgrp, PIDTYPE_PID);
         if (p != NULL)
-                sid = process_session(p);
-
-        read_unlock(&tasklist_lock);
+                sid = task_session(p);
 
         return sid;
 }