[PATCH v2 2/5] exec: Factor unshare_sighand out of de_thread and call it separately

Eric W. Biederman
2020-03-08T21:38:39Z

This makes the code clearer and makes it easier to implement a mutex
that is not taken over any locations that may block indefinitely waiting
for userspace.

Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
---
fs/exec.c | 39 ++++++++++++++++++++++++++-------------
1 file changed, 26 insertions(+), 13 deletions(-)

diff --git a/fs/exec.c b/fs/exec.c
index c3f34791f2f0..ff74b9a74d34 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1194,6 +1194,23 @@ static int de_thread(struct task_struct *tsk)
flush_itimer_signals();
#endif

+ BUG_ON(!thread_group_leader(tsk));
+ return 0;
+
+killed:
+ /* protects against exit_notify() and __exit_signal() */
+ read_lock(&tasklist_lock);
+ sig->group_exit_task = NULL;
+ sig->notify_count = 0;
+ read_unlock(&tasklist_lock);
+ return -EAGAIN;
+}
+
+
+static int unshare_sighand(struct task_struct *me)
+{
+ struct sighand_struct *oldsighand = me->sighand;
+
if (refcount_read(&oldsighand->count) != 1) {
struct sighand_struct *newsighand;
/*
@@ -1210,23 +1227,13 @@ static int de_thread(struct task_struct *tsk)

write_lock_irq(&tasklist_lock);
spin_lock(&oldsighand->siglock);
- rcu_assign_pointer(tsk->sighand, newsighand);
+ rcu_assign_pointer(me->sighand, newsighand);
spin_unlock(&oldsighand->siglock);
write_unlock_irq(&tasklist_lock);

__cleanup_sighand(oldsighand);
}
-
- BUG_ON(!thread_group_leader(tsk));
return 0;
-
-killed:
- /* protects against exit_notify() and __exit_signal() */
- read_lock(&tasklist_lock);
- sig->group_exit_task = NULL;
- sig->notify_count = 0;
- read_unlock(&tasklist_lock);
- return -EAGAIN;
}

char *__get_task_comm(char *buf, size_t buf_size, struct task_struct *tsk)
@@ -1264,13 +1271,19 @@ int flush_old_exec(struct linux_binprm * bprm)
int retval;

/*
- * Make sure we have a private signal table and that
- * we are unassociated from the previous thread group.
+ * Make this the only thread in the thread group.
*/
retval = de_thread(me);
if (retval)
goto out;

+ /*
+ * Make the signal table private.
+ */
+ retval = unshare_sighand(me);
+ if (retval)
+ goto out;
+
/*
* Must be called _before_ exec_mmap() as bprm->mm is
* not visibile until then. This also enables the update
--
2.25.0