exec: simplify the new ->sighand allocation

de_thread() pre-allocates newsighand to make sure that exec() can't fail after
killing all sub-threads. Imho, this buys nothing, but complicates the code:

	- this is (mostly) needed to handle CLONE_SIGHAND without CLONE_THREAD
	  tasks, this is very unlikely (if ever used) case

	- unless we already have some serious problems, GFP_KERNEL allocation
	  should not fail

	- ENOMEM still can happen after de_thread(), ->sighand is not the last
	  object we have to allocate

Change the code to allocate the new ->sighand on demand.

Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Cc: Roland McGrath <roland@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 9 insertions, 15 deletions

diff --git a/fs/exec.c b/fs/exec.c
index 7f325df5e014..c6e91c3dc2a1 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -747,7 +747,7 @@ static int exec_mmap(struct mm_struct *mm)
 static int de_thread(struct task_struct *tsk)
 {
         struct signal_struct *sig = tsk->signal;
-        struct sighand_struct *newsighand, *oldsighand = tsk->sighand;
+        struct sighand_struct *oldsighand = tsk->sighand;
         spinlock_t *lock = &oldsighand->siglock;
         struct task_struct *leader = NULL;
         int count;
@@ -761,10 +761,6 @@ static int de_thread(struct task_struct *tsk)
                 return 0;
         }
 
-        newsighand = kmem_cache_alloc(sighand_cachep, GFP_KERNEL);
-        if (!newsighand)
-                return -ENOMEM;
-
         if (thread_group_empty(tsk))
                 goto no_thread_group;
 
@@ -781,7 +777,6 @@ static int de_thread(struct task_struct *tsk)
                  */
                 spin_unlock_irq(lock);
                 read_unlock(&tasklist_lock);
-                kmem_cache_free(sighand_cachep, newsighand);
                 return -EAGAIN;
         }
 
@@ -899,17 +894,16 @@ no_thread_group:
         if (leader)
                 release_task(leader);
 
-        if (atomic_read(&oldsighand->count) == 1) {
-                /*
-                 * Now that we nuked the rest of the thread group,
-                 * it turns out we are not sharing sighand any more either.
-                 * So we can just keep it.
-                 */
-                kmem_cache_free(sighand_cachep, newsighand);
-        } else {
+        if (atomic_read(&oldsighand->count) != 1) {
+                struct sighand_struct *newsighand;
                 /*
-                 * Move our state over to newsighand and switch it in.
+                 * This ->sighand is shared with the CLONE_SIGHAND
+                 * but not CLONE_THREAD task, switch to the new one.
                  */
+                newsighand = kmem_cache_alloc(sighand_cachep, GFP_KERNEL);
+                if (!newsighand)
+                        return -ENOMEM;
+
                 atomic_set(&newsighand->count, 1);
                 memcpy(newsighand->action, oldsighand->action,
                        sizeof(newsighand->action));