ipc,shm: make shmctl_nolock lockless
Davidlohr Bueso [Wed, 11 Sep 2013 21:26:20 +0000 (14:26 -0700)]
commit c97cb9ccab8c85428ec21eff690642ad2ce1fa8a upstream.

While the INFO cmd doesn't take the ipc lock, the STAT commands do acquire
it unnecessarily.  We can do the permissions and security checks only
holding the rcu lock.

[akpm@linux-foundation.org: coding-style fixes]
Signed-off-by: Davidlohr Bueso <davidlohr.bueso@hp.com>
Tested-by: Sedat Dilek <sedat.dilek@gmail.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: Manfred Spraul <manfred@colorfullife.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mike Galbraith <efault@gmx.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

ipc/shm.c

index ac118be..8fd9ab3 100644 (file)
--- a/ipc/shm.c
+++ b/ipc/shm.c
@@ -882,27 +882,31 @@ static int shmctl_nolock(struct ipc_namespace *ns, int shmid,
                struct shmid64_ds tbuf;
                int result;
 
+               rcu_read_lock();
                if (cmd == SHM_STAT) {
-                       shp = shm_lock(ns, shmid);
+                       shp = shm_obtain_object(ns, shmid);
                        if (IS_ERR(shp)) {
                                err = PTR_ERR(shp);
-                               goto out;
+                               goto out_unlock;
                        }
                        result = shp->shm_perm.id;
                } else {
-                       shp = shm_lock_check(ns, shmid);
+                       shp = shm_obtain_object_check(ns, shmid);
                        if (IS_ERR(shp)) {
                                err = PTR_ERR(shp);
-                               goto out;
+                               goto out_unlock;
                        }
                        result = 0;
                }
+
                err = -EACCES;
                if (ipcperms(ns, &shp->shm_perm, S_IRUGO))
                        goto out_unlock;
+
                err = security_shm_shmctl(shp, cmd);
                if (err)
                        goto out_unlock;
+
                memset(&tbuf, 0, sizeof(tbuf));
                kernel_to_ipc64_perm(&shp->shm_perm, &tbuf.shm_perm);
                tbuf.shm_segsz  = shp->shm_segsz;
@@ -912,8 +916,9 @@ static int shmctl_nolock(struct ipc_namespace *ns, int shmid,
                tbuf.shm_cpid   = shp->shm_cprid;
                tbuf.shm_lpid   = shp->shm_lprid;
                tbuf.shm_nattch = shp->shm_nattch;
-               shm_unlock(shp);
-               if(copy_shmid_to_user (buf, &tbuf, version))
+               rcu_read_unlock();
+
+               if (copy_shmid_to_user(buf, &tbuf, version))
                        err = -EFAULT;
                else
                        err = result;
@@ -924,7 +929,7 @@ static int shmctl_nolock(struct ipc_namespace *ns, int shmid,
        }
 
 out_unlock:
-       shm_unlock(shp);
+       rcu_read_unlock();
 out:
        return err;
 }