[PATCH] document unchecked do_munmaps in ipc/shm.c

From: Manfred Spraul <manfred@colorfullife.com>

There are a few unchecked do_munmap()s in the shm code.  Manfred's comment
explains why they are OK.

ipc/shm.c

@@ -759,6 +759,21 @@ asmlinkage long sys_shmdt(char __user *shmaddr)
 
 	down_write(&mm->mmap_sem);
 
+	/*
+	 * This function tries to be smart and unmap shm segments that
+	 * were modified by partial mlock or munmap calls:
+	 * - It first determines the size of the shm segment that should be
+	 *   unmapped: It searches for a vma that is backed by shm and that
+	 *   started at address shmaddr. It records it's size and then unmaps
+	 *   it.
+	 * - Then it unmaps all shm vmas that started at shmaddr and that
+	 *   are within the initially determined size.
+	 * Errors from do_munmap are ignored: the function only fails if
+	 * it's called with invalid parameters or if it's called to unmap
+	 * a part of a vma. Both calls in this function are for full vmas,
+	 * the parameters are directly copied from the vma itself and always
+	 * valid - therefore do_munmap cannot fail. (famous last words?)
+	 */
 	/*
 	 * If it had been mremap()'d, the starting address would not
 	 * match the usual checks anyway. So assume all vma's are