[PATCH] New System call unshare (try 2)

[Janak Desai <janak@us.ibm.com>]

Hi Chris,

Here is the reworked unshare system call patch which attempts to fix
problems that you identified with the first incarnation. The patch, against
the latest mm tree 2.6.14-rc2-mm2, was tested on single processor i386
system for the following:
	Don't allow namespace unsharing, if sharing fs (CLONE_FS)
	Don't allow sighand unsharing if not unsharing vm
	Don't allow vm unsharing if task cloned with CLONE_THREAD
	Don't allow vm unsharing if the task is performing async io
	Successful unsharing of namespace
		Cloned a task without sharing fs. Performed mounts before
		and after a call to unshare to verify that the namespace
		was indeed unshared.
	Successful unsharing of vm
		Cloned a task using CLONE_VM, CLONE_SIGHAND and CLONE_FILES.
		Used a shared memory segment and futex within it to 
		synchronize execution of threads. Verified shared and 
		unshared vm before and after successful unshare call.

Would appreciate feedback.

Thanks.

-Janak

--------------------------------------------------------------------------
diff -Naurp 2.6.14-rc2-mm2/fs/namespace.c 2.6.14-rc2-mm2+unshare/fs/namespace.c
--- 2.6.14-rc2-mm2/fs/namespace.c	2005-10-12 02:56:08.000000000 +0000
+++ 2.6.14-rc2-mm2+unshare/fs/namespace.c	2005-10-12 11:46:52.000000000 +0000
@@ -1069,9 +1069,6 @@ int copy_namespace(int flags, struct tas
 {
 	struct namespace *namespace = tsk->namespace;
 	struct namespace *new_ns;
-	struct vfsmount *rootmnt = NULL, *pwdmnt = NULL, *altrootmnt = NULL;
-	struct fs_struct *fs = tsk->fs;
-	struct vfsmount *p, *q;
 
 	if (!namespace)
 		return 0;
@@ -1086,6 +1083,28 @@ int copy_namespace(int flags, struct tas
 		return -EPERM;
 	}
 
+	new_ns = dup_namespace_struct(tsk);
+	if (!new_ns)
+		goto out;
+
+	tsk->namespace = new_ns;
+
+	put_namespace(namespace);
+	return 0;
+
+out:
+	put_namespace(namespace);
+	return -ENOMEM;
+}
+
+struct namespace *dup_namespace_struct(struct task_struct *tsk)
+{
+	struct namespace *namespace = tsk->namespace;
+	struct namespace *new_ns;
+	struct vfsmount *rootmnt = NULL, *pwdmnt = NULL, *altrootmnt = NULL;
+	struct fs_struct *fs = tsk->fs;
+	struct vfsmount *p, *q;
+
 	new_ns = kmalloc(sizeof(struct namespace), GFP_KERNEL);
 	if (!new_ns)
 		goto out;
@@ -1134,8 +1153,6 @@ int copy_namespace(int flags, struct tas
 	}
 	up_write(&tsk->namespace->sem);
 
-	tsk->namespace = new_ns;
-
 	if (rootmnt)
 		mntput(rootmnt);
 	if (pwdmnt)
@@ -1143,12 +1160,10 @@ int copy_namespace(int flags, struct tas
 	if (altrootmnt)
 		mntput(altrootmnt);
 
-	put_namespace(namespace);
-	return 0;
+	return new_ns;
 
 out:
-	put_namespace(namespace);
-	return -ENOMEM;
+	return NULL;
 }
 
 asmlinkage long sys_mount(char __user * dev_name, char __user * dir_name,
diff -Naurp 2.6.14-rc2-mm2/include/linux/namespace.h 2.6.14-rc2-mm2+unshare/include/linux/namespace.h
--- 2.6.14-rc2-mm2/include/linux/namespace.h	2005-10-12 02:56:35.000000000 +0000
+++ 2.6.14-rc2-mm2+unshare/include/linux/namespace.h	2005-10-12 11:45:46.000000000 +0000
@@ -14,6 +14,7 @@ struct namespace {
 
 extern int copy_namespace(int, struct task_struct *);
 extern void __put_namespace(struct namespace *namespace);
+extern struct namespace *dup_namespace_struct(struct task_struct *);
 
 static inline void put_namespace(struct namespace *namespace)
 {
diff -Naurp 2.6.14-rc2-mm2/kernel/fork.c 2.6.14-rc2-mm2+unshare/kernel/fork.c
--- 2.6.14-rc2-mm2/kernel/fork.c	2005-10-12 02:56:39.000000000 +0000
+++ 2.6.14-rc2-mm2+unshare/kernel/fork.c	2005-10-12 13:26:05.000000000 +0000
@@ -448,6 +448,55 @@ void mm_release(struct task_struct *tsk,
 	}
 }
 
+/*
+ * Allocate a new mm structure and copy contents from the
+ * mm structure of the passed in task structure.
+ */
+static struct mm_struct *dup_mm_struct(struct task_struct *tsk)
+{
+	struct mm_struct *mm, *oldmm = current->mm;
+	int err;
+
+	if (!oldmm)
+		return NULL;
+
+	mm = allocate_mm();
+	if (!mm)
+		goto fail_nomem;
+
+	memcpy(mm, oldmm, sizeof(*mm));
+
+	if (!mm_init(mm))
+		goto fail_nomem;
+
+	if (init_new_context(tsk, mm))
+		goto fail_nocontext;
+
+	err = dup_mmap(mm, oldmm);
+	if (err)
+		goto free_pt;
+
+	mm->hiwater_rss = get_mm_rss(mm);
+	mm->hiwater_vm = mm->total_vm;
+
+	return mm;
+
+free_pt:
+	mmput(mm);
+
+fail_nomem:
+	return NULL;
+
+fail_nocontext:
+	/*
+	 * If init_new_context() failed, we cannot use mmput() to free the mm
+	 * because it calls destroy_context()
+	 */
+	mm_free_pgd(mm);
+	free_mm(mm);
+	return NULL;
+}
+
 static int copy_mm(unsigned long clone_flags, struct task_struct * tsk)
 {
 	struct mm_struct * mm, *oldmm;
@@ -482,43 +531,17 @@ static int copy_mm(unsigned long clone_f
 	}
 
 	retval = -ENOMEM;
-	mm = allocate_mm();
+	mm = dup_mm_struct(tsk);
 	if (!mm)
 		goto fail_nomem;
 
-	/* Copy the current MM stuff.. */
-	memcpy(mm, oldmm, sizeof(*mm));
-	if (!mm_init(mm))
-		goto fail_nomem;
-
-	if (init_new_context(tsk,mm))
-		goto fail_nocontext;
-
-	retval = dup_mmap(mm, oldmm);
-	if (retval)
-		goto free_pt;
-
-	mm->hiwater_rss = get_mm_rss(mm);
-	mm->hiwater_vm = mm->total_vm;
-
 good_mm:
 	tsk->mm = mm;
 	tsk->active_mm = mm;
 	return 0;
 
-free_pt:
-	mmput(mm);
 fail_nomem:
 	return retval;
-
-fail_nocontext:
-	/*
-	 * If init_new_context() failed, we cannot use mmput() to free the mm
-	 * because it calls destroy_context()
-	 */
-	mm_free_pgd(mm);
-	free_mm(mm);
-	return retval;
 }
 
 static inline struct fs_struct *__copy_fs_struct(struct fs_struct *old)
@@ -753,6 +776,23 @@ int unshare_files(void)
 
 EXPORT_SYMBOL(unshare_files);
 
+/*
+ * Allocate a new sighand structure and copy contents from the
+ * sighand of the passed in task structure
+ */
+static struct sighand_struct *dup_sh_struct(struct task_struct *tsk)
+{
+	struct sighand_struct *sh;
+
+	sh = kmem_cache_alloc(sighand_cachep, GFP_KERNEL);
+	if (!sh)
+		return NULL;
+	spin_lock_init(&sh->siglock);
+	atomic_set(&sh->count, 1);
+	memcpy(sh->action, current->sighand->action, sizeof(sh->action));
+	return sh;
+}
+
 static inline int copy_sighand(unsigned long clone_flags, struct task_struct * tsk)
 {
 	struct sighand_struct *sig;
@@ -761,13 +801,10 @@ static inline int copy_sighand(unsigned 
 		atomic_inc(&current->sighand->count);
 		return 0;
 	}
-	sig = kmem_cache_alloc(sighand_cachep, GFP_KERNEL);
+	sig = dup_sh_struct(tsk);
 	tsk->sighand = sig;
 	if (!sig)
 		return -ENOMEM;
-	spin_lock_init(&sig->siglock);
-	atomic_set(&sig->count, 1);
-	memcpy(sig->action, current->sighand->action, sizeof(sig->action));
 	return 0;
 }
 
@@ -1321,3 +1358,164 @@ void __init proc_caches_init(void)
 			sizeof(struct mm_struct), 0,
 			SLAB_HWCACHE_ALIGN|SLAB_PANIC, NULL, NULL);
 }
+
+/*
+ * Performs sanity checks on the flags passed to the unshare system
+ * call.
+ */
+static inline int check_unshare_flags(unsigned long unshare_flags)
+{
+	int err = -EINVAL;
+
+	if (unshare_flags & ~(CLONE_NEWNS | CLONE_VM | CLONE_SIGHAND))
+		goto errout;
+
+	/*
+	 * Shared signal handlers imply shared VM, so if CLONE_SIGHAND is
+	 * set, CLONE_VM must also be set in the system call argument.
+	 */
+	if ((unshare_flags & CLONE_SIGHAND) && !(unshare_flags & CLONE_VM))
+		goto errout;
+
+	/*
+	 * Cannot unshare namespace if the fs structure is being shared
+	 * through a previous call to clone()
+	 */
+	if ((unshare_flags & CLONE_NEWNS) &&
+	    (atomic_read(&current->fs->count) > 1))
+		goto errout;
+
+	if (unshare_flags & CLONE_VM) {
+		/*
+	 	 * Cannot unshare vm if the signal structure is being shared
+	 	 * through a previous call to clone() with CLONE_THREAD.
+	 	 */
+	    	if (atomic_read(&current->signal->count) > 1)
+			goto errout;
+
+		/*
+		 * Cannot unshare vm only if the sighand structure is being
+		 * shared through a previous call to clone()
+		 */
+		if (!(unshare_flags & CLONE_SIGHAND) &&
+		     (atomic_read(&current->sighand->count) > 1))
+			goto errout;
+
+		/*
+		 * If the task is performing async io, unsharing of vm will
+		 * only allow the parent task, with which the vm was being
+		 * shared, to continue async io operations. The async io
+		 * context is not copied to the new, unshared, copy of vm.
+		 * So don't allow unsharing of vm if the process is setup
+		 * to perform async io.
+		 */
+		if (current->mm->ioctx_list)
+			goto errout;
+	}
+	return 0;
+
+errout:
+	return err;
+
+}
+
+/*
+ * unshare allows a process to 'unshare' part of the process
+ * context which was originally shared using clone.  copy_*
+ * functions used by do_fork() cannot be used here directly
+ * because they modify an inactive task_struct that is being
+ * constructed. Here we are modifying the current, active,
+ * task_struct.
+ */
+asmlinkage long sys_unshare(unsigned long unshare_flags)
+{
+	int err = 0, unshare_ns = 0, unshare_mm = 0, unshare_sh = 0;
+	struct namespace *new_ns = NULL, *ns = current->namespace;
+	struct mm_struct *new_mm = NULL, *active_mm = NULL, *mm = current->mm;
+	struct sighand_struct *new_sh = NULL, *sh = current->sighand;
+
+	err = check_unshare_flags(unshare_flags);
+	if (err)
+		goto unshare_out;
+
+	if (unshare_flags & CLONE_NEWNS) {
+		if (ns && atomic_read(&ns->count) > 1) {
+			err = -EPERM;
+			if (!capable(CAP_SYS_ADMIN))
+				goto unshare_out;
+
+			err = -ENOMEM;
+			new_ns = dup_namespace_struct(current);
+			if (!new_ns)
+				goto unshare_out;
+			else
+				unshare_ns = 1;
+		}
+	}
+
+	if (unshare_flags & CLONE_VM) {
+		if (atomic_read(&mm->mm_users) > 1) {
+			err = -ENOMEM;
+			new_mm = dup_mm_struct(current);
+			if (!new_mm)
+				goto unshare_cleanup_ns;
+			else {
+				unshare_mm = 1;
+				if (unshare_flags & CLONE_SIGHAND) {
+					if (atomic_read(&sh->count) > 1) {
+						new_sh = dup_sh_struct(current);
+						if (!new_sh)
+							goto unshare_cleanup_mm;
+						else
+							unshare_sh = 1;
+					}
+				}
+			}
+		}
+	}
+
+	if (unshare_ns) {
+		task_lock(current);
+		current->namespace = new_ns;
+		task_unlock(current);
+		put_namespace(ns);
+	}
+
+	if (unshare_mm) {
+		task_lock(current);
+		current->min_flt = current->maj_flt = 0;
+		current->nvcsw = current->nivcsw = 0;
+		active_mm = current->active_mm;
+		current->mm = new_mm;
+		current->active_mm = new_mm;
+		activate_mm(active_mm, new_mm);
+		task_unlock(current);
+		mmput(mm);
+	}
+
+	if (unshare_sh) {
+		write_lock_irq(&tasklist_lock);
+		spin_lock(&sh->siglock);
+		spin_lock(&new_sh->siglock);
+		current->sighand = new_sh;
+		spin_unlock(&new_sh->siglock);
+		spin_unlock(&sh->siglock);
+		write_unlock_irq(&tasklist_lock);
+
+		if (atomic_dec_and_test(&sh->count))
+			kmem_cache_free(sighand_cachep, sh);
+	}
+
+	return 0;
+
+unshare_cleanup_mm:
+	if (unshare_mm)
+		mmput(new_mm);
+
+unshare_cleanup_ns:
+	if (unshare_ns)
+		put_namespace(new_ns);
+
+unshare_out:
+	return err;
+}

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