[RFC 02/10] drm: Update file owner during use

[Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>]

From: Tvrtko Ursulin <tvrtko.ursulin@intel.com>

With the typical model where the display server opends the file descriptor
and then hands it over to the client we were showing stale data in
debugfs.

Fix it by updating the drm_file->pid on ioctl access from a different
process.

The field is also made RCU protected to allow for lockless readers. Update
side is protected with dev->filelist_mutex.

Before:

$ cat /sys/kernel/debug/dri/0/clients
             command   pid dev master a   uid      magic
                Xorg  2344   0   y    y     0          0
                Xorg  2344   0   n    y     0          2
                Xorg  2344   0   n    y     0          3
                Xorg  2344   0   n    y     0          4

After:

$ cat /sys/kernel/debug/dri/0/clients
             command  tgid dev master a   uid      magic
                Xorg   830   0   y    y     0          0
       xfce4-session   880   0   n    y     0          1
               xfwm4   943   0   n    y     0          2
           neverball  1095   0   n    y     0          3

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: "Christian König" <christian.koenig@amd.com>
Cc: Daniel Vetter <daniel@ffwll.ch>
---
 drivers/gpu/drm/amd/amdgpu/amdgpu_gem.c |  6 ++--
 drivers/gpu/drm/drm_auth.c              |  3 +-
 drivers/gpu/drm/drm_debugfs.c           | 10 ++++---
 drivers/gpu/drm/drm_file.c              | 40 +++++++++++++++++++++++--
 drivers/gpu/drm/drm_ioctl.c             |  3 ++
 drivers/gpu/drm/nouveau/nouveau_drm.c   |  5 +++-
 drivers/gpu/drm/vmwgfx/vmwgfx_gem.c     |  6 ++--
 include/drm/drm_file.h                  | 13 ++++++--
 8 files changed, 71 insertions(+), 15 deletions(-)

diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_gem.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_gem.c
index 863cb668e000..67ce634992f6 100644
--- a/drivers/gpu/drm/amd/amdgpu/amdgpu_gem.c
+++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_gem.c
@@ -960,6 +960,7 @@ static int amdgpu_debugfs_gem_info_show(struct seq_file *m, void *unused)
 	list_for_each_entry(file, &dev->filelist, lhead) {
 		struct task_struct *task;
 		struct drm_gem_object *gobj;
+		struct pid *pid;
 		int id;
 
 		/*
@@ -969,8 +970,9 @@ static int amdgpu_debugfs_gem_info_show(struct seq_file *m, void *unused)
 		 * Therefore, we need to protect this ->comm access using RCU.
 		 */
 		rcu_read_lock();
-		task = pid_task(file->pid, PIDTYPE_TGID);
-		seq_printf(m, "pid %8d command %s:\n", pid_nr(file->pid),
+		pid = rcu_dereference(file->pid);
+		task = pid_task(pid, PIDTYPE_TGID);
+		seq_printf(m, "pid %8d command %s:\n", pid_nr(pid),
 			   task ? task->comm : "<unknown>");
 		rcu_read_unlock();
 
diff --git a/drivers/gpu/drm/drm_auth.c b/drivers/gpu/drm/drm_auth.c
index cf92a9ae8034..2ed2585ded37 100644
--- a/drivers/gpu/drm/drm_auth.c
+++ b/drivers/gpu/drm/drm_auth.c
@@ -235,7 +235,8 @@ static int drm_new_set_master(struct drm_device *dev, struct drm_file *fpriv)
 static int
 drm_master_check_perm(struct drm_device *dev, struct drm_file *file_priv)
 {
-	if (file_priv->pid == task_pid(current) && file_priv->was_master)
+	if (file_priv->was_master &&
+	    rcu_access_pointer(file_priv->pid) == task_pid(current))
 		return 0;
 
 	if (!capable(CAP_SYS_ADMIN))
diff --git a/drivers/gpu/drm/drm_debugfs.c b/drivers/gpu/drm/drm_debugfs.c
index 4855230ba2c6..b46f5ceb24c6 100644
--- a/drivers/gpu/drm/drm_debugfs.c
+++ b/drivers/gpu/drm/drm_debugfs.c
@@ -90,15 +90,17 @@ static int drm_clients_info(struct seq_file *m, void *data)
 	 */
 	mutex_lock(&dev->filelist_mutex);
 	list_for_each_entry_reverse(priv, &dev->filelist, lhead) {
-		struct task_struct *task;
 		bool is_current_master = drm_is_current_master(priv);
+		struct task_struct *task;
+		struct pid *pid;
 
-		rcu_read_lock(); /* locks pid_task()->comm */
-		task = pid_task(priv->pid, PIDTYPE_TGID);
+		rcu_read_lock(); /* Locks priv->pid and pid_task()->comm! */
+		pid = rcu_dereference(priv->pid);
+		task = pid_task(pid, PIDTYPE_TGID);
 		uid = task ? __task_cred(task)->euid : GLOBAL_ROOT_UID;
 		seq_printf(m, "%20s %5d %3d   %c    %c %5d %10u\n",
 			   task ? task->comm : "<unknown>",
-			   pid_vnr(priv->pid),
+			   pid_vnr(pid),
 			   priv->minor->index,
 			   is_current_master ? 'y' : 'n',
 			   priv->authenticated ? 'y' : 'n',
diff --git a/drivers/gpu/drm/drm_file.c b/drivers/gpu/drm/drm_file.c
index c1018c470047..f2f8175ece15 100644
--- a/drivers/gpu/drm/drm_file.c
+++ b/drivers/gpu/drm/drm_file.c
@@ -156,7 +156,7 @@ struct drm_file *drm_file_alloc(struct drm_minor *minor)
 	if (!file)
 		return ERR_PTR(-ENOMEM);
 
-	file->pid = get_pid(task_tgid(current));
+	rcu_assign_pointer(file->pid, get_pid(task_tgid(current)));
 	file->minor = minor;
 
 	/* for compatibility root is always authenticated */
@@ -196,7 +196,7 @@ struct drm_file *drm_file_alloc(struct drm_minor *minor)
 		drm_syncobj_release(file);
 	if (drm_core_check_feature(dev, DRIVER_GEM))
 		drm_gem_release(dev, file);
-	put_pid(file->pid);
+	put_pid(rcu_access_pointer(file->pid));
 	kfree(file);
 
 	return ERR_PTR(ret);
@@ -287,7 +287,7 @@ void drm_file_free(struct drm_file *file)
 
 	WARN_ON(!list_empty(&file->event_list));
 
-	put_pid(file->pid);
+	put_pid(rcu_access_pointer(file->pid));
 	kfree(file);
 }
 
@@ -501,6 +501,40 @@ int drm_release(struct inode *inode, struct file *filp)
 }
 EXPORT_SYMBOL(drm_release);
 
+void drm_file_update_pid(struct drm_file *filp)
+{
+	struct drm_device *dev;
+	struct pid *pid, *old;
+
+	/*
+	 * Master nodes need to keep the original ownership in order for
+	 * drm_master_check_perm to keep working correctly. (See comment in
+	 * drm_auth.c.)
+	 */
+	if (filp->was_master)
+		return;
+
+	pid = task_tgid(current);
+
+	/*
+	 * Quick unlocked check since the model is a single handover followed by
+	 * exclusive repeated use.
+	 */
+	if (pid == rcu_access_pointer(filp->pid))
+		return;
+
+	dev = filp->minor->dev;
+	mutex_lock(&dev->filelist_mutex);
+	old = rcu_replace_pointer(filp->pid, pid, 1);
+	mutex_unlock(&dev->filelist_mutex);
+
+	if (pid != old) {
+		get_pid(pid);
+		synchronize_rcu();
+		put_pid(old);
+	}
+}
+
 /**
  * drm_release_noglobal - release method for DRM file
  * @inode: device inode
diff --git a/drivers/gpu/drm/drm_ioctl.c b/drivers/gpu/drm/drm_ioctl.c
index 7c9d66ee917d..305b18d9d7b6 100644
--- a/drivers/gpu/drm/drm_ioctl.c
+++ b/drivers/gpu/drm/drm_ioctl.c
@@ -775,6 +775,9 @@ long drm_ioctl_kernel(struct file *file, drm_ioctl_t *func, void *kdata,
 	struct drm_device *dev = file_priv->minor->dev;
 	int retcode;
 
+	/* Update drm_file owner if fd was passed along. */
+	drm_file_update_pid(file_priv);
+
 	if (drm_dev_is_unplugged(dev))
 		return -ENODEV;
 
diff --git a/drivers/gpu/drm/nouveau/nouveau_drm.c b/drivers/gpu/drm/nouveau/nouveau_drm.c
index cc7c5b4a05fd..57aeaf7af613 100644
--- a/drivers/gpu/drm/nouveau/nouveau_drm.c
+++ b/drivers/gpu/drm/nouveau/nouveau_drm.c
@@ -1095,7 +1095,10 @@ nouveau_drm_open(struct drm_device *dev, struct drm_file *fpriv)
 	}
 
 	get_task_comm(tmpname, current);
-	snprintf(name, sizeof(name), "%s[%d]", tmpname, pid_nr(fpriv->pid));
+	rcu_read_lock();
+	snprintf(name, sizeof(name), "%s[%d]",
+		 tmpname, pid_nr(rcu_dereference(fpriv->pid)));
+	rcu_read_unlock();
 
 	if (!(cli = kzalloc(sizeof(*cli), GFP_KERNEL))) {
 		ret = -ENOMEM;
diff --git a/drivers/gpu/drm/vmwgfx/vmwgfx_gem.c b/drivers/gpu/drm/vmwgfx/vmwgfx_gem.c
index c0da89e16e6f..a07e5b7e2f2f 100644
--- a/drivers/gpu/drm/vmwgfx/vmwgfx_gem.c
+++ b/drivers/gpu/drm/vmwgfx/vmwgfx_gem.c
@@ -232,6 +232,7 @@ static int vmw_debugfs_gem_info_show(struct seq_file *m, void *unused)
 	list_for_each_entry(file, &dev->filelist, lhead) {
 		struct task_struct *task;
 		struct drm_gem_object *gobj;
+		struct pid *pid;
 		int id;
 
 		/*
@@ -241,8 +242,9 @@ static int vmw_debugfs_gem_info_show(struct seq_file *m, void *unused)
 		 * Therefore, we need to protect this ->comm access using RCU.
 		 */
 		rcu_read_lock();
-		task = pid_task(file->pid, PIDTYPE_TGID);
-		seq_printf(m, "pid %8d command %s:\n", pid_nr(file->pid),
+		pid = rcu_dereference(file->pid);
+		task = pid_task(pid, PIDTYPE_TGID);
+		seq_printf(m, "pid %8d command %s:\n", pid_nr(pid),
 			   task ? task->comm : "<unknown>");
 		rcu_read_unlock();
 
diff --git a/include/drm/drm_file.h b/include/drm/drm_file.h
index 0d1f853092ab..27d545131d4a 100644
--- a/include/drm/drm_file.h
+++ b/include/drm/drm_file.h
@@ -255,8 +255,15 @@ struct drm_file {
 	/** @master_lookup_lock: Serializes @master. */
 	spinlock_t master_lookup_lock;
 
-	/** @pid: Process that opened this file. */
-	struct pid *pid;
+	/**
+	 * @pid: Process that is using this file.
+	 *
+	 * Must only be dereferenced under a rcu_read_lock or equivalent.
+	 *
+	 * Updates are guarded with dev->filelist_mutex and reference must be
+	 * dropped after a RCU grace period to accommodate lockless readers.
+	 */
+	struct pid __rcu *pid;
 
 	/** @magic: Authentication magic, see @authenticated. */
 	drm_magic_t magic;
@@ -415,6 +422,8 @@ static inline bool drm_is_accel_client(const struct drm_file *file_priv)
 	return file_priv->minor->type == DRM_MINOR_ACCEL;
 }
 
+void drm_file_update_pid(struct drm_file *);
+
 int drm_open(struct inode *inode, struct file *filp);
 int drm_open_helper(struct file *filp, struct drm_minor *minor);
 ssize_t drm_read(struct file *filp, char __user *buffer,
-- 
2.37.2