On Wed, Jun 21, 2023 at 2:48 AM Tvrtko Ursulin
<tvrtko.ursulin@xxxxxxxxxxxxxxx> wrote:
>
> From: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxx>
>
> With the typical model where the display server opens 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
>
> *)
> More detailed and historically accurate description of various handover
> implementation kindly provided by Emil Velikov:
>
> """
> The traditional model, the server was the orchestrator managing the
> primary device node. From the fd, to the master status and
> authentication. But looking at the fd alone, this has varied across
> the years.
>
> IIRC in the DRI1 days, Xorg (libdrm really) would have a list of open
> fd(s) and reuse those whenever needed, DRI2 the client was responsible
> for open() themselves and with DRI3 the fd was passed to the client.
>
> Around the inception of DRI3 and systemd-logind, the latter became
> another possible orchestrator. Whereby Xorg and Wayland compositors
> could ask it for the fd. For various reasons (hysterical and genuine
> ones) Xorg has a fallback path going the open(), whereas Wayland
> compositors are moving to solely relying on logind... some never had
> fallback even.
>
> Over the past few years, more projects have emerged which provide
> functionality similar (be that on API level, Dbus, or otherwise) to
> systemd-logind.
> """
>
> v2:
> * Fixed typo in commit text and added a fine historical explanation
> from Emil.
>
> Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxx>
> Cc: "Christian König" <christian.koenig@xxxxxxx>
> Cc: Daniel Vetter <daniel@xxxxxxxx>
> Acked-by: Christian König <christian.koenig@xxxxxxx>
> Reviewed-by: Emil Velikov <emil.l.velikov@xxxxxxxxx>
Reviewed-by: Rob Clark <robdclark@xxxxxxxxx>
Tested-by: Rob Clark <robdclark@xxxxxxxxx>
> ---
> 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 74055cba3dc9..849097dff02b 100644
> --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_gem.c
> +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_gem.c
> @@ -963,6 +963,7 @@ 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;
>
> /*
> @@ -972,8 +973,9 @@ 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 883d83bc0e3d..e692770ef6d3 100644
> --- a/drivers/gpu/drm/drm_file.c
> +++ b/drivers/gpu/drm/drm_file.c
> @@ -160,7 +160,7 @@ struct drm_file *drm_file_alloc(struct drm_minor *minor)
>
> /* Get a unique identifier for fdinfo: */
> file->client_id = atomic64_inc_return(&ident);
> - 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 */
> @@ -200,7 +200,7 @@ 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);
> @@ -291,7 +291,7 @@ 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);
> }
>
> @@ -505,6 +505,40 @@ 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 51f1918b44d3..e3cb60eb0bc8 100644
> --- a/drivers/gpu/drm/nouveau/nouveau_drm.c
> +++ b/drivers/gpu/drm/nouveau/nouveau_drm.c
> @@ -1101,7 +1101,10 @@ 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 966912053cb0..c76249d5467e 100644
> --- a/include/drm/drm_file.h
> +++ b/include/drm/drm_file.h
> @@ -256,8 +256,15 @@ 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;
>
> /** @client_id: A unique id for fdinfo */
> u64 client_id;
> @@ -420,6 +427,8 @@ 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,
> --
> 2.39.2
>
