Re: [Intel-gfx] [PATCH v2] drm: Update file owner during use

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

On 28/08/2023 20:58, Rob Clark wrote:
> On Wed, Jun 21, 2023 at 2:48 AM Tvrtko Ursulin
> <tvrtko.ursulin@linux.intel.com> wrote:
>>
>> From: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
>>
>> 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@intel.com>
>> Cc: "Christian König" <christian.koenig@amd.com>
>> Cc: Daniel Vetter <daniel@ffwll.ch>
>> Acked-by: Christian König <christian.koenig@amd.com>
>> Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
> 
> Reviewed-by: Rob Clark <robdclark@gmail.com>
> Tested-by: Rob Clark <robdclark@gmail.com>

Thanks. If everyone else is happy with this approach I don't have the 
commit rights for drm-misc.

Regards,

Tvrtko

> 
>> ---
>>   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
>>