From 1c7a387ffef894b1ab3942f0482dac7a6e0a909c Mon Sep 17 00:00:00 2001 From: Tvrtko Ursulin Date: Wed, 21 Jun 2023 10:48:24 +0100 Subject: [PATCH] drm: Update file owner during use MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit 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 Cc: "Christian König" Cc: Daniel Vetter Acked-by: Christian König Reviewed-by: Emil Velikov Reviewed-by: Rob Clark Tested-by: Rob Clark Link: https://patchwork.freedesktop.org/patch/msgid/20230621094824.2348732-1-tvrtko.ursulin@linux.intel.com Signed-off-by: Christian König --- 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 7da871972a8e..c1ee440a06fe 100644 --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_gem.c +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_gem.c @@ -958,6 +958,7 @@ 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; /* @@ -967,8 +968,9 @@ 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 : ""); 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 34c7d1a580e3..44ecd7d0daac 100644 --- a/drivers/gpu/drm/drm_debugfs.c +++ b/drivers/gpu/drm/drm_debugfs.c @@ -92,15 +92,17 @@ 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 : "", - 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 @@ out_prime_destroy: 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 f03ffbacfe9b..77590b0f38fa 100644 --- a/drivers/gpu/drm/drm_ioctl.c +++ b/drivers/gpu/drm/drm_ioctl.c @@ -776,6 +776,9 @@ 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 4396f501b16a..50589f982d1a 100644 --- a/drivers/gpu/drm/nouveau/nouveau_drm.c +++ b/drivers/gpu/drm/nouveau/nouveau_drm.c @@ -1133,7 +1133,10 @@ 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 : ""); rcu_read_unlock(); diff --git a/include/drm/drm_file.h b/include/drm/drm_file.h index 489cc3758a82..e1b5b4282f75 100644 --- a/include/drm/drm_file.h +++ b/include/drm/drm_file.h @@ -254,8 +254,15 @@ 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; @@ -418,6 +425,8 @@ 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,