[PATCH] fdinfo: Add fops flag to allow CAP_PERFMON to see fdinfo

[David Francis <David.Francis@amd.com>]

We want some GPU information to be publicly available to all
processes for basic system-wide profiling (think GPU versions
of top).

This information is available in fdinfo and not easily exposed
by other interfaces.

Allow processes with CAP_PERFMON capability to
- read /proc/<pid>/fd
- follow symlinks in /proc/<pid>/fd, but only if that file has
new file operations flag FOP_PERFMON_FDINFO
- read /proc/<pid>/fdinfo
- read /proc/<pid>/fdinfo/<fd>, but only if that file has
FOP_PERFMON_FDINFO

Signed-off-by: David Francis <David.Francis@amd.com>
Suggested-by: Tvrtko Ursulin <tvrtko.ursulin@igalia.com>
---
 drivers/gpu/drm/amd/amdgpu/amdgpu_drv.c |  2 +-
 fs/proc/base.c                          | 18 ++++++++++++++++
 fs/proc/fd.c                            | 28 ++++++++++++++++++++++++-
 include/linux/fs.h                      |  2 ++
 4 files changed, 48 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_drv.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_drv.c
index 03814a23eb54..d62f8b400258 100644
--- a/drivers/gpu/drm/amd/amdgpu/amdgpu_drv.c
+++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_drv.c
@@ -3022,7 +3022,7 @@ static const struct file_operations amdgpu_driver_kms_fops = {
 #ifdef CONFIG_PROC_FS
 	.show_fdinfo = drm_show_fdinfo,
 #endif
-	.fop_flags = FOP_UNSIGNED_OFFSET,
+	.fop_flags = FOP_UNSIGNED_OFFSET | FOP_PERFMON_FDINFO,
 };
 
 int amdgpu_file_to_fpriv(struct file *filp, struct amdgpu_fpriv **fpriv)
diff --git a/fs/proc/base.c b/fs/proc/base.c
index 6299878e3d97..83182ff6b96d 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -86,6 +86,7 @@
 #include <linux/user_namespace.h>
 #include <linux/fs_parser.h>
 #include <linux/fs_struct.h>
+#include <linux/fdtable.h>
 #include <linux/slab.h>
 #include <linux/sched/autogroup.h>
 #include <linux/sched/mm.h>
@@ -716,6 +717,23 @@ static bool proc_fd_access_allowed(struct inode *inode)
 	task = get_proc_task(inode);
 	if (task) {
 		allowed = ptrace_may_access(task, PTRACE_MODE_READ_FSCREDS);
+		if (!allowed && capable(CAP_PERFMON)) {
+			struct files_struct *files;
+
+			task_lock(task);
+			files = task->files;
+			if (files) {
+				struct file *file;
+
+				spin_lock(&files->file_lock);
+				file = files_lookup_fd_locked(files,
+							      proc_fd(inode));
+				allowed = file && file->f_op->fop_flags &
+						  FOP_PERFMON_FDINFO;
+				spin_unlock(&files->file_lock);
+			}
+			task_unlock(task);
+		}
 		put_task_struct(task);
 	}
 	return allowed;
diff --git a/fs/proc/fd.c b/fs/proc/fd.c
index 9eeccff49b2a..89c1a205148a 100644
--- a/fs/proc/fd.c
+++ b/fs/proc/fd.c
@@ -86,12 +86,35 @@ static int proc_fdinfo_permission(struct mnt_idmap *idmap, struct inode *inode,
 				  int mask)
 {
 	bool allowed = false;
-	struct task_struct *task = get_proc_task(inode);
+	struct task_struct *task;
 
+	task = get_proc_task(inode);
 	if (!task)
 		return -ESRCH;
 
 	allowed = ptrace_may_access(task, PTRACE_MODE_READ_FSCREDS);
+
+	if (!allowed && capable(CAP_PERFMON)) {
+		struct files_struct *files;
+
+		if (S_ISDIR(inode->i_mode)) {
+			allowed = true;
+		} else {
+			task_lock(task);
+			files = task->files;
+			if (files) {
+				struct file *file;
+
+				spin_lock(&files->file_lock);
+				file = files_lookup_fd_locked(files, proc_fd(inode));
+				allowed = file && file->f_op->fop_flags &
+						  FOP_PERFMON_FDINFO;
+				spin_unlock(&files->file_lock);
+			}
+			task_unlock(task);
+		}
+	}
+
 	put_task_struct(task);
 
 	if (!allowed)
@@ -338,6 +361,9 @@ int proc_fd_permission(struct mnt_idmap *idmap,
 	if (rv == 0)
 		return rv;
 
+	if (capable(CAP_PERFMON))
+		return 0;
+
 	rcu_read_lock();
 	p = pid_task(proc_pid(inode), PIDTYPE_PID);
 	if (p && same_thread_group(p, current))
diff --git a/include/linux/fs.h b/include/linux/fs.h
index dd3b57cfadee..bc2826e1cc38 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -2327,6 +2327,8 @@ struct file_operations {
 #define FOP_ASYNC_LOCK		((__force fop_flags_t)(1 << 6))
 /* File system supports uncached read/write buffered IO */
 #define FOP_DONTCACHE		((__force fop_flags_t)(1 << 7))
+/* fdinfo readable with CAP_SYS_PERFMON */
+#define FOP_PERFMON_FDINFO	((__force fop_flags_t)(1 << 8))
 
 /* Wrap a directory iterator that needs exclusive inode access */
 int wrap_directory_iterator(struct file *, struct dir_context *,
-- 
2.34.1