[RFC PATCH] fdinfo: Option for public fdinfo

[David Francis]

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.

Add fd_op flag FOP_PUBLIC_FDINFO, which, if set, makes the fdinfo
for that file available to read for processes without ptrace
permissions.

Note that this makes public how many files each process has open,
and what their fds are.

Signed-off-by: David Francis <David.Francis@xxxxxxx>
---
 drivers/gpu/drm/amd/amdgpu/amdgpu_drv.c |  2 +-
 fs/proc/fd.c                            | 24 ++++++++++++++++++++++--
 include/linux/fs.h                      |  2 ++
 3 files changed, 25 insertions(+), 3 deletions(-)

diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_drv.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_drv.c
index 03814a23eb54..89eabeee5836 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_PUBLIC_FDINFO,
 };
 
 int amdgpu_file_to_fpriv(struct file *filp, struct amdgpu_fpriv **fpriv)
diff --git a/fs/proc/fd.c b/fs/proc/fd.c
index 9eeccff49b2a..e48854f6a28d 100644
--- a/fs/proc/fd.c
+++ b/fs/proc/fd.c
@@ -86,13 +86,34 @@ static int proc_fdinfo_permission(struct mnt_idmap *idmap, struct inode *inode,
 				  int mask)
 {
 	bool allowed = false;
+	struct file *file = NULL;
+	struct files_struct *files = NULL;
 	struct task_struct *task = get_proc_task(inode);
 
 	if (!task)
 		return -ESRCH;
 
-	allowed = ptrace_may_access(task, PTRACE_MODE_READ_FSCREDS);
+	task_lock(task);
+	files = task->files;
+	if (files) {
+		unsigned int fd = proc_fd(inode);
+
+		spin_lock(&files->file_lock);
+		file = files_lookup_fd_locked(files, fd);
+		if (file)
+			get_file(file);
+		spin_unlock(&files->file_lock);
+	}
+	task_unlock(task);
+
+	if (file)
+		allowed = file->f_op->fop_flags & FOP_PUBLIC_FDINFO;
+
+	if (!allowed)
+		allowed = ptrace_may_access(task, PTRACE_MODE_READ_FSCREDS);
+
 	put_task_struct(task);
+	fput(file);
 
 	if (!allowed)
 		return -EACCES;
@@ -401,7 +422,6 @@ static int proc_fdinfo_iterate(struct file *file, struct dir_context *ctx)
 
 const struct inode_operations proc_fdinfo_inode_operations = {
 	.lookup		= proc_lookupfdinfo,
-	.permission	= proc_fdinfo_permission,
 	.setattr	= proc_setattr,
 };
 
diff --git a/include/linux/fs.h b/include/linux/fs.h
index dd3b57cfadee..0444086868d5 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 without ptrace access */
+#define FOP_PUBLIC_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