Re: [PATCH v6 1/3] vfs, fdtable: Add get_task_file helper

On Mon, Dec 23, 2019 at 10:26:46PM +0100, Christian Brauner wrote:

On Mon, Dec 23, 2019 at 09:08:55PM +0000, Sargun Dhillon wrote:

quoted

This introduces a function which can be used to fetch a file, given an
arbitrary task. As long as the user holds a reference (refcnt) to the
task_struct it is safe to call, and will either return NULL on failure,
or a pointer to the file, with a refcnt.

Signed-off-by: Sargun Dhillon <redacted>

Could you please add:

Suggested-by: Oleg Nesterov <oleg@redhat.com>

and add a line like:

"This is based on a patch sent by Oleg (cf. [1]) a while ago."

[1]: Link: https://lore.kernel.org/r/20180915160423.GA31461@redhat.com (local)

apart from a few nits below:

Acked-by: Christian Brauner <redacted>

quoted

---
 fs/file.c            | 22 ++++++++++++++++++++--
 include/linux/file.h |  2 ++
 2 files changed, 22 insertions(+), 2 deletions(-)

diff --git a/fs/file.c b/fs/file.c
index 2f4fcf985079..0ceeb046f4f3 100644
--- a/fs/file.c
+++ b/fs/file.c

@@ -706,9 +706,9 @@ void do_close_on_exec(struct files_struct *files)
 	spin_unlock(&files->file_lock);
 }
 
-static struct file *__fget(unsigned int fd, fmode_t mask, unsigned int refs)
+static struct file *__fget_files(struct files_struct *files, unsigned int fd,
+				 fmode_t mask, unsigned int refs)
 {
-	struct files_struct *files = current->files;
 	struct file *file;
 
 	rcu_read_lock();

@@ -729,6 +729,11 @@ static struct file *__fget(unsigned int fd, fmode_t mask, unsigned int refs)
 	return file;
 }
 
+static struct file *__fget(unsigned int fd, fmode_t mask, unsigned int refs)

This can be:

static inline struct file *__fget(...)

quoted

+{
+	return __fget_files(current->files, fd, mask, refs);
+}
+
 struct file *fget_many(unsigned int fd, unsigned int refs)
 {
 	return __fget(fd, FMODE_PATH, refs);

@@ -746,6 +751,19 @@ struct file *fget_raw(unsigned int fd)
 }
 EXPORT_SYMBOL(fget_raw);
 
+struct file *fget_task(struct task_struct *task, unsigned int fd)
+{
+	struct file *file = NULL;
+
+	task_lock(task);
+	if (task->files)
+		file = __fget_files(task->files, fd, 0, 1);
+
+	task_unlock(task);

Nit: remove the \n:

task_lock(task);
if (task->files)
	file = __fget_files(task->files, fd, 0, 1);
task_unlock(task);

Christian