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

Submitted by Sargun Dhillon on Dec. 23, 2019, 9:08 p.m.

Details

Message ID 20191223210852.GA25101@ircssh-2.c.rugged-nimbus-611.internal
State New
Series "Series without cover letter"
Headers show

Commit Message

Sargun Dhillon Dec. 23, 2019, 9:08 p.m.
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 <sargun@sargun.me>
---
 fs/file.c            | 22 ++++++++++++++++++++--
 include/linux/file.h |  2 ++
 2 files changed, 22 insertions(+), 2 deletions(-)

Patch hide | download patch | download mbox

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)
+{
+	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);
+
+	return file;
+}
+
 /*
  * Lightweight file lookup - no refcnt increment if fd table isn't shared.
  *
diff --git a/include/linux/file.h b/include/linux/file.h
index 3fcddff56bc4..c6c7b24ea9f7 100644
--- a/include/linux/file.h
+++ b/include/linux/file.h
@@ -16,6 +16,7 @@  extern void fput(struct file *);
 extern void fput_many(struct file *, unsigned int);
 
 struct file_operations;
+struct task_struct;
 struct vfsmount;
 struct dentry;
 struct inode;
@@ -47,6 +48,7 @@  static inline void fdput(struct fd fd)
 extern struct file *fget(unsigned int fd);
 extern struct file *fget_many(unsigned int fd, unsigned int refs);
 extern struct file *fget_raw(unsigned int fd);
+extern struct file *fget_task(struct task_struct *task, unsigned int fd);
 extern unsigned long __fdget(unsigned int fd);
 extern unsigned long __fdget_raw(unsigned int fd);
 extern unsigned long __fdget_pos(unsigned int fd);

Comments

Christian Brauner Dec. 23, 2019, 9:26 p.m.
On Mon, Dec 23, 2019 at 09:08:55PM +0000, Sargun Dhillon wrote:
> 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 <sargun@sargun.me>
> ---
>  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)
> +{
> +	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
Christian Brauner Dec. 25, 2019, 4:19 p.m.
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:
> > 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 <sargun@sargun.me>

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

apart from a few nits below:

Acked-by: Christian Brauner <christian.brauner@ubuntu.com>

> > ---
> >  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(...)

> > +{
> > +	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