linux-kernel - Re: [PATCH v6 1/3] vfs, fdtable: Add get_task

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Date:   Mon, 23 Dec 2019 22:26:46 +0100
From:   Christian Brauner <christian.brauner@...ntu.com>
To:     Sargun Dhillon <sargun@...gun.me>
Cc:     linux-kernel@...r.kernel.org,
        containers@...ts.linux-foundation.org, linux-api@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, tycho@...ho.ws, jannh@...gle.com,
        cyphar@...har.com, oleg@...hat.com, luto@...capital.net,
        viro@...iv.linux.org.uk, gpascutto@...illa.com,
        ealvarez@...illa.com, fweimer@...hat.com, jld@...illa.com,
        arnd@...db.de
Subject: Re: [PATCH v6 1/3] vfs, fdtable: Add get_task_file helper

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@...gun.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