[v2,11/24] file: Implement task_lookup_fd_rcu

Submitted by Eric W. Biederman on Nov. 20, 2020, 11:14 p.m.

Details

Message ID 20201120231441.29911-11-ebiederm@xmission.com
State New
Series "exec: Move unshare_files and guarantee files_struct.count is correct"
Headers show

Commit Message

Eric W. Biederman Nov. 20, 2020, 11:14 p.m.
As a companion to lookup_fd_rcu implement task_lookup_fd_rcu for
querying an arbitrary process about a specific file.

Acked-by: Christian Brauner <christian.brauner@ubuntu.com>
v1: https://lkml.kernel.org/r/20200818103713.aw46m7vprsy4vlve@wittgenstein
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
---
 fs/file.c               | 15 +++++++++++++++
 include/linux/fdtable.h |  2 ++
 2 files changed, 17 insertions(+)

Patch hide | download patch | download mbox

diff --git a/fs/file.c b/fs/file.c
index 5861c4f89419..6448523ca29e 100644
--- a/fs/file.c
+++ b/fs/file.c
@@ -865,6 +865,21 @@  struct file *fget_task(struct task_struct *task, unsigned int fd)
 	return file;
 }
 
+struct file *task_lookup_fd_rcu(struct task_struct *task, unsigned int fd)
+{
+	/* Must be called with rcu_read_lock held */
+	struct files_struct *files;
+	struct file *file = NULL;
+
+	task_lock(task);
+	files = task->files;
+	if (files)
+		file = files_lookup_fd_rcu(files, fd);
+	task_unlock(task);
+
+	return file;
+}
+
 /*
  * Lightweight file lookup - no refcnt increment if fd table isn't shared.
  *
diff --git a/include/linux/fdtable.h b/include/linux/fdtable.h
index 2a4a8fed536e..a0558ae9b40c 100644
--- a/include/linux/fdtable.h
+++ b/include/linux/fdtable.h
@@ -110,6 +110,8 @@  static inline struct file *lookup_fd_rcu(unsigned int fd)
 	return files_lookup_fd_rcu(current->files, fd);
 }
 
+struct file *task_lookup_fd_rcu(struct task_struct *task, unsigned int fd);
+
 struct task_struct;
 
 struct files_struct *get_files_struct(struct task_struct *);

Comments

Cyrill Gorcunov Nov. 21, 2020, 6:19 p.m.
On Fri, Nov 20, 2020 at 05:14:28PM -0600, Eric W. Biederman wrote:
> 
> diff --git a/fs/file.c b/fs/file.c
> index 5861c4f89419..6448523ca29e 100644
> --- a/fs/file.c
> +++ b/fs/file.c
> @@ -865,6 +865,21 @@ struct file *fget_task(struct task_struct *task, unsigned int fd)
>  	return file;
>  }
>  
> +struct file *task_lookup_fd_rcu(struct task_struct *task, unsigned int fd)
> +{
> +	/* Must be called with rcu_read_lock held */

Eric, maybe worth to have something like

	RCU_LOCKDEP_WARN(!rcu_read_lock_held(),
			 "suspicious task_lookup_fd_rcu() usage");

here?
Eric W. Biederman Nov. 22, 2020, 1 p.m.
Cyrill Gorcunov <gorcunov@gmail.com> writes:

> On Fri, Nov 20, 2020 at 05:14:28PM -0600, Eric W. Biederman wrote:
>> 
>> diff --git a/fs/file.c b/fs/file.c
>> index 5861c4f89419..6448523ca29e 100644
>> --- a/fs/file.c
>> +++ b/fs/file.c
>> @@ -865,6 +865,21 @@ struct file *fget_task(struct task_struct *task, unsigned int fd)
>>  	return file;
>>  }
>>  
>> +struct file *task_lookup_fd_rcu(struct task_struct *task, unsigned int fd)
>> +{
>> +	/* Must be called with rcu_read_lock held */
>
> Eric, maybe worth to have something like
>
> 	RCU_LOCKDEP_WARN(!rcu_read_lock_held(),
> 			 "suspicious task_lookup_fd_rcu() usage");
>
> here?

That is present in files_lookup_fd_rcu, so this code should
be good from the warning side.

Eric
Cyrill Gorcunov Nov. 22, 2020, 1:52 p.m.
On Sun, Nov 22, 2020 at 07:00:20AM -0600, Eric W. Biederman wrote:
> Cyrill Gorcunov <gorcunov@gmail.com> writes:
...
> That is present in files_lookup_fd_rcu, so this code should
> be good from the warning side.

Indeed, thanks!