[PATCH 09/17] file: Implement fnext_task

[Eric W. Biederman]

As a companion to fget_task and fcheck_task implement fnext_task that
will return the struct file for the first file descriptor show number
is equal or greater than the fd argument value, or NULL if there is
no such struct file.

This allows file descriptors of foreign processes to be iterated through
safely, without needed to increment the count on files_struct.

Signed-off-by: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>
---
 fs/file.c               | 21 +++++++++++++++++++++
 include/linux/fdtable.h |  1 +
 2 files changed, 22 insertions(+)

diff --git a/fs/file.c b/fs/file.c
index 8d4b385055e9..88f9f78869f8 100644
--- a/fs/file.c
+++ b/fs/file.c
@@ -876,6 +876,27 @@ struct file *fcheck_task(struct task_struct *task, unsigned int fd)
 	return file;
 }
 
+struct file *fnext_task(struct task_struct *task, unsigned int *ret_fd)
+{
+	/* Must be called with rcu_read_lock held */
+	struct files_struct *files;
+	unsigned int fd = *ret_fd;
+	struct file *file = NULL;
+
+	task_lock(task);
+	files = task->files;
+	if (files) {
+		for (; fd < files_fdtable(files)->max_fds; fd++) {
+			file = fcheck_files(files, fd);
+			if (file)
+				break;
+		}
+	}
+	task_unlock(task);
+	*ret_fd = fd;
+	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 def9debd2ce2..a3a054084f49 100644
--- a/include/linux/fdtable.h
+++ b/include/linux/fdtable.h
@@ -104,6 +104,7 @@ static inline struct file *fcheck_files(struct files_struct *files, unsigned int
  */
 #define fcheck(fd)	fcheck_files(current->files, fd)
 struct file *fcheck_task(struct task_struct *task, unsigned int fd);
+struct file *fnext_task(struct task_struct *task, unsigned int *fd);
 
 struct task_struct;
 
-- 
2.25.0