[PATCH v7 4/6] files: add a replace_fd_files() function

[Tycho Andersen <tycho@xxxxxxxx>]

Similar to fd_install/__fd_install, we want to be able to replace an fd of
an arbitrary struct files_struct, not just current's. We'll use this in the
next patch to implement the seccomp ioctl that allows inserting fds into a
stopped process' context.

v7: new in v7

Signed-off-by: Tycho Andersen <tycho@xxxxxxxx>
CC: Alexander Viro <viro@xxxxxxxxxxxxxxxxxx>
CC: Kees Cook <keescook@xxxxxxxxxxxx>
CC: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
CC: Oleg Nesterov <oleg@xxxxxxxxxx>
CC: Eric W. Biederman <ebiederm@xxxxxxxxxxxx>
CC: "Serge E. Hallyn" <serge@xxxxxxxxxx>
CC: Christian Brauner <christian.brauner@xxxxxxxxxx>
CC: Tyler Hicks <tyhicks@xxxxxxxxxxxxx>
CC: Akihiro Suda <suda.akihiro@xxxxxxxxxxxxx>
---
 fs/file.c            | 22 +++++++++++++++-------
 include/linux/file.h |  8 ++++++++
 2 files changed, 23 insertions(+), 7 deletions(-)

diff --git a/fs/file.c b/fs/file.c
index 7ffd6e9d103d..3b3c5aadaadb 100644
--- a/fs/file.c
+++ b/fs/file.c
@@ -850,24 +850,32 @@ __releases(&files->file_lock)
 }
 
 int replace_fd(unsigned fd, struct file *file, unsigned flags)
+{
+	return replace_fd_task(current, fd, file, flags);
+}
+
+/*
+ * Same warning as __alloc_fd()/__fd_install() here.
+ */
+int replace_fd_task(struct task_struct *task, unsigned fd,
+		    struct file *file, unsigned flags)
 {
 	int err;
-	struct files_struct *files = current->files;
 
 	if (!file)
-		return __close_fd(files, fd);
+		return __close_fd(task->files, fd);
 
-	if (fd >= rlimit(RLIMIT_NOFILE))
+	if (fd >= task_rlimit(task, RLIMIT_NOFILE))
 		return -EBADF;
 
-	spin_lock(&files->file_lock);
-	err = expand_files(files, fd);
+	spin_lock(&task->files->file_lock);
+	err = expand_files(task->files, fd);
 	if (unlikely(err < 0))
 		goto out_unlock;
-	return do_dup2(files, file, fd, flags);
+	return do_dup2(task->files, file, fd, flags);
 
 out_unlock:
-	spin_unlock(&files->file_lock);
+	spin_unlock(&task->files->file_lock);
 	return err;
 }
 
diff --git a/include/linux/file.h b/include/linux/file.h
index 6b2fb032416c..f94277fee038 100644
--- a/include/linux/file.h
+++ b/include/linux/file.h
@@ -11,6 +11,7 @@
 #include <linux/posix_types.h>
 
 struct file;
+struct task_struct;
 
 extern void fput(struct file *);
 
@@ -79,6 +80,13 @@ static inline void fdput_pos(struct fd f)
 
 extern int f_dupfd(unsigned int from, struct file *file, unsigned flags);
 extern int replace_fd(unsigned fd, struct file *file, unsigned flags);
+/*
+ * Warning! This is only safe if you know the owner of the files_struct is
+ * stopped outside syscall context. It's a very bad idea to use this unless you
+ * have similar guarantees in your code.
+ */
+extern int replace_fd_task(struct task_struct *task, unsigned fd,
+			   struct file *file, unsigned flags);
 extern void set_close_on_exec(unsigned int fd, int flag);
 extern bool get_close_on_exec(unsigned int fd);
 extern int get_unused_fd_flags(unsigned flags);
-- 
2.17.1