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

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 <redacted>
CC: Alexander Viro <viro@zeniv.linux.org.uk>
CC: Kees Cook <redacted>
CC: Andy Lutomirski <luto@amacapital.net>
CC: Oleg Nesterov <oleg@redhat.com>
CC: Eric W. Biederman <redacted>
CC: "Serge E. Hallyn" <serge@hallyn.com>
CC: Christian Brauner <redacted>
CC: Tyler Hicks <redacted>
CC: Akihiro Suda <redacted>
---
 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);