On Thu, Sep 27, 2018 at 06:49:02PM +0200, Jann Horn wrote:

On Thu, Sep 27, 2018 at 5:11 PM Tycho Andersen [off-list ref] wrote:

quoted

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.

[...]

quoted

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;

Why did you remove this? You could just do s/current/task/ instead, right?

No reason, probably just flailing around trying to figure out what
exactly I wanted. I'll make the change, thanks.

quoted

        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);

I think Linux kernel coding style is normally to have comments on the
implementations of functions, not in the headers? Maybe replace the
warning above the implemenation of replace_fd_task() with this
comment.

Will do.

Cheers,

Tycho