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

Tycho Andersen
2018-09-27T15:11:48Z

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@tycho.ws>
CC: Alexander Viro <viro@zeniv.linux.org.uk>
CC: Kees Cook <keescook@chromium.org>
CC: Andy Lutomirski <luto@amacapital.net>
CC: Oleg Nesterov <oleg@redhat.com>
CC: Eric W. Biederman <ebiederm@xmission.com>
CC: "Serge E. Hallyn" <serge@hallyn.com>
CC: Christian Brauner <christian.brauner@ubuntu.com>
CC: Tyler Hicks <tyhicks@canonical.com>
CC: Akihiro Suda <suda.akihiro@lab.ntt.co.jp>
---
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