[PATCH 1/3] [HERE BE DRAGONS - DRAFT - __UNTESTED__] pid: add pidfd_file_create()

[Christian Brauner]

Reserve and fd and pidfile, do stuff that might fail, install fd when
point of no return.

[HERE BE DRAGONS - DRAFT - __UNTESTED__] pid: add pidfd_file_create()

Signed-off-by: Christian Brauner <brauner@xxxxxxxxxx>
---
 include/linux/pid.h |  1 +
 kernel/pid.c        | 45 +++++++++++++++++++++++++++++++++------------
 2 files changed, 34 insertions(+), 12 deletions(-)

diff --git a/include/linux/pid.h b/include/linux/pid.h
index 343abf22092e..c486dbc4d7b6 100644
--- a/include/linux/pid.h
+++ b/include/linux/pid.h
@@ -80,6 +80,7 @@ extern struct pid *pidfd_pid(const struct file *file);
 struct pid *pidfd_get_pid(unsigned int fd, unsigned int *flags);
 struct task_struct *pidfd_get_task(int pidfd, unsigned int *flags);
 int pidfd_create(struct pid *pid, unsigned int flags);
+struct file *pidfd_file_create(struct pid *pid, unsigned int flags, int *pidfd);
 
 static inline struct pid *get_pid(struct pid *pid)
 {
diff --git a/kernel/pid.c b/kernel/pid.c
index 3fbc5e46b721..8d0924f1dbf6 100644
--- a/kernel/pid.c
+++ b/kernel/pid.c
@@ -576,6 +576,32 @@ struct task_struct *pidfd_get_task(int pidfd, unsigned int *flags)
 	return task;
 }
 
+struct file *pidfd_file_create(struct pid *pid, unsigned int flags, int *pidfd)
+{
+	int fd;
+	struct file *pidfile;
+
+	if (!pid || !pid_has_task(pid, PIDTYPE_TGID))
+		return ERR_PTR(-EINVAL);
+
+	if (flags & ~(O_NONBLOCK | O_RDWR | O_CLOEXEC))
+		return ERR_PTR(-EINVAL);
+
+	fd = get_unused_fd_flags(O_RDWR | O_CLOEXEC);
+	if (fd < 0)
+		return ERR_PTR(fd);
+
+	pidfile = anon_inode_getfile("[pidfd]", &pidfd_fops, pid,
+				     flags | O_RDWR | O_CLOEXEC);
+	if (IS_ERR(pidfile)) {
+		put_unused_fd(fd);
+		return pidfile;
+	}
+	get_pid(pid); /* held by pidfile now */
+	*pidfd = fd;
+	return pidfile;
+}
+
 /**
  * pidfd_create() - Create a new pid file descriptor.
  *
@@ -594,20 +620,15 @@ struct task_struct *pidfd_get_task(int pidfd, unsigned int *flags)
  */
 int pidfd_create(struct pid *pid, unsigned int flags)
 {
-	int fd;
+	int pidfd;
+	struct file *pidfile;
 
-	if (!pid || !pid_has_task(pid, PIDTYPE_TGID))
-		return -EINVAL;
+	pidfile = pidfd_file_create(pid, flags, &pidfd);
+	if (IS_ERR(pidfile))
+		return PTR_ERR(pidfile);
 
-	if (flags & ~(O_NONBLOCK | O_RDWR | O_CLOEXEC))
-		return -EINVAL;
-
-	fd = anon_inode_getfd("[pidfd]", &pidfd_fops, get_pid(pid),
-			      flags | O_RDWR | O_CLOEXEC);
-	if (fd < 0)
-		put_pid(pid);
-
-	return fd;
+	fd_install(pidfd, pidfile);
+	return pidfd;
 }
 
 /**
-- 
2.34.1