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Subject[PATCH 1/3] pid: add pidfd_prepare()
Add a new helper that allows to reserve a pidfd and allocates a new
pidfd file that stashes the provided struct pid. This will allow us to
remove places that either open code this function or that call
pidfd_create() but then have to call close_fd() because there are still
failure points after pidfd_create() has been called.

Signed-off-by: Christian Brauner <brauner@kernel.org>
---
include/linux/pid.h | 1 +
kernel/pid.c | 69 +++++++++++++++++++++++++++++++++++++++++++----------
2 files changed, 58 insertions(+), 12 deletions(-)

diff --git a/include/linux/pid.h b/include/linux/pid.h
index 343abf22092e..b75de288a8c2 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);
+int pidfd_prepare(struct pid *pid, unsigned int flags, struct file **ret);

static inline struct pid *get_pid(struct pid *pid)
{
diff --git a/kernel/pid.c b/kernel/pid.c
index 3fbc5e46b721..95e7e01574c8 100644
--- a/kernel/pid.c
+++ b/kernel/pid.c
@@ -576,6 +576,56 @@ struct task_struct *pidfd_get_task(int pidfd, unsigned int *flags)
return task;
}

+/**
+ * pidfd_prepare - allocate a new pidfd_file and reserve a pidfd
+ * @pid: the struct pid for which to create a pidfd
+ * @flags: flags of the new @pidfd
+ * @pidfd: the pidfd to return
+ *
+ * Allocate a new file that stashes @pid and reserve a new pidfd number in the
+ * caller's file descriptor table. The pidfd is reserved but not installed yet.
+ *
+ * If this function returns successfully the caller is responsible to either
+ * call fd_install() passing the returned pidfd and pidfd file as arguments in
+ * order to install the pidfd into its file descriptor table or they must use
+ * put_unused_fd() and fput() on the returned pidfd and pidfd file
+ * respectively.
+ *
+ * This function is useful when a pidfd must already be reserved but there
+ * might still be points of failure afterwards and the caller wants to ensure
+ * that no pidfd is leaked into its file descriptor table.
+ *
+ * Return: On success, a reserved pidfd is returned from the function and a new
+ * pidfd file is returned in the last argument to the function. On
+ * error, a negative error code is returned from the function and the
+ * last argument remains unchanged.
+ */
+int pidfd_prepare(struct pid *pid, unsigned int flags, struct file **ret)
+{
+ int pidfd;
+ struct file *pidfd_file;
+
+ if (!pid || !pid_has_task(pid, PIDTYPE_TGID))
+ return -EINVAL;
+
+ if (flags & ~(O_NONBLOCK | O_RDWR | O_CLOEXEC))
+ return -EINVAL;
+
+ pidfd = get_unused_fd_flags(O_RDWR | O_CLOEXEC);
+ if (pidfd < 0)
+ return pidfd;
+
+ pidfd_file = anon_inode_getfile("[pidfd]", &pidfd_fops, pid,
+ flags | O_RDWR | O_CLOEXEC);
+ if (IS_ERR(pidfd_file)) {
+ put_unused_fd(pidfd);
+ return PTR_ERR(pidfd_file);
+ }
+ get_pid(pid); /* held by pidfd_file now */
+ *ret = pidfd_file;
+ return pidfd;
+}
+
/**
* pidfd_create() - Create a new pid file descriptor.
*
@@ -594,20 +644,15 @@ 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 *pidfd_file;

- if (!pid || !pid_has_task(pid, PIDTYPE_TGID))
- return -EINVAL;
-
- 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);
+ pidfd = pidfd_prepare(pid, flags, &pidfd_file);
+ if (pidfd < 0)
+ return pidfd;

- return fd;
+ fd_install(pidfd, pidfd_file);
+ return pidfd;
}

/**
--
2.34.1

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