The means by which a pid is determined from a pidfd is duplicated, with
some callers holding a reference to the (pid)fd, and others explicitly
pinning the pid.

Introduce __pidfd_get_pid() which abstracts both approaches and provide
optional output parameters for file->f_flags and the fd (the latter of
which, if provided, prevents the function from decrementing the fd's
refernce count).

Additionally, allow the ability to open a pidfd by opening a /proc/<pid>
directory, utilised by the pidfd_send_signal() system call, providing a
pidfd_get_pid_proc() helper function to do so.

Doing this allows us to eliminate open-coded pidfd pid lookup and to
consistently handle this in one place.

This lays the groundwork for a subsequent patch which adds a new sentinel
pidfd to explicitly reference the current process (i.e. thread group
leader) without the need for a pidfd.

Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
---
 include/linux/pid.h | 42 +++++++++++++++++++++++++++++++-
 kernel/pid.c        | 58 ++++++++++++++++++++++++++++++---------------
 kernel/signal.c     | 22 ++++-------------
 3 files changed, 84 insertions(+), 38 deletions(-)

diff --git a/include/linux/pid.h b/include/linux/pid.h
index a3aad9b4074c..68b02eab7509 100644
--- a/include/linux/pid.h
+++ b/include/linux/pid.h
@@ -2,6 +2,7 @@
 #ifndef _LINUX_PID_H
 #define _LINUX_PID_H
 
+#include <linux/file.h>
 #include <linux/pid_types.h>
 #include <linux/rculist.h>
 #include <linux/rcupdate.h>
@@ -72,8 +73,47 @@ extern struct pid init_struct_pid;
 
 struct file;
 
+
+/**
+ * __pidfd_get_pid() - Retrieve a pid associated with the specified pidfd.
+ *
+ * @pidfd:      The pidfd whose pid we want, or the fd of a /proc/<pid> file if
+ *              @alloc_proc is also set.
+ * @pin_pid:    If set, then the reference counter of the returned pid is
+ *              incremented. If not set, then @fd should be provided to pin the
+ *              pidfd.
+ * @allow_proc: If set, then an fd of a /proc/<pid> file can be passed instead
+ *              of a pidfd, and this will be used to determine the pid.
+ * @flags:      Output variable, if non-NULL, then the file->f_flags of the
+ *              pidfd will be set here.
+ * @fd:         Output variable, if non-NULL, then the pidfd reference will
+ *              remain elevated and the caller will need to decrement it
+ *              themselves.
+ *
+ * Returns: If successful, the pid associated with the pidfd, otherwise an
+ *          error.
+ */
+struct pid *__pidfd_get_pid(unsigned int pidfd, bool pin_pid,
+			    bool allow_proc, unsigned int *flags,
+			    struct fd *fd);
+
+static inline struct pid *pidfd_get_pid(unsigned int pidfd, unsigned int *flags)
+{
+	return __pidfd_get_pid(pidfd, /* pin_pid = */ true,
+			       /* allow_proc = */ false,
+			       flags, /* fd = */ NULL);
+}
+
+static inline struct pid *pidfd_to_pid_proc(unsigned int pidfd,
+					    unsigned int *flags,
+					    struct fd *fd)
+{
+	return __pidfd_get_pid(pidfd, /* pin_pid = */ false,
+			       /* allow_proc = */ true,
+			       flags, fd);
+}
+
 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_prepare(struct pid *pid, unsigned int flags, struct file **ret);
 void do_notify_pidfd(struct task_struct *task);
diff --git a/kernel/pid.c b/kernel/pid.c
index 2715afb77eab..25cc1c36a1b1 100644
--- a/kernel/pid.c
+++ b/kernel/pid.c
@@ -36,6 +36,7 @@
 #include <linux/pid_namespace.h>
 #include <linux/init_task.h>
 #include <linux/syscalls.h>
+#include <linux/proc_fs.h>
 #include <linux/proc_ns.h>
 #include <linux/refcount.h>
 #include <linux/anon_inodes.h>
@@ -534,22 +535,46 @@ struct pid *find_ge_pid(int nr, struct pid_namespace *ns)
 }
 EXPORT_SYMBOL_GPL(find_ge_pid);
 
-struct pid *pidfd_get_pid(unsigned int fd, unsigned int *flags)
+struct pid *__pidfd_get_pid(unsigned int pidfd, bool pin_pid,
+			    bool allow_proc, unsigned int *flags,
+			    struct fd *fd)
 {
-	struct fd f;
+	struct file *file;
 	struct pid *pid;
+	struct fd f = fdget(pidfd);
 
-	f = fdget(fd);
-	if (!fd_file(f))
+	file = fd_file(f);
+	if (!file)
 		return ERR_PTR(-EBADF);
 
-	pid = pidfd_pid(fd_file(f));
-	if (!IS_ERR(pid)) {
-		get_pid(pid);
-		*flags = fd_file(f)->f_flags;
+	pid = pidfd_pid(file);
+	/* If we allow opening a pidfd via /proc/<pid>, do so. */
+	if (IS_ERR(pid) && allow_proc)
+		pid = tgid_pidfd_to_pid(file);
+
+	if (IS_ERR(pid)) {
+		fdput(f);
+		return pid;
 	}
 
-	fdput(f);
+	if (pin_pid)
+		get_pid(pid);
+	else
+		WARN_ON_ONCE(!fd); /* Nothing to keep pid/pidfd around? */
+
+	if (flags)
+		*flags = file->f_flags;
+
+	/*
+	 * If the user provides an fd output then it will handle decrementing
+	 * its reference counter.
+	 */
+	if (fd)
+		*fd = f;
+	else
+		/* Otherwise we release it. */
+		fdput(f);
+
 	return pid;
 }
 
@@ -747,23 +772,18 @@ SYSCALL_DEFINE3(pidfd_getfd, int, pidfd, int, fd,
 		unsigned int, flags)
 {
 	struct pid *pid;
-	struct fd f;
 	int ret;
 
 	/* flags is currently unused - make sure it's unset */
 	if (flags)
 		return -EINVAL;
 
-	f = fdget(pidfd);
-	if (!fd_file(f))
-		return -EBADF;
-
-	pid = pidfd_pid(fd_file(f));
+	pid = pidfd_get_pid(pidfd, NULL);
 	if (IS_ERR(pid))
-		ret = PTR_ERR(pid);
-	else
-		ret = pidfd_getfd(pid, fd);
+		return PTR_ERR(pid);
 
-	fdput(f);
+	ret = pidfd_getfd(pid, fd);
+
+	put_pid(pid);
 	return ret;
 }
diff --git a/kernel/signal.c b/kernel/signal.c
index 4344860ffcac..868bfa674c62 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -3875,17 +3875,6 @@ static int copy_siginfo_from_user_any(kernel_siginfo_t *kinfo,
 	return copy_siginfo_from_user(kinfo, info);
 }
 
-static struct pid *pidfd_to_pid(const struct file *file)
-{
-	struct pid *pid;
-
-	pid = pidfd_pid(file);
-	if (!IS_ERR(pid))
-		return pid;
-
-	return tgid_pidfd_to_pid(file);
-}
-
 #define PIDFD_SEND_SIGNAL_FLAGS                            \
 	(PIDFD_SIGNAL_THREAD | PIDFD_SIGNAL_THREAD_GROUP | \
 	 PIDFD_SIGNAL_PROCESS_GROUP)
@@ -3908,10 +3897,11 @@ SYSCALL_DEFINE4(pidfd_send_signal, int, pidfd, int, sig,
 		siginfo_t __user *, info, unsigned int, flags)
 {
 	int ret;
-	struct fd f;
 	struct pid *pid;
 	kernel_siginfo_t kinfo;
 	enum pid_type type;
+	unsigned int f_flags;
+	struct fd f;
 
 	/* Enforce flags be set to 0 until we add an extension. */
 	if (flags & ~PIDFD_SEND_SIGNAL_FLAGS)
@@ -3921,12 +3911,8 @@ SYSCALL_DEFINE4(pidfd_send_signal, int, pidfd, int, sig,
 	if (hweight32(flags & PIDFD_SEND_SIGNAL_FLAGS) > 1)
 		return -EINVAL;
 
-	f = fdget(pidfd);
-	if (!fd_file(f))
-		return -EBADF;
-
 	/* Is this a pidfd? */
-	pid = pidfd_to_pid(fd_file(f));
+	pid = pidfd_to_pid_proc(pidfd, &f_flags, &f);
 	if (IS_ERR(pid)) {
 		ret = PTR_ERR(pid);
 		goto err;
@@ -3939,7 +3925,7 @@ SYSCALL_DEFINE4(pidfd_send_signal, int, pidfd, int, sig,
 	switch (flags) {
 	case 0:
 		/* Infer scope from the type of pidfd. */
-		if (fd_file(f)->f_flags & PIDFD_THREAD)
+		if (f_flags & PIDFD_THREAD)
 			type = PIDTYPE_PID;
 		else
 			type = PIDTYPE_TGID;
-- 
2.46.2