[PATCH v2 2/5] pid: add pidfd_open()

@@ -433,3 +433,4 @@
 425	i386	io_uring_setup		sys_io_uring_setup		__ia32_sys_io_uring_setup
 426	i386	io_uring_enter		sys_io_uring_enter		__ia32_sys_io_uring_enter
 427	i386	io_uring_register	sys_io_uring_register		__ia32_sys_io_uring_register
+428	i386	pidfd_open		sys_pidfd_open			__ia32_sys_pidfd_open

@@ -349,6 +349,7 @@
 425	common	io_uring_setup		__x64_sys_io_uring_setup
 426	common	io_uring_enter		__x64_sys_io_uring_enter
 427	common	io_uring_register	__x64_sys_io_uring_register
+428	common	pidfd_open		__x64_sys_pidfd_open
 
 #
 # x32-specific system call numbers start at 512 to avoid cache impact

@@ -66,6 +66,8 @@ struct pid
 
 extern struct pid init_struct_pid;
 
+extern const struct file_operations pidfd_fops;
+
 static inline struct pid *get_pid(struct pid *pid)
 {
 	if (pid)

@@ -929,6 +929,7 @@ asmlinkage long sys_clock_adjtime32(clockid_t which_clock,
 				struct old_timex32 __user *tx);
 asmlinkage long sys_syncfs(int fd);
 asmlinkage long sys_setns(int fd, int nstype);
+asmlinkage long sys_pidfd_open(pid_t pid, unsigned int flags);
 asmlinkage long sys_sendmmsg(int fd, struct mmsghdr __user *msg,
 			     unsigned int vlen, unsigned flags);
 asmlinkage long sys_process_vm_readv(pid_t pid,

@@ -18,5 +18,7 @@
 #define P_PID		1
 #define P_PGID		2
 
+/* Get a file descriptor for /proc/<pid> of the corresponding pidfd */
+#define PIDFD_GET_PROCFD _IOR('p', 1, int)
 
 #endif /* _UAPI_LINUX_WAIT_H */

@@ -26,8 +26,10 @@
  *
  */
 
+#include <linux/anon_inodes.h>
 #include <linux/mm.h>
 #include <linux/export.h>
+#include <linux/fsnotify.h>
 #include <linux/slab.h>
 #include <linux/init.h>
 #include <linux/rculist.h>

@@ -40,6 +42,7 @@
 #include <linux/proc_fs.h>
 #include <linux/sched/task.h>
 #include <linux/idr.h>
+#include <linux/wait.h>
 
 struct pid init_struct_pid = {
 	.count 		= ATOMIC_INIT(1),

@@ -451,6 +454,184 @@ struct pid *find_ge_pid(int nr, struct pid_namespace *ns)
 	return idr_get_next(&ns->idr, &nr);
 }
 
+#ifdef CONFIG_PROC_FS
+static struct pid_namespace *pidfd_get_proc_pid_ns(const struct file *file)
+{
+	struct inode *inode;
+	struct super_block *sb;
+
+	inode = file_inode(file);
+	sb = inode->i_sb;
+	if (sb->s_magic != PROC_SUPER_MAGIC)
+		return ERR_PTR(-EINVAL);
+
+	if (inode->i_ino != PROC_ROOT_INO)
+		return ERR_PTR(-EINVAL);
+
+	return get_pid_ns(inode->i_sb->s_fs_info);
+}
+
+static struct pid *pidfd_get_pid(const struct file *file)
+{
+	if (file->f_op != &pidfd_fops)
+		return ERR_PTR(-EINVAL);
+
+	return get_pid(file->private_data);
+}
+
+static struct file *pidfd_open_proc_pid(const struct file *procf, pid_t pid,
+					const struct pid *pidfd_pid)
+{
+	char name[12]; /* int to strlen + \0 but with */
+	struct file *file;
+	struct pid *proc_pid;
+
+	snprintf(name, sizeof(name), "%d", pid);
+	file = file_open_root(procf->f_path.dentry, procf->f_path.mnt, name,
+			      O_DIRECTORY | O_RDONLY | O_NOFOLLOW, 0);
+	if (IS_ERR(file))
+		return file;
+
+	proc_pid = tgid_pidfd_to_pid(file);
+	if (IS_ERR(proc_pid)) {
+		filp_close(file, NULL);
+		return ERR_CAST(proc_pid);
+	}
+
+	if (pidfd_pid != proc_pid) {
+		filp_close(file, NULL);
+		return ERR_PTR(-ESRCH);
+	}
+
+	return file;
+}
+
+static inline int pidfd_to_procfd(int procfd, struct file *pidfd_file)
+{
+	long fd;
+	pid_t ns_pid;
+	struct fd fdproc;
+	struct file *file = NULL;
+	struct pid *pidfd_pid = NULL;
+	struct pid_namespace *proc_pid_ns = NULL;
+
+	fdproc = fdget(procfd);
+	if (!fdproc.file)
+		return -EBADF;
+
+	proc_pid_ns = pidfd_get_proc_pid_ns(fdproc.file);
+	if (IS_ERR(proc_pid_ns)) {
+		fd = PTR_ERR(proc_pid_ns);
+		proc_pid_ns = NULL;
+		goto err;
+	}
+
+	pidfd_pid = pidfd_get_pid(pidfd_file);
+	if (IS_ERR(pidfd_pid)) {
+		fd = PTR_ERR(pidfd_pid);
+		pidfd_pid = NULL;
+		goto err;
+	}
+
+	ns_pid = pid_nr_ns(pidfd_pid, proc_pid_ns);
+	if (!ns_pid) {
+		fd = -ESRCH;
+		goto err;
+	}
+
+	file = pidfd_open_proc_pid(fdproc.file, ns_pid, pidfd_pid);
+	if (IS_ERR(file)) {
+		fd = PTR_ERR(file);
+		file = NULL;
+		goto err;
+	}
+
+	fd = get_unused_fd_flags(O_CLOEXEC);
+	if (fd < 0)
+		goto err;
+
+	fsnotify_open(file);
+	fd_install(fd, file);
+	file = NULL;
+
+err:
+	fdput(fdproc);
+	if (proc_pid_ns)
+		put_pid_ns(proc_pid_ns);
+	put_pid(pidfd_pid);
+	if (file)
+		filp_close(file, NULL);
+
+	return fd;
+}
+#else
+static inline int pidfd_to_procfd(int procfd, struct file *pidfd_file)
+{
+	return -EOPNOTSUPP;
+}
+#endif /* CONFIG_PROC_FS */
+
+static long pidfd_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
+{
+	int procfd = arg;
+
+	switch (cmd) {
+	case PIDFD_GET_PROCFD:
+		return pidfd_to_procfd(procfd, file);
+	default:
+		return -ENOTTY;
+	}
+}
+
+static int pidfd_release(struct inode *inode, struct file *file)
+{
+	struct pid *pid = file->private_data;
+
+	if (pid) {
+		file->private_data = NULL;
+		put_pid(pid);
+	}
+
+	return 0;
+}
+
+const struct file_operations pidfd_fops = {
+	.release = pidfd_release,
+	.unlocked_ioctl = pidfd_ioctl,
+};
+
+static int pidfd_create_fd_cloexec(pid_t pid)
+{
+	int fd;
+	struct pid *p;
+
+	p = find_get_pid(pid);
+	if (!p)
+		return -ESRCH;
+
+	fd = anon_inode_getfd("pidfd", &pidfd_fops, p, O_RDWR | O_CLOEXEC);
+	if (fd < 0)
+		put_pid(p);
+
+	return fd;
+}
+
+/*
+ * pidfd_open - open a pidfd
+ * @pid:    pid for which to retrieve a pidfd
+ * @flags:  flags to pass
+ */
+SYSCALL_DEFINE2(pidfd_open, pid_t, pid, unsigned int, flags)
+{
+	if (flags)
+		return -EINVAL;
+
+	if (pid <= 0)
+		return -EINVAL;
+
+	return pidfd_create_fd_cloexec(pid);
+}
+
 void __init pid_idr_init(void)
 {
 	/* Verify no one has done anything silly: */