Add fget_remote()

The function holds and returns struct file for a file descriptor index
in the given process.

Reviewed by:	brooks, markj
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week
Differential revision:	https://reviews.freebsd.org/D43518

sys/kern/kern_descrip.c

@@ -2960,6 +2960,38 @@ fget_cap(struct thread *td, int fd, cap_rights_t *needrightsp,
 }
 #endif
 
+int
+fget_remote(struct thread *td, struct proc *p, int fd, struct file **fpp)
+{
+	struct filedesc *fdp;
+	struct file *fp;
+	int error;
+
+	if (p == td->td_proc)	/* curproc */
+		return (fget_unlocked(td, fd, &cap_no_rights, fpp));
+
+	PROC_LOCK(p);
+	fdp = fdhold(p);
+	PROC_UNLOCK(p);
+	if (fdp == NULL)
+		return (ENOENT);
+	FILEDESC_SLOCK(fdp);
+	if (refcount_load(&fdp->fd_refcnt) != 0) {
+		fp = fget_noref(fdp, fd);
+		if (fp != NULL && fhold(fp)) {
+			*fpp = fp;
+			error = 0;
+		} else {
+			error = EBADF;
+		}
+	} else {
+		error = ENOENT;
+	}
+	FILEDESC_SUNLOCK(fdp);
+	fddrop(fdp);
+	return (error);
+}
+
 #ifdef CAPABILITIES
 int
 fgetvp_lookup_smr(struct nameidata *ndp, struct vnode **vpp, bool *fsearch)

sys/sys/file.h

@@ -261,6 +261,7 @@ int fget_write(struct thread *td, int fd, cap_rights_t *rightsp,
 int fget_fcntl(struct thread *td, int fd, cap_rights_t *rightsp,
     int needfcntl, struct file **fpp);
 int _fdrop(struct file *fp, struct thread *td);
+int fget_remote(struct thread *td, struct proc *p, int fd, struct file **fpp);
 
 fo_rdwr_t	invfo_rdwr;
 fo_truncate_t	invfo_truncate;