Here's another refactoring to properly reference count a 'struct file *'
just after calling fd_getfile().

Instead of incrementing `f_count' by hand, give the current reference
to finishdup() like it is done in other places in the same file.

Ok?

Index: kern/kern_descrip.c
===================================================================
RCS file: /cvs/src/sys/kern/kern_descrip.c,v
retrieving revision 1.144
diff -u -p -r1.144 kern_descrip.c
--- kern/kern_descrip.c 3 Apr 2018 09:00:03 -0000       1.144
+++ kern/kern_descrip.c 3 Apr 2018 13:30:47 -0000
@@ -1274,6 +1274,8 @@ dupfdopen(struct proc *p, int indx, int 
        struct filedesc *fdp = p->p_fd;
        int dupfd = p->p_dupfd;
        struct file *wfp;
+       int error, flags;
+       register_t dummy;
 
        fdpassertlocked(fdp);
 
@@ -1297,24 +1299,26 @@ dupfdopen(struct proc *p, int indx, int 
         */
        if ((wfp = fd_getfile(fdp, dupfd)) == NULL)
                return (EBADF);
+       FREF(wfp);
 
        /*
         * Check that the mode the file is being opened for is a
         * subset of the mode of the existing descriptor.
         */
-       if (((mode & (FREAD|FWRITE)) | wfp->f_flag) != wfp->f_flag)
+       if (((mode & (FREAD|FWRITE)) | wfp->f_flag) != wfp->f_flag) {
+               FRELE(wfp, p);
                return (EACCES);
-       if (wfp->f_count == LONG_MAX-2)
-               return (EDEADLK);
+       }
 
-       fdp->fd_ofiles[indx] = wfp;
-       fdp->fd_ofileflags[indx] = (fdp->fd_ofileflags[indx] & UF_EXCLOSE) |
-           (fdp->fd_ofileflags[dupfd] & ~UF_EXCLOSE);
+       flags = fdp->fd_ofileflags[indx] & UF_EXCLOSE;
        if (ISSET(p->p_p->ps_flags, PS_PLEDGE))
-               fdp->fd_ofileflags[indx] |= UF_PLEDGED;
-       wfp->f_count++;
-       fd_used(fdp, indx);
-       return (0);
+               flags |= UF_PLEDGED;
+
+       /* finishdup() does FRELE */
+       error = finishdup(p, wfp, dupfd, indx, &dummy, 1);
+       if (error == 0)
+               fdp->fd_ofileflags[indx] |= flags;
+       return (error);
 }
 
 /*

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