kern: dup: do not assume oldfde is valid

oldfde may be invalidated if the table has grown due to the operation that
we're performing, either via fdalloc() or a direct fdgrowtable_exp().

This was technically OK before rS367927 because the old table remained valid
until the filedesc became unused, but now it may be freed immediately if
it's an unshared table in a single-threaded process, so it is no longer a
good assumption to make.

This fixes dup/dup2 invocations that grow the file table; in the initial
report, it manifested as a kernel panic in devel/gmake's configure script.

Reported by:	Guy Yur <guyyur gmail com>
Reviewed by:	rew
Differential Revision:	https://reviews.freebsd.org/D27319

sys/kern/kern_descrip.c

@@ -870,7 +870,7 @@ kern_dup(struct thread *td, u_int mode, int flags, int old, int new)
 	struct filedesc *fdp;
 	struct filedescent *oldfde, *newfde;
 	struct proc *p;
-	struct file *delfp;
+	struct file *delfp, *oldfp;
 	u_long *oioctls, *nioctls;
 	int error, maxfd;
 
@@ -910,7 +910,8 @@ kern_dup(struct thread *td, u_int mode, int flags, int old, int new)
 	}
 
 	oldfde = &fdp->fd_ofiles[old];
-	if (!fhold(oldfde->fde_file))
+	oldfp = oldfde->fde_file;
+	if (!fhold(oldfp))
 		goto unlock;
 
 	/*
@@ -922,14 +923,14 @@ kern_dup(struct thread *td, u_int mode, int flags, int old, int new)
 	case FDDUP_NORMAL:
 	case FDDUP_FCNTL:
 		if ((error = fdalloc(td, new, &new)) != 0) {
-			fdrop(oldfde->fde_file, td);
+			fdrop(oldfp, td);
 			goto unlock;
 		}
 		break;
 	case FDDUP_MUSTREPLACE:
 		/* Target file descriptor must exist. */
 		if (fget_locked(fdp, new) == NULL) {
-			fdrop(oldfde->fde_file, td);
+			fdrop(oldfp, td);
 			goto unlock;
 		}
 		break;
@@ -948,7 +949,7 @@ kern_dup(struct thread *td, u_int mode, int flags, int old, int new)
 				error = racct_set_unlocked(p, RACCT_NOFILE, new + 1);
 				if (error != 0) {
 					error = EMFILE;
-					fdrop(oldfde->fde_file, td);
+					fdrop(oldfp, td);
 					goto unlock;
 				}
 			}
@@ -964,6 +965,12 @@ kern_dup(struct thread *td, u_int mode, int flags, int old, int new)
 
 	KASSERT(old != new, ("new fd is same as old"));
 
+	/* Refetch oldfde because the table may have grown and old one freed. */
+	oldfde = &fdp->fd_ofiles[old];
+	KASSERT(oldfp == oldfde->fde_file,
+	    ("fdt_ofiles shift from growth observed at fd %d",
+	    old));
+
 	newfde = &fdp->fd_ofiles[new];
 	delfp = newfde->fde_file;