Added error for writing to /dev/ on Linux.

Starting in Linux 5.10, trying to write to /dev/ files errors out.
Prefer to inform people when this happens rather than hoping they
guess what's wrong.

Signed-off-by: Rich Ercolani <[email protected]>

cmd/zfs/zfs_main.c

@@ -727,6 +727,29 @@ finish_progress(char *done)
 	pt_header = NULL;
 }
 
+static int
+is_dev_fs(int fd)
+{
+	int nfd = open("/dev/null", O_WRONLY);
+	struct stat st, nst;
+	fstat(fd, &st);
+	fstat(nfd, &nst);
+	int res = (st.st_dev == nst.st_dev);
+	close(nfd);
+	return (res);
+}
+
+static int
+is_dev_error(int err, int fd)
+{
+#ifdef __linux__
+	if (err == EINVAL && is_dev_fs(fd)) {
+		return (1);
+	}
+#endif
+	return (0);
+}
+
 static int
 zfs_mount_and_share(libzfs_handle_t *hdl, const char *dataset, zfs_type_t type)
 {
@@ -4563,19 +4586,30 @@ zfs_do_send(int argc, char **argv)
 		    "You must redirect standard output.\n"));
 		return (1);
 	}
-
 	if (flags.saved) {
 		zhp = zfs_open(g_zfs, argv[0], ZFS_TYPE_DATASET);
 		if (zhp == NULL)
 			return (1);
 
 		err = zfs_send_saved(zhp, &flags, STDOUT_FILENO,
 		    resume_token);
+		if (err != 0 && is_dev_error(errno, STDOUT_FILENO)) {
+			(void) fprintf(stderr,
+			    gettext("Error: Stream cannot be written to /dev/"
+			    " files.\n"));
+
+		}
 		zfs_close(zhp);
 		return (err != 0);
 	} else if (resume_token != NULL) {
-		return (zfs_send_resume(g_zfs, &flags, STDOUT_FILENO,
-		    resume_token));
+		err = zfs_send_resume(g_zfs, &flags, STDOUT_FILENO,
+		    resume_token);
+		if (err != 0 && is_dev_error(errno, STDOUT_FILENO)) {
+			(void) fprintf(stderr,
+			    gettext("Error: Stream cannot be written to /dev/"
+			    " files.\n"));
+		}
+		return (err);
 	}
 
 	if (flags.skipmissing && !flags.replicate) {
@@ -4626,6 +4660,11 @@ zfs_do_send(int argc, char **argv)
 		err = zfs_send_one(zhp, fromname, STDOUT_FILENO, &flags,
 		    redactbook);
 		zfs_close(zhp);
+		if (err != 0 && is_dev_error(errno, STDOUT_FILENO)) {
+			(void) fprintf(stderr,
+			    gettext("Error: Stream cannot be written to /dev/"
+			    " files.\n"));
+		}
 		return (err != 0);
 	}
 
@@ -4702,6 +4741,11 @@ zfs_do_send(int argc, char **argv)
 		nvlist_free(dbgnv);
 	}
 	zfs_close(zhp);
+	if (err != 0 && is_dev_error(errno, STDOUT_FILENO)) {
+		(void) fprintf(stderr,
+		    gettext("Error: Stream cannot be written to /dev/ "
+		    "files.\n"));
+	}
 
 	return (err != 0);
 }