Added error for writing to /dev/ on Linux

Starting in Linux 5.10, trying to write to /dev/{null,zero} 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 +727,38 @@ finish_progress(char *done)
 	pt_header = NULL;
 }
 
+/* This function checks if the passed fd refers to /dev/null or /dev/zero */
+#ifdef __linux__
+static int
+is_dev_files(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);
+	if (res != 1) {
+		nfd = open("/dev/zero", O_WRONLY);
+		fstat(nfd, &nst);
+		res = (st.st_dev == nst.st_dev);
+		close(nfd);
+	}
+	return (res);
+}
+#endif
+
+static int
+is_dev_error(int err, int fd)
+{
+#ifdef __linux__
+	if (err == EINVAL && is_dev_files(fd)) {
+		return (1);
+	}
+#endif
+	return (0);
+}
+
 static int
 zfs_mount_and_share(libzfs_handle_t *hdl, const char *dataset, zfs_type_t type)
 {
@@ -4571,11 +4603,23 @@ zfs_do_send(int argc, char **argv)
 
 		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/{null,zero} 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/{null,zero} files.\n"));
+		}
+		return (err);
 	}
 
 	if (flags.skipmissing && !flags.replicate) {
@@ -4626,6 +4670,11 @@ 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/{null,zero} files.\n"));
+		}
 		return (err != 0);
 	}
 
@@ -4702,6 +4751,11 @@ zfs_do_send(int argc, char **argv)
 		nvlist_free(dbgnv);
 	}
 	zfs_close(zhp);
+	if (is_dev_error(errno, STDOUT_FILENO)) {
+		(void) fprintf(stderr,
+		    gettext("Error: Stream cannot be written to "
+		    "/dev/{null,zero} files.\n"));
+	}
 
 	return (err != 0);
 }

lib/libzfs/libzfs_sendrecv.c

@@ -849,6 +849,7 @@ dump_ioctl(zfs_handle_t *zhp, const char *fromsnap, uint64_t fromsnap_obj,
 		case ERANGE:
 		case EFAULT:
 		case EROFS:
+		case EINVAL:
 			zfs_error_aux(hdl, strerror(errno));
 			return (zfs_error(hdl, EZFS_BADBACKUP, errbuf));