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.

Reviewed-by: Antonio Russo <aerusso@aerusso.net>
Reviewed-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: John Kennedy <john.kennedy@delphix.com>
Signed-off-by: Rich Ercolani <rincebrain@gmail.com>
Closes:  #11991

cmd/zfs/zfs_main.c

@@ -730,6 +730,32 @@ finish_progress(char *done)
 	pt_header = NULL;
 }
 
+/* This function checks if the passed fd refers to /dev/null or /dev/zero */
+#ifdef __linux__
+static boolean_t
+is_dev_nullzero(int fd)
+{
+	struct stat st;
+	fstat(fd, &st);
+	return (major(st.st_rdev) == 1 && (minor(st.st_rdev) == 3 /* null */ ||
+	    minor(st.st_rdev) == 5 /* zero */));
+}
+#endif
+
+static void
+note_dev_error(int err, int fd)
+{
+#ifdef __linux__
+	if (err == EINVAL && is_dev_nullzero(fd)) {
+		(void) fprintf(stderr,
+		    gettext("Error: Writing directly to /dev/{null,zero} files"
+		    " on certain kernels is not currently implemented.\n"
+		    "(As a workaround, "
+		    "try \"zfs send [...] | cat > /dev/null\")\n"));
+	}
+#endif
+}
+
 static int
 zfs_mount_and_share(libzfs_handle_t *hdl, const char *dataset, zfs_type_t type)
 {
@@ -4448,11 +4474,16 @@ zfs_do_send(int argc, char **argv)
 
 		err = zfs_send_saved(zhp, &flags, STDOUT_FILENO,
 		    resume_token);
+		if (err != 0)
+			note_dev_error(errno, STDOUT_FILENO);
 		zfs_close(zhp);
 		return (err != 0);
 	} else if (resume_token != NULL) {
-		return (zfs_send_resume(g_zfs, &flags, STDOUT_FILENO,
+		err = zfs_send_resume(g_zfs, &flags, STDOUT_FILENO,
-		    resume_token));
+		    resume_token);
+		if (err != 0)
+			note_dev_error(errno, STDOUT_FILENO);
+		return (err);
 	}
 
 	/*
@@ -4496,6 +4527,8 @@ zfs_do_send(int argc, char **argv)
 		err = zfs_send_one(zhp, fromname, STDOUT_FILENO, &flags,
 		    redactbook);
 		zfs_close(zhp);
+		if (err != 0)
+			note_dev_error(errno, STDOUT_FILENO);
 		return (err != 0);
 	}
 
@@ -4572,6 +4605,7 @@ zfs_do_send(int argc, char **argv)
 		nvlist_free(dbgnv);
 	}
 	zfs_close(zhp);
+	note_dev_error(errno, STDOUT_FILENO);
 
 	return (err != 0);
 }

lib/libzfs/libzfs_sendrecv.c

@@ -847,7 +847,8 @@ dump_ioctl(zfs_handle_t *zhp, const char *fromsnap, uint64_t fromsnap_obj,
 		case ERANGE:
 		case EFAULT:
 		case EROFS:
-			zfs_error_aux(hdl, strerror(errno));
+		case EINVAL:
+			zfs_error_aux(hdl, "%s", strerror(errno));
 			return (zfs_error(hdl, EZFS_BADBACKUP, errbuf));
 
 		default: