Add support for boot environment data to be stored in the label

Modern bootloaders leverage data stored in the root filesystem to 
enable some of their powerful features. GRUB specifically has a grubenv 
file which can store large amounts of configuration data that can be 
read and written at boot time and during normal operation. This allows 
sysadmins to configure useful features like automated failover after 
failed boot attempts. Unfortunately, due to the Copy-on-Write nature 
of ZFS, the standard behavior of these tools cannot handle writing to
ZFS files safely at boot time. We need an alternative way to store 
data that allows the bootloader to make changes to the data.

This work is very similar to work that was done on Illumos to enable 
similar functionality in the FreeBSD bootloader. This patch is different 
in that the data being stored is a raw grubenv file; this file can store 
arbitrary variables and values, and the scripting provided by grub is 
powerful enough that special structures are not required to implement 
advanced behavior.

We repurpose the second padding area in each label to store the grubenv 
file, protected by an embedded checksum. We add two ioctls to get and 
set this data, and libzfs_core and libzfs functions to access them more 
easily. There are no direct command line interfaces to these functions; 
these will be added directly to the bootloader utilities.

Reviewed-by: Pavel Zakharov <[email protected]>
Reviewed-by: Matthew Ahrens <[email protected]>
Reviewed-by: Brian Behlendorf <[email protected]>
Signed-off-by: Paul Dagnelie <[email protected]>
Closes #10009 

2 files changed, 60 insertions, 2 deletions

diff --git a/lib/libzfs/libzfs_pool.c b/lib/libzfs/libzfs_pool.c
index 9122b3ee1..2b21787ee 100644
--- a/lib/libzfs/libzfs_pool.c
+++ b/lib/libzfs/libzfs_pool.c
@@ -22,7 +22,7 @@
 /*
  * Copyright 2015 Nexenta Systems, Inc.  All rights reserved.
  * Copyright (c) 2005, 2010, Oracle and/or its affiliates. All rights reserved.
- * Copyright (c) 2011, 2018 by Delphix. All rights reserved.
+ * Copyright (c) 2011, 2020 by Delphix. All rights reserved.
  * Copyright 2016 Igor Kozhukhov <[email protected]>
  * Copyright (c) 2018 Datto Inc.
  * Copyright (c) 2017 Open-E, Inc. All Rights Reserved.
@@ -429,7 +429,7 @@ zpool_get_prop(zpool_handle_t *zhp, zpool_prop_t prop, char *buf,
  * Assuming bootfs is a valid dataset name.
  */
 static boolean_t
-bootfs_name_valid(const char *pool, char *bootfs)
+bootfs_name_valid(const char *pool, const char *bootfs)
 {
 	int len = strlen(pool);
 	if (bootfs[0] == '\0')
@@ -4459,3 +4459,39 @@ zpool_wait_status(zpool_handle_t *zhp, zpool_wait_activity_t activity,
 
 	return (error);
 }
+
+int
+zpool_set_bootenv(zpool_handle_t *zhp, const char *envmap)
+{
+	int error = lzc_set_bootenv(zhp->zpool_name, envmap);
+	if (error != 0) {
+		(void) zpool_standard_error_fmt(zhp->zpool_hdl, error,
+		    dgettext(TEXT_DOMAIN,
+		    "error setting bootenv in pool '%s'"), zhp->zpool_name);
+	}
+
+	return (error);
+}
+
+int
+zpool_get_bootenv(zpool_handle_t *zhp, char *outbuf, size_t size, off_t offset)
+{
+	nvlist_t *nvl = NULL;
+	int error = lzc_get_bootenv(zhp->zpool_name, &nvl);
+	if (error != 0) {
+		(void) zpool_standard_error_fmt(zhp->zpool_hdl, error,
+		    dgettext(TEXT_DOMAIN,
+		    "error getting bootenv in pool '%s'"), zhp->zpool_name);
+		return (-1);
+	}
+	char *envmap = fnvlist_lookup_string(nvl, "envmap");
+	if (offset >= strlen(envmap)) {
+		fnvlist_free(nvl);
+		return (0);
+	}
+
+	strlcpy(outbuf, envmap + offset, size);
+	int bytes = MIN(strlen(envmap + offset), size);
+	fnvlist_free(nvl);
+	return (bytes);
+}
diff --git a/lib/libzfs_core/libzfs_core.c b/lib/libzfs_core/libzfs_core.c
index 4e83b624b..22996fc9b 100644
--- a/lib/libzfs_core/libzfs_core.c
+++ b/lib/libzfs_core/libzfs_core.c
@@ -1619,3 +1619,25 @@ lzc_wait_fs(const char *fs, zfs_wait_activity_t activity, boolean_t *waited)
 
 	return (error);
 }
+
+/*
+ * Set the bootenv contents for the given pool.
+ */
+int
+lzc_set_bootenv(const char *pool, const char *env)
+{
+	nvlist_t *args = fnvlist_alloc();
+	fnvlist_add_string(args, "envmap", env);
+	int error = lzc_ioctl(ZFS_IOC_SET_BOOTENV, pool, args, NULL);
+	fnvlist_free(args);
+	return (error);
+}
+
+/*
+ * Get the contents of the bootenv of the given pool.
+ */
+int
+lzc_get_bootenv(const char *pool, nvlist_t **outnvl)
+{
+	return (lzc_ioctl(ZFS_IOC_GET_BOOTENV, pool, NULL, outnvl));
+}