[RFC v3 13/21] PKRAM: prevent inadvertent use of a stale superblock

[Anthony Yznaga <anthony.yznaga@oracle.com>]

When pages have been saved to be preserved by the current boot, set
a magic number on the super block to be validated by the next kernel.

Signed-off-by: Anthony Yznaga <anthony.yznaga@oracle.com>
---
 mm/pkram.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/mm/pkram.c b/mm/pkram.c
index c63b27bb711b..befdffc76940 100644
--- a/mm/pkram.c
+++ b/mm/pkram.c
@@ -22,6 +22,7 @@
 
 #include "internal.h"
 
+#define PKRAM_MAGIC		0x706B726D
 
 /*
  * Represents a reference to a data page saved to PKRAM.
@@ -110,6 +111,8 @@ struct pkram_region_list {
  * The structure occupies a memory page.
  */
 struct pkram_super_block {
+	__u32	magic;
+
 	__u64	node_pfn;		/* first element of the node list */
 	__u64	region_list_pfn;
 	__u64	nr_regions;
@@ -179,6 +182,11 @@ void __init pkram_reserve(void)
 		err = PTR_ERR(pkram_sb);
 		goto out;
 	}
+	if (pkram_sb->magic != PKRAM_MAGIC) {
+		pr_err("PKRAM: invalid super block\n");
+		err = -EINVAL;
+		goto out;
+	}
 	/* An empty pkram_sb is not an error */
 	if (!pkram_sb->node_pfn) {
 		pkram_sb = NULL;
@@ -1012,6 +1020,7 @@ static void __pkram_reboot(void)
 	 */
 	memset(pkram_sb, 0, PAGE_SIZE);
 	if (!err && node_pfn) {
+		pkram_sb->magic = PKRAM_MAGIC;
 		pkram_sb->node_pfn = node_pfn;
 		pkram_sb->region_list_pfn = rl_pfn;
 		pkram_sb->nr_regions = nr_regions;
-- 
1.9.4