[PATCH-v3] erofs: code for verifying superblock checksum of an erofs image.

[Pratik Shinde <pratikshinde320@gmail.com>]

Patch for kernel side changes of checksum feature.Used kernel's
crc32c library for calculating the checksum.

Signed-off-by: Pratik Shinde <pratikshinde320@gmail.com>
---
 fs/erofs/erofs_fs.h |  5 +++--
 fs/erofs/internal.h |  3 ++-
 fs/erofs/super.c    | 44 ++++++++++++++++++++++++++++++++++++++++++++
 3 files changed, 49 insertions(+), 3 deletions(-)

diff --git a/fs/erofs/erofs_fs.h b/fs/erofs/erofs_fs.h
index b1ee565..4d8097a 100644
--- a/fs/erofs/erofs_fs.h
+++ b/fs/erofs/erofs_fs.h
@@ -17,6 +17,7 @@
  */
 #define EROFS_FEATURE_INCOMPAT_LZ4_0PADDING	0x00000001
 #define EROFS_ALL_FEATURE_INCOMPAT		EROFS_FEATURE_INCOMPAT_LZ4_0PADDING
+#define EROFS_FEATURE_COMPAT_SB_CHKSUM 		0x00000001
 
 /* 128-byte erofs on-disk super block */
 struct erofs_super_block {
@@ -37,8 +38,8 @@ struct erofs_super_block {
 	__u8 uuid[16];          /* 128-bit uuid for volume */
 	__u8 volume_name[16];   /* volume name */
 	__le32 feature_incompat;
-
-	__u8 reserved2[44];
+	__le32 chksum_blocks;	/* number of blocks used for checksum */
+	__u8 reserved2[40];
 };
 
 /*
diff --git a/fs/erofs/internal.h b/fs/erofs/internal.h
index 544a453..cec27ca 100644
--- a/fs/erofs/internal.h
+++ b/fs/erofs/internal.h
@@ -86,7 +86,7 @@ struct erofs_sb_info {
 	u8 uuid[16];                    /* 128-bit uuid for volume */
 	u8 volume_name[16];             /* volume name */
 	u32 feature_incompat;
-
+	u32 feature_compat;
 	unsigned int mount_opt;
 };
 
@@ -426,6 +426,7 @@ static inline void z_erofs_exit_zip_subsystem(void) {}
 #endif	/* !CONFIG_EROFS_FS_ZIP */
 
 #define EFSCORRUPTED    EUCLEAN         /* Filesystem is corrupted */
+#define EFSBADCRC	EBADMSG		/* Bad crc found */
 
 #endif	/* __EROFS_INTERNAL_H */
 
diff --git a/fs/erofs/super.c b/fs/erofs/super.c
index 0e36949..a2a638a 100644
--- a/fs/erofs/super.c
+++ b/fs/erofs/super.c
@@ -9,6 +9,7 @@
 #include <linux/statfs.h>
 #include <linux/parser.h>
 #include <linux/seq_file.h>
+#include <linux/crc32c.h>
 #include "xattr.h"
 
 #define CREATE_TRACE_POINTS
@@ -46,6 +47,42 @@ void _erofs_info(struct super_block *sb, const char *function,
 	va_end(args);
 }
 
+static int erofs_validate_sb_chksum(void  *sbdata, struct super_block *sb)
+{
+	u32 disk_chksum, nblocks, crc = 0;
+	void *kaddr;
+	struct page *page;
+	struct erofs_super_block *dsb;
+	char *buf;
+	int i, ret = 0;
+
+	buf = kmalloc(EROFS_BLKSIZ, GFP_KERNEL);
+	if (!buf)
+		return -ENOMEM;
+	memcpy(buf, sbdata, EROFS_BLKSIZ);
+	dsb = (struct erofs_super_block *)(buf + EROFS_SUPER_OFFSET);
+	disk_chksum = le32_to_cpu(dsb->checksum);
+	nblocks = le32_to_cpu(dsb->chksum_blocks);
+	dsb->checksum = 0;
+	crc = crc32c(0, buf, EROFS_BLKSIZ);
+	for (i = 1; i < nblocks; i++) {
+		page = erofs_get_meta_page(sb, i);
+		if (IS_ERR(page)) {
+			ret = PTR_ERR(page);
+			goto out;
+		}
+		kaddr = kmap_atomic(page);
+		crc = crc32c(crc, kaddr, EROFS_BLKSIZ);
+		kunmap_atomic(kaddr);
+		unlock_page(page);
+		put_page(page);
+	}
+	if (crc != disk_chksum)
+		ret = -EFSBADCRC;
+out:	kfree(buf);
+	return ret;
+}
+
 static void erofs_inode_init_once(void *ptr)
 {
 	struct erofs_inode *vi = ptr;
@@ -121,6 +158,13 @@ static int erofs_read_superblock(struct super_block *sb)
 		goto out;
 	}
 
+	if (dsb->feature_compat & EROFS_FEATURE_COMPAT_SB_CHKSUM) {
+		ret = erofs_validate_sb_chksum(data, sb);
+		if (ret < 0) {
+			erofs_err(sb, "super block checksum incorrect");
+			goto out;
+		}
+	}
 	blkszbits = dsb->blkszbits;
 	/* 9(512 bytes) + LOG_SECTORS_PER_BLOCK == LOG_BLOCK_SIZE */
 	if (blkszbits != LOG_BLOCK_SIZE) {
-- 
2.9.3