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

[Pratik Shinde <pratikshinde320@gmail.com>]

Patch for kernel side changes of checksum feature.I 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 |  2 +-
 fs/erofs/super.c    | 50 ++++++++++++++++++++++++++++++++++++++++++++++----
 3 files changed, 50 insertions(+), 7 deletions(-)

diff --git a/fs/erofs/erofs_fs.h b/fs/erofs/erofs_fs.h
index b1ee565..bab5506 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_SB_CHKSUM 0x0001
 
 /* 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..cd3af45 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 features;
 	unsigned int mount_opt;
 };
 
diff --git a/fs/erofs/super.c b/fs/erofs/super.c
index 0e36949..94e1d6a 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 +47,45 @@ void _erofs_info(struct super_block *sb, const char *function,
 	va_end(args);
 }
 
+static int erofs_validate_sb_chksum(struct erofs_super_block *dsb,
+				       struct super_block *sb)
+{
+	u32 disk_chksum = le32_to_cpu(dsb->checksum);
+	u32 nblocks = le32_to_cpu(dsb->chksum_blocks);
+	u32 crc;
+	struct erofs_super_block *dsb2;
+	char *buf;
+	unsigned int off = 0;
+	void *kaddr;
+	struct page *page;
+	int i, ret = -EINVAL;
+
+	buf = kmalloc(nblocks * EROFS_BLKSIZ, GFP_KERNEL);
+	if (!buf)
+		goto out;
+	for (i = 0; i < nblocks; i++) {
+		page = erofs_get_meta_page(sb, i);
+		if (IS_ERR(page))
+			goto out;
+		kaddr = kmap_atomic(page);
+		(void) memcpy(buf + off, kaddr, EROFS_BLKSIZ);
+		kunmap_atomic(kaddr);
+		unlock_page(page);
+		/* first page will be released by erofs_read_superblock */
+		if (i != 0)
+			put_page(page);
+		off += EROFS_BLKSIZ;
+	}
+	dsb2 = (struct erofs_super_block *)(buf + EROFS_SUPER_OFFSET);
+	dsb2->checksum = 0;
+	crc = crc32c(0, buf, nblocks * EROFS_BLKSIZ);
+	if (crc != disk_chksum)
+		goto out;
+	ret = 0;
+out:	kfree(buf);
+	return ret;
+}
+
 static void erofs_inode_init_once(void *ptr)
 {
 	struct erofs_inode *vi = ptr;
@@ -109,18 +149,20 @@ static int erofs_read_superblock(struct super_block *sb)
 		erofs_err(sb, "cannot read erofs superblock");
 		return PTR_ERR(page);
 	}
-
 	sbi = EROFS_SB(sb);
-
 	data = kmap_atomic(page);
 	dsb = (struct erofs_super_block *)(data + EROFS_SUPER_OFFSET);
-
 	ret = -EINVAL;
 	if (le32_to_cpu(dsb->magic) != EROFS_SUPER_MAGIC_V1) {
 		erofs_err(sb, "cannot find valid erofs superblock");
 		goto out;
 	}
-
+	if (dsb->feature_compat & EROFS_FEATURE_SB_CHKSUM) {
+		if (erofs_validate_sb_chksum(dsb, sb)) {
+			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