[PATCH] erofs: micro-optimize superblock checksum

[Gao Xiang]

Just verify the remaining unknown on-disk data instead of allocating a
temporary buffer for the whole superblock and zeroing out the checksum
field since .magic(EROFS_SUPER_MAGIC_V1) is verified and .checksum(0)
is fixed.

Signed-off-by: Gao Xiang <hsiangkao at linux.alibaba.com>
---
 fs/erofs/erofs_fs.h |  3 ++-
 fs/erofs/super.c    | 31 ++++++++++++-------------------
 2 files changed, 14 insertions(+), 20 deletions(-)

diff --git a/fs/erofs/erofs_fs.h b/fs/erofs/erofs_fs.h
index c8f2ae845bd2..199395ed1c1f 100644
--- a/fs/erofs/erofs_fs.h
+++ b/fs/erofs/erofs_fs.h
@@ -9,6 +9,7 @@
 #ifndef __EROFS_FS_H
 #define __EROFS_FS_H
 
+/* to allow for x86 boot sectors and other oddities. */
 #define EROFS_SUPER_OFFSET      1024
 
 #define EROFS_FEATURE_COMPAT_SB_CHKSUM          0x00000001
@@ -54,7 +55,7 @@ struct erofs_deviceslot {
 /* erofs on-disk super block (currently 128 bytes) */
 struct erofs_super_block {
 	__le32 magic;           /* file system magic number */
-	__le32 checksum;        /* crc32c(super_block) */
+	__le32 checksum;        /* crc32c to avoid unexpected on-disk overlap */
 	__le32 feature_compat;
 	__u8 blkszbits;         /* filesystem block size in bit shift */
 	__u8 sb_extslots;	/* superblock size = 128 + sb_extslots * 16 */
diff --git a/fs/erofs/super.c b/fs/erofs/super.c
index 9166054370aa..6cc0c5ea5ff5 100644
--- a/fs/erofs/super.c
+++ b/fs/erofs/super.c
@@ -6,6 +6,7 @@
  */
 #include <linux/statfs.h>
 #include <linux/seq_file.h>
+#include <linux/crc32.h>
 #include <linux/crc32c.h>
 #include <linux/fs_context.h>
 #include <linux/fs_parser.h>
@@ -39,29 +40,21 @@ void _erofs_printk(struct super_block *sb, const char *fmt, ...)
 
 static int erofs_superblock_csum_verify(struct super_block *sb, void *sbdata)
 {
-	size_t len = 1 << EROFS_SB(sb)->blkszbits;
-	struct erofs_super_block *dsb;
-	u32 expected_crc, crc;
+	struct erofs_super_block *dsb = sbdata + EROFS_SUPER_OFFSET;
+	u32 len = 1 << EROFS_SB(sb)->blkszbits, crc;
 
 	if (len > EROFS_SUPER_OFFSET)
 		len -= EROFS_SUPER_OFFSET;
+	len -= offsetof(struct erofs_super_block, checksum) +
+			sizeof(dsb->checksum);
 
-	dsb = kmemdup(sbdata + EROFS_SUPER_OFFSET, len, GFP_KERNEL);
-	if (!dsb)
-		return -ENOMEM;
-
-	expected_crc = le32_to_cpu(dsb->checksum);
-	dsb->checksum = 0;
-	/* to allow for x86 boot sectors and other oddities. */
-	crc = crc32c(~0, dsb, len);
-	kfree(dsb);
-
-	if (crc != expected_crc) {
-		erofs_err(sb, "invalid checksum 0x%08x, 0x%08x expected",
-			  crc, expected_crc);
-		return -EBADMSG;
-	}
-	return 0;
+	/* skip .magic(pre-verified) and .checksum(0) fields */
+	crc = crc32c(0x5045B54A, (&dsb->checksum) + 1, len);
+	if (crc == le32_to_cpu(dsb->checksum))
+		return 0;
+	erofs_err(sb, "invalid checksum 0x%08x, 0x%08x expected",
+		  crc, le32_to_cpu(dsb->checksum));
+	return -EBADMSG;
 }
 
 static void erofs_inode_init_once(void *ptr)
-- 
2.43.5