[PATCH v2 2/2] udf: support 2048-byte spacing of VRS descriptors on 4K media

["Steven J. Magnani" <steve.magnani@digidescorp.com>]

Some UDF creators (specifically Microsoft, but perhaps others) mishandle
the ECMA-167 corner case that requires descriptors within a Volume
Recognition Sequence to be placed at 4096-byte intervals on media where
the block size is 4K. Instead, the descriptors are placed at the 2048-
byte interval mandated for media with smaller blocks. This nonconformity
currently prevents Linux from recognizing the filesystem as UDF.

Modify the driver to tolerate a misformatted VRS on 4K media.

Signed-off-by: Steven J. Magnani <steve@digidescorp.com>

--- a/fs/udf/super.c	2019-07-10 20:55:33.334359446 -0500
+++ b/fs/udf/super.c	2019-07-10 21:20:58.138382326 -0500
@@ -741,6 +741,7 @@ static int udf_check_vsd(struct super_bl
 	int sectorsize;
 	struct buffer_head *bh = NULL;
 	int nsr = 0;
+	int quirk_nsr = 0;
 	struct udf_sb_info *sbi;
 
 	sbi = UDF_SB(sb);
@@ -780,11 +781,27 @@ static int udf_check_vsd(struct super_bl
 		if (vsd_id > nsr)
 			nsr = vsd_id;
 
+		/* Special handling for improperly formatted VRS (e.g., Win10)
+		 * where components are separated by 2048 bytes
+		 * even though sectors are 4K
+		 */
+		if ((sb->s_blocksize == 4096) && (quirk_nsr < 2)) {
+			vsd_id = identify_vsd(vsd + 1);
+			if ((nsr == 1) || (quirk_nsr == 1)) {
+				/* BEA01 has been seen, allow quirk NSR */
+				if (vsd_id > quirk_nsr)
+					quirk_nsr = vsd_id;
+			} else if (vsd_id > 3)
+				quirk_nsr = vsd_id;  /* 0 -> 255 */
+		}
+
 		brelse(bh);
 	}
 
 	if ((nsr >= 2) && (nsr <= 3))
 		return nsr;
+	else if ((quirk_nsr >= 2) && (quirk_nsr <= 3))
+		return quirk_nsr;
 	else if (!bh && sector - (sbi->s_session << sb->s_blocksize_bits) ==
 			VSD_FIRST_SECTOR_OFFSET)
 		return -1;