svn commit: r336158 - stable/11/sbin/fsck_msdosfs

[Xin LI]

Author: delphij
Date: Tue Jul 10 02:46:31 2018
New Revision: 336158
URL: https://svnweb.freebsd.org/changeset/base/336158

Log:
  MFC r335655:
  
  Fix division by zero when reading boot block by postponing division
  until it is necessary and after we validated bytes per sector is non-
  zero.
  
  Obtained from:	Android https://android-review.googlesource.com/c/platform/external/fsck_msdos/+/36362

Modified:
  stable/11/sbin/fsck_msdosfs/boot.c
Directory Properties:
  stable/11/   (props changed)

Modified: stable/11/sbin/fsck_msdosfs/boot.c
==============================================================================
--- stable/11/sbin/fsck_msdosfs/boot.c	Tue Jul 10 02:43:22 2018	(r336157)
+++ stable/11/sbin/fsck_msdosfs/boot.c	Tue Jul 10 02:46:31 2018	(r336158)
@@ -178,12 +178,8 @@ readboot(int dosfs, struct bootblock *boot)
 		/* Check backup bpbFSInfo?					XXX */
 	}
 
-	boot->ClusterOffset = (boot->bpbRootDirEnts * 32 +
-	    boot->bpbBytesPerSec - 1) / boot->bpbBytesPerSec +
-	    boot->bpbResSectors + boot->bpbFATs * boot->FATsecs -
-	    CLUST_FIRST * boot->bpbSecPerClust;
-
-	if (boot->bpbBytesPerSec % DOSBOOTBLOCKSIZE_REAL != 0) {
+	if (boot->bpbBytesPerSec % DOSBOOTBLOCKSIZE_REAL != 0 ||
+	    boot->bpbBytesPerSec == 0) {
 		pfatal("Invalid sector size: %u", boot->bpbBytesPerSec);
 		return FSFATAL;
 	}
@@ -196,6 +192,10 @@ readboot(int dosfs, struct bootblock *boot)
 		boot->NumSectors = boot->bpbSectors;
 	} else
 		boot->NumSectors = boot->bpbHugeSectors;
+	boot->ClusterOffset = (boot->bpbRootDirEnts * 32 +
+	    boot->bpbBytesPerSec - 1) / boot->bpbBytesPerSec +
+	    boot->bpbResSectors + boot->bpbFATs * boot->FATsecs -
+	    CLUST_FIRST * boot->bpbSecPerClust;
 	boot->NumClusters = (boot->NumSectors - boot->ClusterOffset) /
 	    boot->bpbSecPerClust;