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Correct off-by-two issue when determining FAT type.

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In the code we used NumClusters as the upper (non-inclusive) boundary
of valid cluster number, so the actual value was 2 (CLUST_FIRST) more
than the real number of clusters. This causes a FAT16 media with
65524 clusters be treated as FAT32 and might affect FAT12 media with
4084 clusters as well.

To fix this, we increment NumClusters by CLUST_FIRST after the type
determination.

PR:		243179
MFC after:	2 weeks
Differential Revision:	https://reviews.freebsd.org/D23082
This commit is contained in:
Xin LI 2020-01-11 17:41:20 +00:00
parent ae5b45c86e
commit d3dd66792b
1 changed files with 14 additions and 3 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

17
sbin/fsck_msdosfs/boot.c
View File

@ -266,8 +266,11 @@ readboot(int dosfs, struct bootblock *boot)
return FSFATAL;
}
boot->NumClusters = (boot->NumSectors - boot->FirstCluster) / boot->bpbSecPerClust +
CLUST_FIRST;
/*
* The number of clusters is derived from available data sectors, divided
* by sectors per cluster.
*/
boot->NumClusters = (boot->NumSectors - boot->FirstCluster) / boot->bpbSecPerClust;
if (boot->flags & FAT32) {
if (boot->NumClusters > (CLUST_RSRVD & CLUST32_MASK)) {
@ -310,11 +313,19 @@ readboot(int dosfs, struct bootblock *boot)
break;
}
if (boot->NumFatEntries < boot->NumClusters - CLUST_FIRST) {
if (boot->NumFatEntries < boot->NumClusters) {
pfatal("FAT size too small, %u entries won't fit into %u sectors\n",
boot->NumClusters, boot->FATsecs);
return FSFATAL;
}
/*
* There are two reserved clusters. To avoid adding CLUST_FIRST every time
* when we perform boundary checks, we increment the NumClusters by 2,
* which is CLUST_FIRST to denote the first out-of-range cluster number.
*/
boot->NumClusters += CLUST_FIRST;
boot->ClusterSize = boot->bpbBytesPerSec * boot->bpbSecPerClust;
boot->NumFiles = 1;