FAT: fix FAT16/32 next cluster calculation, make more similar to FAT12

The FAT16/32 next cluster calculations were changed to be somewhat
more like the FAT12 one, but that was done incorrectly: we would end
up reading off the end of the FAT sector we were looking at.  Make it
structurally more similar to the FAT12 calculation, and actually make
it compute correctly.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>

1 files changed, 16 insertions, 10 deletions

diff --git a/core/fs/fat/fat.c b/core/fs/fat/fat.c
index 79439e09..23a55120 100644
--- a/core/fs/fat/fat.c
+++ b/core/fs/fat/fat.c
@@ -70,17 +70,20 @@ static uint32_t get_next_cluster(struct fs_info *fs, uint32_t clust_num)
 	break;
 	
     case FAT16:
-	fat_sector = clust_num >> (SECTOR_SHIFT(fs) - 1);
-	offset = clust_num & sector_mask;
+	offset = clust_num << 1;
+	fat_sector = offset >> SECTOR_SHIFT(fs);
+	offset &= sector_mask;
 	cs = get_fat_sector(fs, fat_sector);
-	next_cluster = ((uint16_t *)cs->data)[offset];
+	next_cluster = *(uint16_t *)(cs->data + offset);
 	break;
 	
     case FAT32:
-	fat_sector = clust_num >> (SECTOR_SHIFT(fs) - 2);
-	offset = clust_num & sector_mask;
+	offset = clust_num << 2;
+	fat_sector = offset >> SECTOR_SHIFT(fs);
+	offset &= sector_mask;
 	cs = get_fat_sector(fs, fat_sector);
-	next_cluster = ((uint32_t *)cs->data)[offset] & 0x0fffffff;
+	next_cluster = *(uint32_t *)(cs->data + offset);
+	next_cluster &= 0x0fffffff;
 	break;
     }
     
@@ -99,24 +102,27 @@ static sector_t get_next_sector(struct fs_info* fs, uint32_t sector)
     if (sector < data_area) {
 	/* Root directory sector... */
 	sector++;
-	if (sector == data_area)
+	if (sector >= data_area)
 	    sector = 0; /* Ran out of root directory, return EOF */
 	return sector;
     }
     
     data_sector = sector - data_area;
     if ((data_sector + 1) & sbi->clust_mask)  /* Still in the same cluster */
-	return sector+1;		      /* Next sector inside cluster */
+	return sector + 1;		      /* Next sector inside cluster */
 
     /* get a new cluster */
     cluster = data_sector >> clust_shift;
     cluster = get_next_cluster(fs, cluster + 2) - 2;
 
-    if (cluster >= sbi->clusters)
+    if (cluster >= sbi->clusters) {
+	printf("Logical cluster = 0x%x, total clusters = 0x%x\n",
+	       cluster + 2, sbi->clusters);
 	return 0;
+    }
     
     /* return the start of the new cluster */
-    sector = (cluster << sbi->clust_shift) + data_area;
+    sector = (cluster << clust_shift) + data_area;
     return sector;
 }