vvfat: Fix vvfat_write() for writes before the root directory

The calculation in sector2cluster() is done relative to the offset of
the root directory. Any writes to blocks before the start of the root
directory (in particular, writes to the FAT) result in negative values,
which are not handled correctly in vvfat_write().

This changes sector2cluster() to return a signed value, and makes sure
that vvfat_write() doesn't try to find mappings for negative cluster
number. It clarifies the code in vvfat_write() to make it more obvious
that the cluster numbers can be negative.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Message-Id: <20211209152231.23756-1-kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

1 files changed, 22 insertions, 8 deletions

diff --git a/block/vvfat.c b/block/vvfat.c
index 36e73d4..b2b58d9 100644
--- a/block/vvfat.c
+++ b/block/vvfat.c
@@ -882,7 +882,7 @@ static int read_directory(BDRVVVFATState* s, int mapping_index)
     return 0;
 }
 
-static inline uint32_t sector2cluster(BDRVVVFATState* s,off_t sector_num)
+static inline int32_t sector2cluster(BDRVVVFATState* s,off_t sector_num)
 {
     return (sector_num - s->offset_to_root_dir) / s->sectors_per_cluster;
 }
@@ -2981,6 +2981,7 @@ static int vvfat_write(BlockDriverState *bs, int64_t sector_num,
 {
     BDRVVVFATState *s = bs->opaque;
     int i, ret;
+    int first_cluster, last_cluster;
 
 DLOG(checkpoint());
 
@@ -2999,9 +3000,20 @@ DLOG(checkpoint());
     if (sector_num < s->offset_to_fat)
         return -1;
 
-    for (i = sector2cluster(s, sector_num);
-            i <= sector2cluster(s, sector_num + nb_sectors - 1);) {
-        mapping_t* mapping = find_mapping_for_cluster(s, i);
+    /*
+     * Values will be negative for writes to the FAT, which is located before
+     * the root directory.
+     */
+    first_cluster = sector2cluster(s, sector_num);
+    last_cluster = sector2cluster(s, sector_num + nb_sectors - 1);
+
+    for (i = first_cluster; i <= last_cluster;) {
+        mapping_t *mapping = NULL;
+
+        if (i >= 0) {
+            mapping = find_mapping_for_cluster(s, i);
+        }
+
         if (mapping) {
             if (mapping->read_only) {
                 fprintf(stderr, "Tried to write to write-protected file %s\n",
@@ -3041,8 +3053,9 @@ DLOG(checkpoint());
                 }
             }
             i = mapping->end;
-        } else
+        } else {
             i++;
+        }
     }
 
     /*
@@ -3056,10 +3069,11 @@ DLOG(fprintf(stderr, "Write to qcow backend: %d + %d\n", (int)sector_num, nb_sec
         return ret;
     }
 
-    for (i = sector2cluster(s, sector_num);
-            i <= sector2cluster(s, sector_num + nb_sectors - 1); i++)
-        if (i >= 0)
+    for (i = first_cluster; i <= last_cluster; i++) {
+        if (i >= 0) {
             s->used_clusters[i] |= USED_ALLOCATED;
+        }
+    }
 
 DLOG(checkpoint());
     /* TODO: add timeout */