diff --git a/block/vvfat.c b/block/vvfat.c
index 8ffe8b3b9bfaf..a7040248cdead 100644
--- a/block/vvfat.c
+++ b/block/vvfat.c
@@ -242,7 +242,7 @@ typedef struct mbr_t {
 } QEMU_PACKED mbr_t;
 
 typedef struct direntry_t {
-    uint8_t name[8 + 3];
+    uint8_t name[8 + 3]; /* file name + extension */
     uint8_t attributes;
     uint8_t reserved[2];
     uint16_t ctime;
@@ -426,6 +426,7 @@ static direntry_t *create_long_filename(BDRVVVFATState *s, const char *filename)
         else if(offset<22) offset=14+offset-10;
         else offset=28+offset-22;
         entry=array_get(&(s->directory),s->directory.next-1-(i/26));
+        offset = MIN(offset, sizeof(entry->name) - 1);
         if (i >= 2 * length + 2) {
             entry->name[offset] = 0xff;
         } else if (i % 2 == 0) {