[PATCH v2 1/2] vvfat: allow some writes to bootsector

["Hervé Poussineau" <hpoussin@reactos.org>]

'reserved1' field in bootsector is used to mark volume dirty, or need to verify.
Allow writes to bootsector which only changes the 'reserved1' field.

This fixes I/O errors on Windows guests.

Resolves: https://bugs.launchpad.net/qemu/+bug/1889421
Signed-off-by: Hervé Poussineau <hpoussin@reactos.org>
---
 block/vvfat.c | 26 +++++++++++++++++++++++++-
 1 file changed, 25 insertions(+), 1 deletion(-)

diff --git a/block/vvfat.c b/block/vvfat.c
index d6dd919683d..ae53f0d7283 100644
--- a/block/vvfat.c
+++ b/block/vvfat.c
@@ -2993,11 +2993,35 @@ DLOG(checkpoint());
 
     vvfat_close_current_file(s);
 
+    if (sector_num == s->offset_to_bootsector && nb_sectors == 1) {
+        /*
+         * Write on bootsector. Allow only changing the reserved1 field,
+         * used to mark volume dirtiness
+         */
+        unsigned char *bootsector = s->first_sectors
+                                    + s->offset_to_bootsector * 0x200;
+        /*
+         * LATER TODO: if FAT32, this is wrong (see init_directories(),
+         * which always creates a FAT16 bootsector)
+         */
+        const int reserved1_offset = offsetof(bootsector_t, u.fat16.reserved1);
+
+        for (i = 0; i < 0x200; i++) {
+            if (i != reserved1_offset && bootsector[i] != buf[i]) {
+                fprintf(stderr, "Tried to write to protected bootsector\n");
+                return -1;
+            }
+        }
+
+        /* Update bootsector with the only updatable byte, and return success */
+        bootsector[reserved1_offset] = buf[reserved1_offset];
+        return 0;
+    }
+
     /*
      * Some sanity checks:
      * - do not allow writing to the boot sector
      */
-
     if (sector_num < s->offset_to_fat)
         return -1;
 
-- 
2.36.2