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

[Kevin Wolf <kwolf@redhat.com>]

Am 03.09.2022 um 18:23 hat Hervé Poussineau geschrieben:
> '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 | 18 +++++++++++++++++-
>  1 file changed, 17 insertions(+), 1 deletion(-)
> 
> diff --git a/block/vvfat.c b/block/vvfat.c
> index d6dd919683d..35057a51c67 100644
> --- a/block/vvfat.c
> +++ b/block/vvfat.c
> @@ -2993,11 +2993,27 @@ 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
> +         */
> +        const unsigned char *initial = s->first_sectors
> +                                       + s->offset_to_bootsector * 0x200;
> +        for (i = 0; i < 0x200; i++) {
> +            if (i != offsetof(bootsector_t, u.fat16.reserved1) &&

I think you need to check the FAT version (s->fat_type) before accessing
u.fat16. For FAT32, the "reserved" field is at a different offset (but
seems to have the same meaning).

> +                initial[i] != buf[i]) {
> +                fprintf(stderr, "Tried to write to protected bootsector\n");
> +                return -1;
> +            }
> +        }
> +        return 0;
> +    }

Should we update s->first_sectors with the new value so that the guest
would actually read back what it wrote instead of having the change
disappear magically?

>      /*
>       * Some sanity checks:
>       * - do not allow writing to the boot sector
>       */
> -
>      if (sector_num < s->offset_to_fat)
>          return -1;

Kevin