Re: [PATCH] nbd: Avoid off-by-one in long export name truncation

[Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>]

23.06.2020 00:03, Eric Blake wrote:
> When snprintf returns the same value as the buffer size, the final
> byte was truncated to ensure a NUL terminator.  Fortunately, such long
> export names are unusual enough, with no real impact other than what
> is displayed to the user.
> 
> Fixes: 5c86bdf12089

Oh, I remember myself checking snprintf spec when reviewing this, but
still I missed this thing :(

> Reported-by: Max Reitz <mreitz@redhat.com>
> Signed-off-by: Eric Blake <eblake@redhat.com>

Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>

> ---
>   block/nbd.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/block/nbd.c b/block/nbd.c
> index eed160c5cda1..8301fcac0589 100644
> --- a/block/nbd.c
> +++ b/block/nbd.c
> @@ -2009,7 +2009,7 @@ static void nbd_refresh_filename(BlockDriverState *bs)
>           len = snprintf(bs->exact_filename, sizeof(bs->exact_filename),
>                          "nbd://%s:%s", host, port);
>       }
> -    if (len > sizeof(bs->exact_filename)) {
> +    if (len >= sizeof(bs->exact_filename)) {
>           /* Name is too long to represent exactly, so leave it empty. */
>           bs->exact_filename[0] = '\0';
>       }
> 

Hmm, interesting, that in case of len=0 (no one branch was executed from previous
if-else-if, which seems possible) we do nothing. Shouldn't this if actually be
if (!len || len >= sizeof(bs->exact_filename)) {
   /* Name is too long or we have nothing to report */
   bs->exact_filename[0] = '\0';
}

Probably not, as bdrv_refresh_filename() does it anyway prior to calling drv->bdrv_refresh_filename().


-- 
Best regards,
Vladimir