[PATCH] libbb/loop: fix existence check for LOOP_CONFIGURE ioctl

[Denys Vlasenko]

Applied, thank you

On Tue, Jan 20, 2026 at 10:29 AM <wouter.franken_ext at softathome.com> wrote:
>
> The LOOP_CONFIGURE ioctl is supported in 5.8 kernels and up. To have
> backwards compatibility there is a config option CONFIG_TRY_LOOP_CONFIGURE
> that will check if the ioctl exists and if not fall back to old way of
> configuring loop devices.
>
> Normally errno will be set to EINVAL when this ioctl does not exist.
> However, when kernel config CONFIG_COMPAT is enabled, then compat_ioctl
> is called. In that case -ENOIOCTLCMD is returned by loop device driver
> and generic ioctl wrapper will set errno to ENOTTY. Because busybox
> does not expect this it will fail to mount loop devices in this case.
>
> This patch fixes the check for the existence of the ioctl LOOP_CONFIGURE
> by checking if errno is one of both: EINVAL or ENOTTY.
>
> Signed-off-by: Wouter Franken <wouter.franken at mind.be>
> Signed-off-by: Wouter Franken <wouter.franken_ext at softathome.com>
> ---
>  libbb/loop.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/libbb/loop.c b/libbb/loop.c
> index a0c5d02..b603d0d 100644
> --- a/libbb/loop.c
> +++ b/libbb/loop.c
> @@ -158,7 +158,7 @@ static int set_loopdev_params(int lfd,
>                 if (rc == 0)
>                         return rc; /* SUCCESS! */
>  # if ENABLE_TRY_LOOP_CONFIGURE
> -               if (errno != EINVAL)
> +               if (errno != EINVAL && errno != ENOTTY)
>                         return rc; /* error other than old kernel */
>                 /* Old kernel, fall through into old way to do it: */
>  # endif
> --
> 2.34.1
>
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