linux-kernel - Re: [PATCH v2] userfaultfd: fix UFFDIO

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Date:   Wed, 23 Jun 2021 05:42:12 +0300
From:   "Dmitry V. Levin" <ldv@...linux.org>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Gleb Fotengauer-Malinovskiy <glebfm@...linux.org>,
        Axel Rasmussen <axelrasmussen@...gle.com>,
        Peter Xu <peterx@...hat.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Lokesh Gidra <lokeshgidra@...gle.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] userfaultfd: fix UFFDIO_CONTINUE ioctl request
 definition

Hi,

On Tue, Jun 01, 2021 at 05:34:32PM +0300, Gleb Fotengauer-Malinovskiy wrote:
> This ioctl request reads from uffdio_continue structure which justifies
> _IOC_READ flag.
> See NOTEs in include/uapi/asm-generic/ioctl.h for more information.

These ioctl flag usage rules seem to be a perpetual source of mistakes.
I suggest a slightly different wording of the commit message which is
hopefully a bit easier to follow:

"This ioctl request reads from uffdio_continue structure written by
userspace which justifies _IOC_WRITE flag.  It also writes back to that
structure which justifies _IOC_READ flag.

See NOTEs in include/uapi/asm-generic/ioctl.h for more information."
 
Please make sure this patch is merged before v5.13 is released, otherwise
the UAPI bug would have to stay for backwards ABI compatibility.

> Fixes: f619147104c8 ("userfaultfd: add UFFDIO_CONTINUE ioctl")
> Signed-off-by: Gleb Fotengauer-Malinovskiy <glebfm@...linux.org>
> Acked-by: Peter Xu <peterx@...hat.com>
> ---
>  include/uapi/linux/userfaultfd.h | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> v2 fix commit message
> 
> diff --git a/include/uapi/linux/userfaultfd.h b/include/uapi/linux/userfaultfd.h
> index bafbeb1a2624..650480f41f1d 100644
> --- a/include/uapi/linux/userfaultfd.h
> +++ b/include/uapi/linux/userfaultfd.h
> @@ -80,8 +80,8 @@
>  				      struct uffdio_zeropage)
>  #define UFFDIO_WRITEPROTECT	_IOWR(UFFDIO, _UFFDIO_WRITEPROTECT, \
>  				      struct uffdio_writeprotect)
> -#define UFFDIO_CONTINUE		_IOR(UFFDIO, _UFFDIO_CONTINUE,	\
> -				     struct uffdio_continue)
> +#define UFFDIO_CONTINUE		_IOWR(UFFDIO, _UFFDIO_CONTINUE,	\
> +				      struct uffdio_continue)
>  
>  /* read() structure */
>  struct uffd_msg {

Reviewed-by: Dmitry V. Levin <ldv@...linux.org>


-- 
ldv