linux-kernel - Re: [PATCH v2 3/5] ioctl_userfaultfd.2: clarify the state of the uffdio

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Date:   Sun, 8 Oct 2023 23:52:56 +0200
From:   Alejandro Colomar <alx@...nel.org>
To:     Axel Rasmussen <axelrasmussen@...gle.com>,
        Mike Rapoport <rppt@...nel.org>
Cc:     Peter Xu <peterx@...hat.com>, linux-man@...r.kernel.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 3/5] ioctl_userfaultfd.2: clarify the state of the
 uffdio_api structure on error

Hi Axel,

On Tue, Oct 03, 2023 at 12:45:45PM -0700, Axel Rasmussen wrote:
> The old FIXME noted that the zeroing was done to differentiate the two
> EINVAL cases. It's possible something like this was true historically,
> but in current Linux we zero it in *both* EINVAL cases, so this is at
> least no longer true.
> 
> After reading the code, I can't determine any clear reason why we zero
> it in some cases but not in others. So, some simple advice we can give
> userspace is: if an error occurs, treat the contents of the structure as
> unspecified. Just re-initialize it before retrying UFFDIO_API again.
> 
> Signed-off-by: Axel Rasmussen <axelrasmussen@...gle.com>
> ---
>  man2/ioctl_userfaultfd.2 | 16 ++++++++--------
>  1 file changed, 8 insertions(+), 8 deletions(-)
> 
> diff --git a/man2/ioctl_userfaultfd.2 b/man2/ioctl_userfaultfd.2
> index 28dd2fcdd..2ee6a0532 100644
> --- a/man2/ioctl_userfaultfd.2
> +++ b/man2/ioctl_userfaultfd.2
> @@ -248,6 +248,14 @@ operation returns 0 on success.
>  On error, \-1 is returned and
>  .I errno
>  is set to indicate the error.
> +If an error occurs,
> +the kernel may zero the provided
> +.I uffdio_api
> +structure.
> +The caller should treat its contents as unspecified,
> +and reinitialize it before re-attempting another
> +.B UFFDIO_API
> +call.
>  Possible errors include:
>  .TP
>  .B EFAULT
> @@ -281,14 +289,6 @@ feature was enabled,
>  but the calling process doesn't have the
>  .B CAP_SYS_PTRACE
>  capability.
> -.\" FIXME In the above error case, the returned 'uffdio_api' structure is
> -.\" zeroed out. Why is this done? This should be explained in the manual page.
> -.\"
> -.\" Mike Rapoport:
> -.\"     In my understanding the uffdio_api
> -.\"     structure is zeroed to allow the caller
> -.\"     to distinguish the reasons for -EINVAL.
> -.\"

I've added Mike to the thread in case he wants to comment.

Thanks,
Alex

>  .SS UFFDIO_REGISTER
>  (Since Linux 4.3.)
>  Register a memory address range with the userfaultfd object.
> -- 
> 2.42.0.609.gbb76f46606-goog
> 

-- 
<https://www.alejandro-colomar.es/>

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