Re: [PATCH v3 1/3] ioctl_userfaultfd.2: clarify the state of the uffdio_api structure on error

Alejandro Colomar
2023-10-17T23:07:44Z

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On Tue, Oct 17, 2023 at 04:01:08PM -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.
>
> Reviewed-by: Mike Rapoport (IBM) <rppt@kernel.org>
> Signed-off-by: Axel Rasmussen <axelrasmussen@google.com>

Hi Axel,

Patch applied. Thanks, and thank you too Mike for the review.

Cheers,
Alex

> ---
> 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 e68085262..82aee667c 100644
> --- a/man2/ioctl_userfaultfd.2
> +++ b/man2/ioctl_userfaultfd.2
> @@ -272,6 +272,14 @@ 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
> @@ -305,14 +313,6 @@ 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.
> -.\"
> .SS UFFDIO_REGISTER
> (Since Linux 4.3.)
> Register a memory address range with the userfaultfd object.
> --
> 2.42.0.655.g421f12c284-goog
>

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

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