Re: [PATCH 08/10] ioctl_userfaultfd.2: clarify the state of the uffdio_api structure on error

[Mike Rapoport <rppt@kernel.org>]

On Tue, Sep 19, 2023 at 12:02:04PM -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.

In old kernels (e.g. 4.20 and I didn't go to check when this changed) we
had two -EINVALS: one when UFFDIO_API was called when

	state != UFFD_STATE_WAIT_API

and another for API version or features mismatch and we
zeroed uffd_api struct only in the second case.

In the current code the first case does not exits anymore.

> Signed-off-by: Axel Rasmussen <axelrasmussen@google.com>

Reviewed-by: Mike Rapoport (IBM) <rppt@kernel.org>

> ---
>  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 1aa9654be..29dca1f6b 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 @@ twice,
>  the first time with no features set,
>  is explicitly allowed
>  as per the two-step feature detection handshake.
> -.\" 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.
> -- 
> 2.42.0.459.ge4e396fd5e-goog
> 
> 

-- 
Sincerely yours,
Mike.