linux-kernel - Re: [PATCH 07/10] ioctl_userfaultfd.2: correct and update UFFDIO

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Message-ID: <5ddepg57wqnidtvsio2pse44dot6pvr3rcmhwld6ml3sflwcz3@ijd3h4teqblr>
Date:   Tue, 26 Sep 2023 01:52:34 +0200
From:   Alejandro Colomar <alx@...nel.org>
To:     Axel Rasmussen <axelrasmussen@...gle.com>
Cc:     Peter Xu <peterx@...hat.com>, linux-man@...r.kernel.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 07/10] ioctl_userfaultfd.2: correct and update UFFDIO_API
 ioctl error codes

Hi Axel,

On Tue, Sep 19, 2023 at 12:02:03PM -0700, Axel Rasmussen wrote:
> First, it is not correct that repeated UFFDIO_API calls result in
> EINVAL. This is true *if both calls enable features*, but in the case
> where we're doing a two-step feature detection handshake, the kernel
> explicitly expects 2 calls (one with no features set). So, correct this
> description.
> 
> Then, some new error cases have been added to the kernel recently, and
> the man page wasn't updated to note these. So, add in descriptions of
> these new error cases.
> 
> Signed-off-by: Axel Rasmussen <axelrasmussen@...gle.com>
> ---
>  man2/ioctl_userfaultfd.2 | 24 +++++++++++++++++++-----
>  1 file changed, 19 insertions(+), 5 deletions(-)
> 
> diff --git a/man2/ioctl_userfaultfd.2 b/man2/ioctl_userfaultfd.2
> index 53b1f473f..1aa9654be 100644
> --- a/man2/ioctl_userfaultfd.2
> +++ b/man2/ioctl_userfaultfd.2
> @@ -280,17 +280,31 @@ refers to an address that is outside the calling process's
>  accessible address space.
>  .TP
>  .B EINVAL
> -The userfaultfd has already been enabled by a previous
> -.B UFFDIO_API
> -operation.
> -.TP
> -.B EINVAL
>  The API version requested in the
>  .I api
>  field is not supported by this kernel, or the
>  .I features
>  field passed to the kernel includes feature bits that are not supported
>  by the current kernel version.
> +.TP
> +.B EPERM

This EPERM should probably be at the end.  Unless you have a good reason
to break alphabetic order.

Thanks,
Alex

> +The
> +.B UFFD_FEATURE_EVENT_FORK
> +feature was enabled,
> +but the calling process doesn't have the
> +.B CAP_SYS_PTRACE
> +capability.
> +.TP
> +.B EINVAL
> +A previous
> +.B UFFDIO_API
> +call already enabled one or more features for this userfaultfd.
> +Calling
> +.B UFFDIO_API
> +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.
>  .\"
> -- 
> 2.42.0.459.ge4e396fd5e-goog
> 

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