linux-kernel - Re: [PATCH 1/2] Fix userfaultfd_api to return EINVAL as expected

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Date: Wed, 8 May 2024 09:39:10 +0200
From: David Hildenbrand <david@...hat.com>
To: Audra Mitchell <audra@...hat.com>, viro@...iv.linux.org.uk
Cc: brauner@...nel.org, jack@...e.cz, linux-fsdevel@...r.kernel.org,
 linux-kernel@...r.kernel.org, akpm@...ux-foundation.org, shuah@...nel.org,
 linux-kselftest@...r.kernel.org, linux-mm@...ck.org, raquini@...hat.com,
 Peter Xu <peterx@...hat.com>
Subject: Re: [PATCH 1/2] Fix userfaultfd_api to return EINVAL as expected

On 07.05.24 21:55, Audra Mitchell wrote:
> Currently if we request a feature that is not set in the Kernel
> config we fail silently and return the available features. However, the
> documentation indicates we should return an EINVAL.

I assume you are referencing

"EINVAL The API version requested in the api field is not supported by 
this kernel, or  the  features  field passed to the kernel includes 
feature bits that are not supported by the current kernel version."

and

"To  enable  userfaultfd features the application should set a bit 
corresponding to each feature it wants to enable in the features field. 
If the kernel supports all the requested features it will enable them. 
Otherwise it will zero out the returned uffdio_api structure and return 
EINVAL.
"

in which case I agree.

> 
> We need to fix this issue since we can end up with a Kernel warning
> should a program request the feature UFFD_FEATURE_WP_UNPOPULATED on
> a kernel with the config not set with this feature.

Can you mention which exact one? Is it a WARN* or a pr_warn() ?

Likely we want "Fixes:" here.

> 
> Signed-off-by: Audra Mitchell <audra@...hat.com>
> ---
>   fs/userfaultfd.c | 5 +++++
>   1 file changed, 5 insertions(+)
> 
> diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
> index 60dcfafdc11a..17210558de79 100644
> --- a/fs/userfaultfd.c
> +++ b/fs/userfaultfd.c
> @@ -2073,6 +2073,11 @@ static int userfaultfd_api(struct userfaultfd_ctx *ctx,
>   	uffdio_api.features &= ~UFFD_FEATURE_WP_UNPOPULATED;
>   	uffdio_api.features &= ~UFFD_FEATURE_WP_ASYNC;
>   #endif
> +
> +	ret = -EINVAL;
> +	if (features & ~uffdio_api.features)
> +		goto err_out;
> +
>   	uffdio_api.ioctls = UFFD_API_IOCTLS;
>   	ret = -EFAULT;
>   	if (copy_to_user(buf, &uffdio_api, sizeof(uffdio_api)))

CCing Peter.

-- 
Cheers,

David / dhildenb