linux-kernel - Re: [PATCH 2/2] vmalloc: reject vmap with VM_FLUSH_RESET

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Date:   Fri, 23 Dec 2022 10:24:25 +0000
From:   Lorenzo Stoakes <lstoakes@...il.com>
To:     Christoph Hellwig <hch@....de>
Cc:     Andy Gross <agross@...nel.org>,
        Bjorn Andersson <andersson@...nel.org>,
        Konrad Dybcio <konrad.dybcio@...aro.org>,
        Mathieu Poirier <mathieu.poirier@...aro.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Uladzislau Rezki <urezki@...il.com>,
        linux-arm-msm@...r.kernel.org, linux-remoteproc@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        iommu@...ts.linux.dev
Subject: Re: [PATCH 2/2] vmalloc: reject vmap with VM_FLUSH_RESET_PERMS

On Fri, Dec 23, 2022 at 10:27:03AM +0100, Christoph Hellwig wrote:
> VM_FLUSH_RESET_PERMS is just for use with vmalloc as it is tied to freeing
> the underlying pages.
>
> Signed-off-by: Christoph Hellwig <hch@....de>
> ---
>  mm/vmalloc.c | 3 +++
>  1 file changed, 3 insertions(+)
>
> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> index 9e30f0b3920325..88a644cde9fb12 100644
> --- a/mm/vmalloc.c
> +++ b/mm/vmalloc.c
> @@ -2849,6 +2849,9 @@ void *vmap(struct page **pages, unsigned int count,
>
>  	might_sleep();
>
> +	if (WARN_ON_ONCE(flags & VM_FLUSH_RESET_PERMS))
> +		return NULL;
> +

Might it be worth adding a specific vmap mask that explicitly indicates what
flags are permissible on vmap()? Then this could become e.g.:-

	if (WARN_ON_ONCE(flags & ~VM_VMAP_PERMITTED_MASK))
		return NULL;

And would be self-documenting as to why we are disallowing flags (i.e. they are
not part of the permitted vmap mask).