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Date: Wed, 18 Apr 2018 18:17:04 -0400
From: Boris Ostrovsky <boris.ostrovsky@...cle.com>
To: Laura Abbott <labbott@...hat.com>, Juergen Gross <jgross@...e.com>
Cc: Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
"H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
xen-devel@...ts.xenproject.org, linux-kernel@...r.kernel.org,
kernel-hardening@...ts.openwall.com
Subject: Re: [PATCHv2] x86/xen: Remove use of VLAs
On 04/18/2018 01:08 PM, Laura Abbott wrote:
> There's an ongoing effort to remove VLAs[1] from the kernel to eventually
> turn on -Wvla. It turns out, the few VLAs in use in Xen produce only a
> single entry array that is always bounded by GDT_SIZE. Clean up the code to
> get rid of the VLA and the loop.
>
> [1] https://lkml.org/lkml/2018/3/7/621
>
> Signed-off-by: Laura Abbott <labbott@...hat.com>
> ---
> v2: Updated the code to reflect that we know size is always bounded by
> GDT_SIZE. This gets rid of the array and the loop. I can throw a few
> more comments in there if someone thinks they need to be updated.
> ---
> arch/x86/xen/enlighten_pv.c | 84 ++++++++++++++++-----------------------------
> 1 file changed, 29 insertions(+), 55 deletions(-)
>
> diff --git a/arch/x86/xen/enlighten_pv.c b/arch/x86/xen/enlighten_pv.c
> index c36d23aa6c35..1254f2fa3a89 100644
> --- a/arch/x86/xen/enlighten_pv.c
> +++ b/arch/x86/xen/enlighten_pv.c
> @@ -421,45 +421,32 @@ static void xen_load_gdt(const struct desc_ptr *dtr)
> {
> unsigned long va = dtr->address;
> unsigned int size = dtr->size + 1;
> - unsigned pages = DIV_ROUND_UP(size, PAGE_SIZE);
> - unsigned long frames[pages];
> - int f;
> -
> - /*
> - * A GDT can be up to 64k in size, which corresponds to 8192
> - * 8-byte entries, or 16 4k pages..
> - */
> + unsigned long pfn, mfn;
> + int level;
> + pte_t *ptep;
> + void *virt;
>
> - BUG_ON(size > 65536);
> + BUG_ON(size > GDT_SIZE);
I'd probably BUG_ON(size>PAGE_SIZE) because that's what we are really
trying to avoid. Maybe with a comment that we expect GDT_SIZE at most,
and it is less than PAGE_SIZE.
I can fix it while committing if you don't object.
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@...cle.com>
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