linux-kernel - Re: [PATCH] x86/xen: Remove use of VLAs

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Message-ID: <7cf66c00-74c2-62bb-81f5-4332313a5a6e@oracle.com>
Date:   Mon, 16 Apr 2018 09:27:24 -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: [PATCH] x86/xen: Remove use of VLAs

On 04/13/2018 06:11 PM, Laura Abbott wrote:
> There's an ongoing effort to remove VLAs[1] from the kernel to eventually
> turn on -Wvla. The few VLAs in use have an upper bound based on a size
> of 64K. This doesn't produce an excessively large stack so just switch
> the upper bound.
>
> [1] https://lkml.org/lkml/2018/3/7/621
>
> Signed-off-by: Laura Abbott <labbott@...hat.com>
> ---
>  arch/x86/xen/enlighten_pv.c | 6 ++----
>  1 file changed, 2 insertions(+), 4 deletions(-)
>
> diff --git a/arch/x86/xen/enlighten_pv.c b/arch/x86/xen/enlighten_pv.c
> index c36d23aa6c35..d96a5a535cbb 100644
> --- a/arch/x86/xen/enlighten_pv.c
> +++ b/arch/x86/xen/enlighten_pv.c
> @@ -421,8 +421,7 @@ 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);



Isn't dtr->size always either GDT_SIZE or 0?

-boris




> -	unsigned long frames[pages];
> +	unsigned long frames[DIV_ROUND_UP(SZ_64K, PAGE_SIZE)];
>  	int f;
>  
>  	/*
> @@ -470,8 +469,7 @@ static void __init xen_load_gdt_boot(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];
> +	unsigned long frames[DIV_ROUND_UP(SZ_64K, PAGE_SIZE)];
>  	int f;
>  
>  	/*