linux-kernel - Re: [PATCH 5/5] arm64/vdso: Restrict splitting VVAR VMA

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Date:   Thu, 20 Feb 2020 12:22:52 +0000
From:   Vincenzo Frascino <vincenzo.frascino@....com>
To:     Andrei Vagin <avagin@...il.com>
Cc:     linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Dmitry Safonov <dima@...sta.com>
Subject: Re: [PATCH 5/5] arm64/vdso: Restrict splitting VVAR VMA

Hi Andrei,

On 04/02/2020 17:59, Andrei Vagin wrote:
> Forbid splitting VVAR VMA resulting in a stricter ABI and reducing the
> amount of corner-cases to consider while working further on VDSO time
> namespace support.
> 
> As the offset from timens to VVAR page is computed compile-time, the pages
> in VVAR should stay together and not being partically mremap()'ed.
> 

I agree on the concept, but why do we need to redefine mremap?
special_mapping_mremap() (mm/mmap.c +3317) seems doing already the same thing if
we leave mremap == NULL as is.

> Signed-off-by: Andrei Vagin <avagin@...il.com>
> ---
>  arch/arm64/kernel/vdso.c | 13 +++++++++++++
>  1 file changed, 13 insertions(+)
> 
> diff --git a/arch/arm64/kernel/vdso.c b/arch/arm64/kernel/vdso.c
> index 2e553468b183..e6ebdc184c1e 100644
> --- a/arch/arm64/kernel/vdso.c
> +++ b/arch/arm64/kernel/vdso.c
> @@ -229,6 +229,17 @@ static vm_fault_t vvar_fault(const struct vm_special_mapping *sm,
>  	return vmf_insert_pfn(vma, vmf->address, pfn);
>  }
>  
> +static int vvar_mremap(const struct vm_special_mapping *sm,
> +		       struct vm_area_struct *new_vma)
> +{
> +	unsigned long new_size = new_vma->vm_end - new_vma->vm_start;
> +
> +	if (new_size != VVAR_NR_PAGES * PAGE_SIZE)
> +		return -EINVAL;
> +
> +	return 0;
> +}
> +
>  static int __setup_additional_pages(enum arch_vdso_type arch_index,
>  				    struct mm_struct *mm,
>  				    struct linux_binprm *bprm,
> @@ -311,6 +322,7 @@ static struct vm_special_mapping aarch32_vdso_spec[C_PAGES] = {
>  	{
>  		.name = "[vvar]",
>  		.fault = vvar_fault,
> +		.mremap = vvar_mremap,
>  	},
>  	{
>  		.name = "[vdso]",
> @@ -493,6 +505,7 @@ static struct vm_special_mapping vdso_spec[A_PAGES] __ro_after_init = {
>  	{
>  		.name	= "[vvar]",
>  		.fault = vvar_fault,
> +		.mremap = vvar_mremap,
>  	},
>  	{
>  		.name	= "[vdso]",
> 

-- 
Regards,
Vincenzo

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