Re: [PATCH 1/6] arm64/vdso: use the fault callback to map vvar pages

[Mark Rutland]

On Tue, Jun 16, 2020 at 12:55:40AM -0700, Andrei Vagin wrote:
> This is required to support time namespaces where a time namespace data
> page is different for each namespace.

Can you please give a bit more of an introduction to the changes here?
As-is, this doesn't give reviewers the necessary context to understand
the change, nor does it justify it.

Ideally, a commit message for this should look something like:

| Currently the vdso has no awareness of time namespaces, which may
| apply distinct offsets to processes in different namespaces. To handle
| this within the vdso, we'll need to expose a per-namespace data page.
|
| As a preparatory step, this patch separates the vdso data page from
| the code pages, and has it faulted in via its own fault callback.
| Subsquent patches will extend this to support distinct pages per time
| namespace.

Otherwise, I have a few minor comments below.

> Reviewed-by: Vincenzo Frascino <vincenzo.frascino@xxxxxxx>
> Reviewed-by: Dmitry Safonov <dima@xxxxxxxxxx>
> Signed-off-by: Andrei Vagin <avagin@xxxxxxxxx>
> ---
>  arch/arm64/kernel/vdso.c | 24 +++++++++++++++---------
>  1 file changed, 15 insertions(+), 9 deletions(-)
> 
> diff --git a/arch/arm64/kernel/vdso.c b/arch/arm64/kernel/vdso.c
> index 4e016574bd91..b0aec4e8c9b4 100644
> --- a/arch/arm64/kernel/vdso.c
> +++ b/arch/arm64/kernel/vdso.c
> @@ -108,28 +108,32 @@ static int __vdso_init(enum vdso_abi abi)
>  			PAGE_SHIFT;
>  
>  	/* Allocate the vDSO pagelist, plus a page for the data. */
> -	vdso_pagelist = kcalloc(vdso_info[abi].vdso_pages + 1,
> +	vdso_pagelist = kcalloc(vdso_info[abi].vdso_pages,
>  				sizeof(struct page *),
>  				GFP_KERNEL);

The comment above is now stale. Can you please update it, or (event
better) delete it entirely?

>  	if (vdso_pagelist == NULL)
>  		return -ENOMEM;
>  
> -	/* Grab the vDSO data page. */
> -	vdso_pagelist[0] = phys_to_page(__pa_symbol(vdso_data));
> -
> -
>  	/* Grab the vDSO code pages. */
>  	pfn = sym_to_pfn(vdso_info[abi].vdso_code_start);
>  
>  	for (i = 0; i < vdso_info[abi].vdso_pages; i++)
> -		vdso_pagelist[i + 1] = pfn_to_page(pfn + i);
> +		vdso_pagelist[i] = pfn_to_page(pfn + i);
>  
> -	vdso_info[abi].dm->pages = &vdso_pagelist[0];
> -	vdso_info[abi].cm->pages = &vdso_pagelist[1];
> +	vdso_info[abi].cm->pages = vdso_pagelist;
>  
>  	return 0;
>  }
>  
> +static vm_fault_t vvar_fault(const struct vm_special_mapping *sm,
> +			     struct vm_area_struct *vma, struct vm_fault *vmf)
> +{
> +	if (vmf->pgoff == 0)
> +		return vmf_insert_pfn(vma, vmf->address,
> +				sym_to_pfn(vdso_data));
> +	return VM_FAULT_SIGBUS;
> +}

This might look better as:

|	if (vmf->pgoff != 0)
|		return VM_FAULT_SIGBUS;
|	
|	return vmf_insert_pfn(vma, vmf->address, sym_to_pfn(vdso_data));

> +
>  static int __setup_additional_pages(enum vdso_abi abi,
>  				    struct mm_struct *mm,
>  				    struct linux_binprm *bprm,
> @@ -150,7 +154,7 @@ static int __setup_additional_pages(enum vdso_abi abi,
>  	}
>  
>  	ret = _install_special_mapping(mm, vdso_base, PAGE_SIZE,
> -				       VM_READ|VM_MAYREAD,
> +				       VM_READ|VM_MAYREAD|VM_PFNMAP,

This change needs to be explained in the commit message. WHy is it
necessary, and why only so for the data page?

Thanks,
Mark.

>  				       vdso_info[abi].dm);
>  	if (IS_ERR(ret))
>  		goto up_fail;
> @@ -209,6 +213,7 @@ static struct vm_special_mapping aarch32_vdso_maps[] = {
>  #ifdef CONFIG_COMPAT_VDSO
>  	[AA32_MAP_VVAR] = {
>  		.name = "[vvar]",
> +		.fault = vvar_fault,
>  	},
>  	[AA32_MAP_VDSO] = {
>  		.name = "[vdso]",
> @@ -376,6 +381,7 @@ enum aarch64_map {
>  static struct vm_special_mapping aarch64_vdso_maps[] __ro_after_init = {
>  	[AA64_MAP_VVAR] = {
>  		.name	= "[vvar]",
> +		.fault = vvar_fault,
>  	},
>  	[AA64_MAP_VDSO] = {
>  		.name	= "[vdso]",
> -- 
> 2.24.1
>