Re: [PATCH] arm64/vdso: Support mremap() for vDSO

[Will Deacon]

On Wed, Jul 26, 2017 at 08:07:37PM +0300, Dmitry Safonov wrote:
> vDSO VMA address is saved in mm_context for the purpose of using
> restorer from vDSO page to return to userspace after signal handling.
> 
> In Checkpoint Restore in Userspace (CRIU) project we place vDSO VMA
> on restore back to the place where it was on the dump.
> With the exception for x86 (where there is API to map vDSO with
> arch_prctl()), we move vDSO inherited from CRIU task to restoree
> position by mremap().
> 
> CRIU does support arm64 architecture, but kernel doesn't update
> context.vdso pointer after mremap(). Which results in translation
> fault after signal handling on restored application:
> https://github.com/xemul/criu/issues/288
> 
> Make vDSO code track the VMA address by supplying .mremap() fops
> the same way it's done for x86 and arm32 by:
> commit b059a453b1cf ("x86/vdso: Add mremap hook to vm_special_mapping")
> commit 280e87e98c09 ("ARM: 8683/1: ARM32: Support mremap() for sigpage/vDSO").
> 
> Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
> Cc: Will Deacon <will.deacon@xxxxxxx>
> Cc: Russell King <rmk+kernel@xxxxxxxxxxxxxxx>
> Cc: linux-arm-kernel@xxxxxxxxxxxxxxxxxxx
> Cc: Cyrill Gorcunov <gorcunov@xxxxxxxxxx>
> Cc: Pavel Emelyanov <xemul@xxxxxxxxxxxxx>
> Cc: Christopher Covington <cov@xxxxxxxxxxxxxx>
> Signed-off-by: Dmitry Safonov <dsafonov@xxxxxxxxxxxxx>
> ---
>  arch/arm64/kernel/vdso.c | 15 +++++++++++++++
>  1 file changed, 15 insertions(+)
> 
> diff --git a/arch/arm64/kernel/vdso.c b/arch/arm64/kernel/vdso.c
> index e8f759f764f2..2d419006ad43 100644
> --- a/arch/arm64/kernel/vdso.c
> +++ b/arch/arm64/kernel/vdso.c
> @@ -110,12 +110,27 @@ int aarch32_setup_vectors_page(struct linux_binprm *bprm, int uses_interp)
>  }
>  #endif /* CONFIG_COMPAT */
>  
> +static int vdso_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;
> +	unsigned long vdso_size = vdso_end - vdso_start;

You might be able to use vdso_pages here, but it depends on my question
below.

> +
> +	if (vdso_size != new_size)
> +		return -EINVAL;
> +
> +	current->mm->context.vdso = (void *)new_vma->vm_start;
> +
> +	return 0;
> +}
> +
>  static struct vm_special_mapping vdso_spec[2] __ro_after_init = {
>  	{
>  		.name	= "[vvar]",
>  	},
>  	{
>  		.name	= "[vdso]",
> +		.mremap = vdso_mremap,

Does this mean we move the vdso text, but not the data page? How does that
work?

Will