Re: [PATCH] arm/vdso: introduce vdso_mremap hook

[Christopher Covington]

Hi Dmitry,

On 11/01/2016 01:22 PM, Dmitry Safonov wrote:
>   Add vdso_mremap hook which will fix context.vdso pointer after mremap()
> on vDSO vma. This is needed for correct landing after syscall execution.
> Primary goal of this is for CRIU on arm - we need to restore vDSO image
> at the exactly same place where the vma was in dumped application. With
> the help of this hook we'll move vDSO at the new position.
>   The CRIU code handles situations like when vDSO of dumped application
> was different from vDSO on restoring system. This usally happens when
> some new symbols are being added to vDSO. In these situations CRIU
> inserts jump trampolines from old vDSO blob to new vDSO on restore.
> By that reason even if on restore vDSO blob lies on the same address as
> blob in dumped application - we still need to move it if it differs.
> 
>   There was previously attempt to add this functionality for arm64 by
> arch_mremap hook [1], while this patch introduces this with minimal
> effort - the same way I've added it to x86:
> commit b059a453b1cf ("x86/vdso: Add mremap hook to vm_special_mapping")
> 
>   At this moment, vdso restoring code is disabled for arm/arm64 arch
> in CRIU [2], so C/R is only working for !CONFIG_VDSO kernels. This patch
> is aimed to fix that.
>   The same hook may be introduced for arm64 kernel, but at this moment
> arm64 vdso code is actively reworked by Kevin, so we can do it on top.
>   Separately, I've refactored arch_remap hook out from ppc64 [3].
> 
> [1]: https://marc.info/?i=1448455781-26660-1-git-send-email-cov@xxxxxxxxxxxxxx
> [2]: https://github.com/xemul/criu/blob/master/Makefile#L39
> [3]: https://marc.info/?i=20161027170948.8279-1-dsafonov@xxxxxxxxxxxxx
> 
> Cc: Kevin Brodsky <kevin.brodsky@xxxxxxx>
> Cc: Christopher Covington <cov@xxxxxxxxxxxxxx>
> Cc: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
> Cc: Oleg Nesterov <oleg@xxxxxxxxxx>
> Cc: Russell King <linux@xxxxxxxxxxxxxxx>
> Cc: Will Deacon <will.deacon@xxxxxxx>
> Cc: linux-arm-kernel@xxxxxxxxxxxxxxxxxxx
> Cc: linux-mm@xxxxxxxxx
> Cc: Cyrill Gorcunov <gorcunov@xxxxxxxxxx>
> Cc: Pavel Emelyanov <xemul@xxxxxxxxxxxxx>
> Signed-off-by: Dmitry Safonov <dsafonov@xxxxxxxxxxxxx>
> ---
>  arch/arm/kernel/vdso.c | 21 +++++++++++++++++++++
>  1 file changed, 21 insertions(+)
> 
> diff --git a/arch/arm/kernel/vdso.c b/arch/arm/kernel/vdso.c
> index 53cf86cf2d1a..d1001f87c2f6 100644
> --- a/arch/arm/kernel/vdso.c
> +++ b/arch/arm/kernel/vdso.c
> @@ -54,8 +54,11 @@ static const struct vm_special_mapping vdso_data_mapping = {
>  	.pages = &vdso_data_page,
>  };
>  
> +static int vdso_mremap(const struct vm_special_mapping *sm,
> +		struct vm_area_struct *new_vma);
>  static struct vm_special_mapping vdso_text_mapping __ro_after_init = {
>  	.name = "[vdso]",
> +	.mremap = vdso_mremap,
>  };
>  
>  struct elfinfo {
> @@ -254,6 +257,24 @@ void arm_install_vdso(struct mm_struct *mm, unsigned long addr)
>  		mm->context.vdso = addr;
>  }
>  
> +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_total_pages - 1) << PAGE_SHIFT;
> +
> +	/* Disallow partial vDSO blob remap */
> +	if (vdso_size != new_size)
> +		return -EINVAL;
> +
> +	if (WARN_ON_ONCE(current->mm != new_vma->vm_mm))
> +		return -EFAULT;
> +
> +	current->mm->context.vdso = new_vma->vm_start;
> +
> +	return 0;
> +}
> +
>  static void vdso_write_begin(struct vdso_data *vdata)
>  {
>  	++vdso_data->seq_count;
> 

What do you think about putting this code somewhere generic (not under
arch/*), so that powerpc and arm64 can reuse it once the cosmetic changes
to make them compatible are made? My thought was that it could be defined
underneath CONFIG_GENERIC_VDSO, which architectures could select as they
became compatible.

Thanks,
Cov

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