Message-ID: <58a3ad23-cbbd-40ff-0b5f-b1128674fccb@arm.com>
Date:   Mon, 12 Apr 2021 13:58:49 +0100
From:   Vincenzo Frascino <vincenzo.frascino@....com>
To:     Christophe Leroy <christophe.leroy@...roup.eu>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>
Cc:     linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        dima@...sta.com, avagin@...il.com, arnd@...db.de,
        tglx@...utronix.de, luto@...nel.org, linux-arch@...r.kernel.org
Subject: Re: [PATCH RESEND v1 3/4] powerpc/vdso: Separate vvar vma from vdso



On 3/31/21 5:48 PM, Christophe Leroy wrote:
> From: Dmitry Safonov <dima@...sta.com>
> 
> Since commit 511157ab641e ("powerpc/vdso: Move vdso datapage up front")
> VVAR page is in front of the VDSO area. In result it breaks CRIU
> (Checkpoint Restore In Userspace) [1], where CRIU expects that "[vdso]"
> from /proc/../maps points at ELF/vdso image, rather than at VVAR data page.
> Laurent made a patch to keep CRIU working (by reading aux vector).
> But I think it still makes sence to separate two mappings into different
> VMAs. It will also make ppc64 less "special" for userspace and as
> a side-bonus will make VVAR page un-writable by debugger (which previously
> would COW page and can be unexpected).
> 
> I opportunistically Cc stable on it: I understand that usually such
> stuff isn't a stable material, but that will allow us in CRIU have
> one workaround less that is needed just for one release (v5.11) on
> one platform (ppc64), which we otherwise have to maintain.
> I wouldn't go as far as to say that the commit 511157ab641e is ABI
> regression as no other userspace got broken, but I'd really appreciate
> if it gets backported to v5.11 after v5.12 is released, so as not
> to complicate already non-simple CRIU-vdso code. Thanks!
> 
> Cc: Andrei Vagin <avagin@...il.com>
> Cc: Andy Lutomirski <luto@...nel.org>
> Cc: Benjamin Herrenschmidt <benh@...nel.crashing.org>
> Cc: Christophe Leroy <christophe.leroy@...roup.eu>
> Cc: Laurent Dufour <ldufour@...ux.ibm.com>
> Cc: Michael Ellerman <mpe@...erman.id.au>
> Cc: Paul Mackerras <paulus@...ba.org>
> Cc: linuxppc-dev@...ts.ozlabs.org
> Cc: stable@...r.kernel.org # v5.11
> [1]: https://github.com/checkpoint-restore/criu/issues/1417
> Signed-off-by: Dmitry Safonov <dima@...sta.com>
> Tested-by: Christophe Leroy <christophe.leroy@...roup.eu>
> Signed-off-by: Christophe Leroy <christophe.leroy@...roup.eu>

Reviewed-by: Vincenzo Frascino <vincenzo.frascino@....com> # vDSO parts.

> ---
>  arch/powerpc/include/asm/mmu_context.h |  2 +-
>  arch/powerpc/kernel/vdso.c             | 54 +++++++++++++++++++-------
>  2 files changed, 40 insertions(+), 16 deletions(-)
> 
> diff --git a/arch/powerpc/include/asm/mmu_context.h b/arch/powerpc/include/asm/mmu_context.h
> index 652ce85f9410..4bc45d3ed8b0 100644
> --- a/arch/powerpc/include/asm/mmu_context.h
> +++ b/arch/powerpc/include/asm/mmu_context.h
> @@ -263,7 +263,7 @@ extern void arch_exit_mmap(struct mm_struct *mm);
>  static inline void arch_unmap(struct mm_struct *mm,
>  			      unsigned long start, unsigned long end)
>  {
> -	unsigned long vdso_base = (unsigned long)mm->context.vdso - PAGE_SIZE;
> +	unsigned long vdso_base = (unsigned long)mm->context.vdso;
>  
>  	if (start <= vdso_base && vdso_base < end)
>  		mm->context.vdso = NULL;
> diff --git a/arch/powerpc/kernel/vdso.c b/arch/powerpc/kernel/vdso.c
> index e839a906fdf2..b14907209822 100644
> --- a/arch/powerpc/kernel/vdso.c
> +++ b/arch/powerpc/kernel/vdso.c
> @@ -55,10 +55,10 @@ static int vdso_mremap(const struct vm_special_mapping *sm, struct vm_area_struc
>  {
>  	unsigned long new_size = new_vma->vm_end - new_vma->vm_start;
>  
> -	if (new_size != text_size + PAGE_SIZE)
> +	if (new_size != text_size)
>  		return -EINVAL;
>  
> -	current->mm->context.vdso = (void __user *)new_vma->vm_start + PAGE_SIZE;
> +	current->mm->context.vdso = (void __user *)new_vma->vm_start;
>  
>  	return 0;
>  }
> @@ -73,6 +73,10 @@ static int vdso64_mremap(const struct vm_special_mapping *sm, struct vm_area_str
>  	return vdso_mremap(sm, new_vma, &vdso64_end - &vdso64_start);
>  }
>  
> +static struct vm_special_mapping vvar_spec __ro_after_init = {
> +	.name = "[vvar]",
> +};
> +
>  static struct vm_special_mapping vdso32_spec __ro_after_init = {
>  	.name = "[vdso]",
>  	.mremap = vdso32_mremap,
> @@ -89,11 +93,11 @@ static struct vm_special_mapping vdso64_spec __ro_after_init = {
>   */
>  static int __arch_setup_additional_pages(struct linux_binprm *bprm, int uses_interp)
>  {
> -	struct mm_struct *mm = current->mm;
> +	unsigned long vdso_size, vdso_base, mappings_size;
>  	struct vm_special_mapping *vdso_spec;
> +	unsigned long vvar_size = PAGE_SIZE;
> +	struct mm_struct *mm = current->mm;
>  	struct vm_area_struct *vma;
> -	unsigned long vdso_size;
> -	unsigned long vdso_base;
>  
>  	if (is_32bit_task()) {
>  		vdso_spec = &vdso32_spec;
> @@ -110,8 +114,8 @@ static int __arch_setup_additional_pages(struct linux_binprm *bprm, int uses_int
>  		vdso_base = 0;
>  	}
>  
> -	/* Add a page to the vdso size for the data page */
> -	vdso_size += PAGE_SIZE;
> +	mappings_size = vdso_size + vvar_size;
> +	mappings_size += (VDSO_ALIGNMENT - 1) & PAGE_MASK;
>  
>  	/*
>  	 * pick a base address for the vDSO in process space. We try to put it
> @@ -119,9 +123,7 @@ static int __arch_setup_additional_pages(struct linux_binprm *bprm, int uses_int
>  	 * and end up putting it elsewhere.
>  	 * Add enough to the size so that the result can be aligned.
>  	 */
> -	vdso_base = get_unmapped_area(NULL, vdso_base,
> -				      vdso_size + ((VDSO_ALIGNMENT - 1) & PAGE_MASK),
> -				      0, 0);
> +	vdso_base = get_unmapped_area(NULL, vdso_base, mappings_size, 0, 0);
>  	if (IS_ERR_VALUE(vdso_base))
>  		return vdso_base;
>  
> @@ -133,7 +135,13 @@ static int __arch_setup_additional_pages(struct linux_binprm *bprm, int uses_int
>  	 * install_special_mapping or the perf counter mmap tracking code
>  	 * will fail to recognise it as a vDSO.
>  	 */
> -	mm->context.vdso = (void __user *)vdso_base + PAGE_SIZE;
> +	mm->context.vdso = (void __user *)vdso_base + vvar_size;
> +
> +	vma = _install_special_mapping(mm, vdso_base, vvar_size,
> +				       VM_READ | VM_MAYREAD | VM_IO |
> +				       VM_DONTDUMP | VM_PFNMAP, &vvar_spec);
> +	if (IS_ERR(vma))
> +		return PTR_ERR(vma);
>  
>  	/*
>  	 * our vma flags don't have VM_WRITE so by default, the process isn't
> @@ -145,9 +153,12 @@ static int __arch_setup_additional_pages(struct linux_binprm *bprm, int uses_int
>  	 * It's fine to use that for setting breakpoints in the vDSO code
>  	 * pages though.
>  	 */
> -	vma = _install_special_mapping(mm, vdso_base, vdso_size,
> +	vma = _install_special_mapping(mm, vdso_base + vvar_size, vdso_size,
>  				       VM_READ | VM_EXEC | VM_MAYREAD |
>  				       VM_MAYWRITE | VM_MAYEXEC, vdso_spec);
> +	if (IS_ERR(vma))
> +		do_munmap(mm, vdso_base, vvar_size, NULL);
> +
>  	return PTR_ERR_OR_ZERO(vma);
>  }
>  
> @@ -249,11 +260,22 @@ static struct page ** __init vdso_setup_pages(void *start, void *end)
>  	if (!pagelist)
>  		panic("%s: Cannot allocate page list for VDSO", __func__);
>  
> -	pagelist[0] = virt_to_page(vdso_data);
> -
>  	for (i = 0; i < pages; i++)
> -		pagelist[i + 1] = virt_to_page(start + i * PAGE_SIZE);
> +		pagelist[i] = virt_to_page(start + i * PAGE_SIZE);
> +
> +	return pagelist;
> +}
> +
> +static struct page ** __init vvar_setup_pages(void)
> +{
> +	struct page **pagelist;
>  
> +	/* .pages is NULL-terminated */
> +	pagelist = kcalloc(2, sizeof(struct page *), GFP_KERNEL);
> +	if (!pagelist)
> +		panic("%s: Cannot allocate page list for VVAR", __func__);
> +
> +	pagelist[0] = virt_to_page(vdso_data);
>  	return pagelist;
>  }
>  
> @@ -295,6 +317,8 @@ static int __init vdso_init(void)
>  	if (IS_ENABLED(CONFIG_PPC64))
>  		vdso64_spec.pages = vdso_setup_pages(&vdso64_start, &vdso64_end);
>  
> +	vvar_spec.pages = vvar_setup_pages();
> +
>  	smp_wmb();
>  
>  	return 0;
> 

-- 
Regards,
Vincenzo

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