linux-kernel - Re: [PATCH] riscv: vdso: map data page before vDSO code

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Date:   Sun, 29 Aug 2021 13:16:49 +0300
From:   Sergey Larin <cerg2010cerg2010@...l.ru>
To:     Kefeng Wang <wangkefeng.wang@...wei.com>
Cc:     Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] riscv: vdso: map data page before vDSO code

On Sun, Aug 29, 2021 at 05:59:04PM +0800, Kefeng Wang wrote:
> Hi Sergey
> 
> There is already one fix,
> 
> https://patchwork.kernel.org/project/linux-riscv/list/?series=534877
> 

Oh, I missed it. That one looks cleaner. Thanks anyway!

> On 2021/8/29 17:47, Sergey Larin wrote:
> > Current vDSO implementation assumes that the code size always fits in
> > single page, and the data page follows it:
> > 
> > 	PROVIDE(_vdso_data = . + PAGE_SIZE);
> > 
> > However, this was not the case with my kernel build - the
> > shared object had the size of 4800 bytes. This, obviously, is more than
> > 4096 and requires second page for the rest of the data.
> > 
> > CLOCK_REALTIME_COARSE clock became broken. It was always returning 0
> > because vDSO code was reading the second code page, not the
> > data page. Glibc uses this clock for the time() function.
> > 
> > So instead of computing the offset for the data page (it is necessary to
> > do in runtime - you can't know the size of the binary while you're
> > building it) simply move it behind the code like the ARM does:
> > 
> > 	PROVIDE(_vdso_data = . - PAGE_SIZE);
> > 
> > This commit also fixes arch_vma_name for the data page - it was
> > reporting the same '[vdso]' name for it in my case.
> > 
> > Since I don't have the real hardware, the change was debugged with KGDB
> > in RVVM and also verified in QEMU.
> > 
> > Signed-off-by: Sergey Larin <cerg2010cerg2010@...l.ru>
> > ---
> >   arch/riscv/kernel/vdso.c          | 22 +++++++++++-----------
> >   arch/riscv/kernel/vdso/vdso.lds.S |  2 +-
> >   2 files changed, 12 insertions(+), 12 deletions(-)
> > 
> > diff --git a/arch/riscv/kernel/vdso.c b/arch/riscv/kernel/vdso.c
> > index 25a3b8849599..0c49390e9be3 100644
> > --- a/arch/riscv/kernel/vdso.c
> > +++ b/arch/riscv/kernel/vdso.c
> > @@ -44,13 +44,13 @@ static int __init vdso_init(void)
> >   		return -ENOMEM;
> >   	}
> > +	vdso_pagelist[0] = virt_to_page(vdso_data);
> >   	for (i = 0; i < vdso_pages; i++) {
> >   		struct page *pg;
> >   		pg = virt_to_page(vdso_start + (i << PAGE_SHIFT));
> > -		vdso_pagelist[i] = pg;
> > +		vdso_pagelist[i + 1] = pg;
> >   	}
> > -	vdso_pagelist[i] = virt_to_page(vdso_data);
> >   	return 0;
> >   }
> > @@ -77,21 +77,21 @@ int arch_setup_additional_pages(struct linux_binprm *bprm,
> >   	 * install_special_mapping or the perf counter mmap tracking code
> >   	 * will fail to recognise it as a vDSO (since arch_vma_name fails).
> >   	 */
> > -	mm->context.vdso = (void *)vdso_base;
> > +	mm->context.vdso = (void *)vdso_base + PAGE_SIZE;
> > -	ret =
> > -	   install_special_mapping(mm, vdso_base, vdso_pages << PAGE_SHIFT,
> > -		(VM_READ | VM_EXEC | VM_MAYREAD | VM_MAYWRITE | VM_MAYEXEC),
> > -		vdso_pagelist);
> > +	ret = install_special_mapping(mm, vdso_base, PAGE_SIZE,
> > +		(VM_READ | VM_MAYREAD), &vdso_pagelist[0]);
> >   	if (unlikely(ret)) {
> >   		mm->context.vdso = NULL;
> >   		goto end;
> >   	}
> > -	vdso_base += (vdso_pages << PAGE_SHIFT);
> > -	ret = install_special_mapping(mm, vdso_base, PAGE_SIZE,
> > -		(VM_READ | VM_MAYREAD), &vdso_pagelist[vdso_pages]);
> > +	vdso_base += PAGE_SIZE;
> > +	ret =
> > +	   install_special_mapping(mm, vdso_base, vdso_pages << PAGE_SHIFT,
> > +		(VM_READ | VM_EXEC | VM_MAYREAD | VM_MAYWRITE | VM_MAYEXEC),
> > +		&vdso_pagelist[1]);
> >   	if (unlikely(ret))
> >   		mm->context.vdso = NULL;
> > @@ -105,7 +105,7 @@ const char *arch_vma_name(struct vm_area_struct *vma)
> >   	if (vma->vm_mm && (vma->vm_start == (long)vma->vm_mm->context.vdso))
> >   		return "[vdso]";
> >   	if (vma->vm_mm && (vma->vm_start ==
> > -			   (long)vma->vm_mm->context.vdso + PAGE_SIZE))
> > +			   (long)vma->vm_mm->context.vdso - PAGE_SIZE))
> >   		return "[vdso_data]";
> >   	return NULL;
> >   }
> > diff --git a/arch/riscv/kernel/vdso/vdso.lds.S b/arch/riscv/kernel/vdso/vdso.lds.S
> > index e6f558bca71b..fd8a31075256 100644
> > --- a/arch/riscv/kernel/vdso/vdso.lds.S
> > +++ b/arch/riscv/kernel/vdso/vdso.lds.S
> > @@ -8,7 +8,7 @@ OUTPUT_ARCH(riscv)
> >   SECTIONS
> >   {
> > -	PROVIDE(_vdso_data = . + PAGE_SIZE);
> > +	PROVIDE(_vdso_data = . - PAGE_SIZE);
> >   	. = SIZEOF_HEADERS;
> >   	.hash		: { *(.hash) }			:text