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

[Kefeng Wang]

Hi Sergey

There is already one fix,

https://patchwork.kernel.org/project/linux-riscv/list/?series=534877

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@xxxxxxx>
> ---
>   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