Re: [PATCH v1 1/8] xen/riscv: make sure that identity mapping isn't bigger then page size

[Jan Beulich <jbeulich@suse.com>]

On 06.06.2023 21:55, Oleksii Kurochko wrote:
> Signed-off-by: Oleksii Kurochko <oleksii.kurochko@gmail.com>

Such commits without description are worrying. This may be okay for
entirely trivial and obvious changes, but that's going to be the
exception.

> --- a/xen/arch/riscv/xen.lds.S
> +++ b/xen/arch/riscv/xen.lds.S
> @@ -20,6 +20,7 @@ SECTIONS
>      . = XEN_VIRT_START;
>      _start = .;
>      .text : {
> +        _idmap_start = .;
>          _stext = .;            /* Text section */
>          *(.text.header)
>  
> @@ -35,6 +36,7 @@ SECTIONS
>          *(.gnu.warning)
>          . = ALIGN(POINTER_ALIGN);
>          _etext = .;             /* End of text section */
> +        _idmap_end = .;
>      } :text

So this covers all of .text. Why is it expected that .text will be (and
remain) ...

> @@ -174,3 +176,10 @@ ASSERT(!SIZEOF(.got),      ".got non-empty")
>  ASSERT(!SIZEOF(.got.plt),  ".got.plt non-empty")
>  
>  ASSERT(_end - _start <= MB(2), "Xen too large for early-boot assumptions")
> +
> +/*
> + * We require that Xen is loaded at a page boundary, so this ensures that any
> + * code running on the identity map cannot cross a page boundary.
> + */
> +ASSERT(IS_ALIGNED(_idmap_start, PAGE_SIZE), "_idmap_start should be page-aligned")
> +ASSERT(_idmap_end - _idmap_start <= PAGE_SIZE, "Identity mapped code is larger than a page size")

... less than 4k in size? And why is only .text of interest, but not
other sections?

I find the other assertion a little puzzling too: Isn't that merely
checking that XEN_VIRT_START is page aligned?

Jan