Re: [PATCH 2/7] powerpc: move __end_rodata to cover arch read-only sections

["Nicholas Piggin" <npiggin@gmail.com>]

On Thu Sep 15, 2022 at 10:47 PM AEST, Michael Ellerman wrote:
> Nicholas Piggin <npiggin@gmail.com> writes:
> > powerpc has a number of read-only sections and tables that are put
> > after RO_DATA(). Move the __end_rodata symbol to cover these as well.
> >
> > Setting memory to read-only at boot is done using __init_begin,
> > change that that to use __end_rodata.
>
> Did you just do that because it seems logical?

I actually was looking at moving init so runtime code and data is
closer.

> Because it does seem logical, but it leaves a RWX region in the gap
> between __end_rodata and __init_begin, which is bad.
>
> This is the current behaviour, on radix:
>
> ---[ Start of kernel VM ]---
> 0xc000000000000000-0xc000000001ffffff  0x0000000000000000        32M         r      X   pte  valid  present        dirty  accessed
> 0xc000000002000000-0xc00000007fffffff  0x0000000002000000      2016M         r  w       pte  valid  present        dirty  accessed
>
> And with your change:
>
> ---[ Start of kernel VM ]---
> 0xc000000000000000-0xc0000000013fffff  0x0000000000000000        20M         r      X   pte  valid  present        dirty  accessed
> 0xc000000001400000-0xc000000001ffffff  0x0000000001400000        12M         r  w   X   pte  valid  present        dirty  accessed
> 0xc000000002000000-0xc00000007fffffff  0x0000000002000000      2016M         r  w       pte  valid  present        dirty  accessed
>
>
> On radix the 16M alignment is larger than we need, but we need to chose
> a value at build time that works for radix and hash.
>
> We could make the code smarter on radix, to mark those pages in between
> __end_rodata and __init_begin as RW_ and use them for data. But that
> would be a more involved change.

Ah, yes Christophe pointed out it's broken too. We could just align
__end_rodata to STRICT_ALIGN_SIZE for this patch?

Thanks,
Nick