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

From: Michael Ellerman <[email-masked]>
Date: Thu, 15 Sep 2022 22:47:50 +1000

Nicholas Piggin <[email-masked]> 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?

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.

I think if we just drop the changes to the C files this patch is OK to
go in.

cheers