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

On Thu Sep 15, 2022 at 10:47 PM AEST, Michael Ellerman wrote:

Nicholas Piggin [off-list ref] writes:

quoted

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