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From: Michael Ellerman <mpe@ellerman.id.au>
To: Nicholas Piggin <npiggin@gmail.com>, linuxppc-dev@lists.ozlabs.org
Subject: Re: [PATCH 2/7] powerpc: move __end_rodata to cover arch read-only sections
Date: Fri, 16 Sep 2022 16:35:14 +1000	[thread overview]
Message-ID: <87zgezltct.fsf@mpe.ellerman.id.au> (raw)
In-Reply-To: <CMXEFN5FQ3F8.3QA0U4XPG6JN0@bobo>

"Nicholas Piggin" <npiggin@gmail.com> writes:
> 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?

Yeah that should work.

I'd be happier if we had something more explicit to document that
boundary, I'll send a patch.

cheers

  reply	other threads:[~2022-09-16  6:35 UTC|newest]

Thread overview: 15+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-09-14 15:47 [PATCH 0/7] powerpc: build / linker improvements Nicholas Piggin
2022-09-14 15:47 ` [PATCH 1/7] powerpc/build: put sys_call_table in .data.rel.ro if RELOCATABLE Nicholas Piggin
2022-09-15  5:53   ` Christophe Leroy
2022-09-15 12:51     ` Michael Ellerman
2022-09-16  0:30       ` Nicholas Piggin
2022-09-14 15:47 ` [PATCH 2/7] powerpc: move __end_rodata to cover arch read-only sections Nicholas Piggin
2022-09-15  5:50   ` Christophe Leroy
2022-09-15 12:47   ` Michael Ellerman
2022-09-16  0:28     ` Nicholas Piggin
2022-09-16  6:35       ` Michael Ellerman [this message]
2022-09-14 15:47 ` [PATCH 3/7] powerpc/32/build: move got1/got2 sections out of text Nicholas Piggin
2022-09-14 15:47 ` [PATCH 4/7] powerpc/build: move got, toc, plt, branch_lt sections to read-only Nicholas Piggin
2022-09-14 15:47 ` [PATCH 5/7] powerpc/build: move .data.rel.ro, .sdata2 " Nicholas Piggin
2022-09-14 15:47 ` [PATCH 6/7] powerpc/64/build: only include .opd with ELFv1 Nicholas Piggin
2022-09-14 15:47 ` [PATCH 7/7] powerpc/64/build: merge .got and .toc input sections Nicholas Piggin

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