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Date: Mon, 20 Apr 2020 12:08:36 +1000
From: Nicholas Piggin <npiggin@gmail.com>
Subject: Re: [musl] Powerpc Linux 'scv' system call ABI proposal take 2
To: Rich Felker <dalias@libc.org>
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Cc: libc-dev@lists.llvm.org, Szabolcs Nagy <nsz@port70.net>,
 Nicholas Piggin via Libc-alpha <libc-alpha@sourceware.org>,
 linuxppc-dev@lists.ozlabs.org, musl@lists.openwall.com
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Excerpts from Rich Felker's message of April 20, 2020 11:29 am:
> On Mon, Apr 20, 2020 at 10:27:58AM +1000, Nicholas Piggin wrote:
>> Excerpts from Szabolcs Nagy's message of April 16, 2020 7:58 pm:
>> > * Nicholas Piggin via Libc-alpha <libc-alpha@sourceware.org> [2020-04-=
16 10:16:54 +1000]:
>> >> Well it would have to test HWCAP and patch in or branch to two=20
>> >> completely different sequences including register save/restores yes.
>> >> You could have the same asm and matching clobbers to put the sequence
>> >> inline and then you could patch the one sc/scv instruction I suppose.
>> >=20
>> > how would that 'patch' work?
>> >=20
>> > there are many reasons why you don't
>> > want libc to write its .text
>>=20
>> I guess I don't know what I'm talking about when it comes to libraries.=20
>> Shame if there is no good way to load-time patch libc. It's orthogonal
>> to the scv selection though -- if you don't patch you have to=20
>> conditional or indirect branch however you implement it.
>=20
> Patched pages cannot be shared. The whole design of PIC and shared
> libraries is that the code("text")/rodata is immutable and shared and
> that only a minimal amount of data, packed tightly together (the GOT)
> has to exist per-instance.

Yeah the pages which were patched couldn't be shared across exec, which
is a significant downside, unless you could group all patch sites into
their own section and similarly pack it together (which has issues of
being out of line).

>=20
> Also, allowing patching of executable pages is generally frowned upon
> these days because W^X is a desirable hardening property.

Right, it would want be write-protected after being patched.

Thanks,
Nick