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Date: Sun, 19 Apr 2020 21:29:04 -0400
From: Rich Felker <dalias@libc.org>
To: Nicholas Piggin <npiggin@gmail.com>
Subject: Re: [musl] Powerpc Linux 'scv' system call ABI proposal take 2
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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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 <linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org>

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 
> >> 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.
> > 
> > how would that 'patch' work?
> > 
> > there are many reasons why you don't
> > want libc to write its .text
> 
> I guess I don't know what I'm talking about when it comes to libraries. 
> 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 
> conditional or indirect branch however you implement it.

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.

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

Rich