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Date: Sun, 19 Apr 2020 21:29:04 -0400
From: Rich Felker <dalias@...c.org>
To: Nicholas Piggin <npiggin@...il.com>
Cc: Nicholas Piggin via Libc-alpha <libc-alpha@...rceware.org>,
Szabolcs Nagy <nsz@...t70.net>, libc-dev@...ts.llvm.org,
linuxppc-dev@...ts.ozlabs.org, musl@...ts.openwall.com
Subject: Re: Powerpc Linux 'scv' system call ABI proposal take 2
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@...rceware.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
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