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From: Florian Weimer <fw@deneb.enyo.de>
To: Nicholas Piggin <npiggin@gmail.com>
Subject: Re: [musl] Powerpc Linux 'scv' system call ABI proposal take 2
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Date: Tue, 21 Apr 2020 11:57:00 +0200
In-Reply-To: <20200420211751.GF23945@port70.net> (Szabolcs Nagy's message of
 "Mon, 20 Apr 2020 23:17:51 +0200")
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Cc: libc-dev@lists.llvm.org, Rich Felker <dalias@libc.org>,
 Nicholas Piggin via Libc-alpha <libc-alpha@sourceware.org>,
 linuxppc-dev@lists.ozlabs.org, musl@lists.openwall.com
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* Szabolcs Nagy:

> * Nicholas Piggin <npiggin@gmail.com> [2020-04-20 12:08:36 +1000]:
>> Excerpts from Rich Felker's message of April 20, 2020 11:29 am:
>> > 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.
>
> "frowned upon" means that users may have to update
> their security policy setting in pax, selinux, apparmor,
> seccomp bpf filters and who knows what else that may
> monitor and flag W&X mprotect.
>
> libc update can break systems if the new libc does W&X.

It's possible to map over pre-compiled alternative implementations,
though.  Basically, we would do the patching and build time and store
the results in the file.

It works best if the variance is concentrated on a few pages, and
there are very few alternatives.  For example, having two syscall APIs
and supporting threading and no-threading versions would need four
code versions in total, which is likely excessive.