[libc-dev] [musl] Powerpc Linux 'scv' system call ABI proposal take 2

[Nicholas Piggin via libc-dev]

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 at 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.

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).

> 
> 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