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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