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Message-ID: <CAFdMc-3vNEawj9cjdqCGq4FB21zhhs6KgKCeY1prxQejBW=hHQ@mail.gmail.com>
Date: Fri, 8 Aug 2025 00:50:03 -0300
From: Daniel Gutson <danielgutson@...il.com>
To: musl@...ts.openwall.com
Cc: David Edelsohn <dje.gcc@...il.com>, Sertonix <sertonix@...teo.net>
Subject: Re: ctr registry after syscall on powerpc
El jue, 7 de ago de 2025, 16:47, Rich Felker <dalias@...c.org> escribió:
> On Thu, Aug 07, 2025 at 03:27:18PM -0400, Rich Felker wrote:
> > On Wed, Aug 06, 2025 at 11:23:50AM -0400, David Edelsohn wrote:
> > > On Wed, Aug 6, 2025 at 11:07 AM Sertonix <sertonix@...teo.net> wrote:
> > >
> > > > (Sorry for the noise, I don't know any better place to ask)
> > > >
> > > > With GCC 15 I get a compiled musl libc that stores a value in the ctr
> > > > registry, doing a syscall which changes the ctr registry and then
> > > > reading back garbage data when trying to read the original value. I
> > > > unfortunatly couldn't find any information do determine if this is an
> > > > issue in musl, gcc or the kernel.
> > > >
> > > > The relevant code from src/malloc/mallocng/malloc.c:
> > > >
> > > > size_t pagesize = PGSZ; // stored into ctr registry
> > > > ...
> > > > ctx.brk = brk(0); // mess up ctr registry
> > > > ...
> > > > ctx.brk += -ctx.brk & (pagesize-1); // try to read back ctr
> > > > registry
> > > >
> > > > When I mark ctr as clobber in __syscall1 I don't see this issue. Is
> > > > that a correct fix?
> > > >
> > >
> > > The CTR register is volatile in the ELFv2 (and other) PowerPC ABIs, and
> > > that behavior is not changed by the Linux kernel system call ABI. It's
> > > unusual that GCC is allocating / spilling a value to CTR and that it is
> > > assuming the register is valid across a call. There must be more that
> is
> > > affecting the dataflow analysis and causing this strange behavior.
> > >
> > > I would open a GCC bug. Please include details about exactly which GCC
> > > release and vendor build is being used, and the pre-processed source
> code.
> >
> > It's not across a[n exteral] call. It's just across an (inlineable)
> > syscall. So apparently this is a bug in our syscall asm constraints.
>
> Proposed patch. Anything else like this missing?
>
What about XER?
> Rich
>
Who’s got the sweetest disposition?
One guess, that’s who?
Who’d never, ever start an argument?
Who never shows a bit of temperament?
Who's never wrong but always right?
Who'd never dream of starting a fight?
Who get stuck with all the bad luck?
>
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