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Message-ID: <20250807194737.GQ1827@brightrain.aerifal.cx>
Date: Thu, 7 Aug 2025 15:47:38 -0400
From: Rich Felker <dalias@...c.org>
To: David Edelsohn <dje.gcc@...il.com>
Cc: musl@...ts.openwall.com, Sertonix <sertonix@...teo.net>
Subject: Re: ctr registry after syscall on powerpc
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?
Rich
View attachment "0001-powerpc-64-fix-missing-ctr-register-in-syscall-asm-c.patch" of type "text/plain" (6244 bytes)
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