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Message-ID: <20250807192718.GP1827@brightrain.aerifal.cx>
Date: Thu, 7 Aug 2025 15:27:18 -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 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.
Rich
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