Re: [PATCH] powerpc64: use a type for mcontext_t regs field

[Rich Felker <dalias@libc.org>]

On Tue, Feb 12, 2019 at 12:05:17PM -0500, David Edelsohn wrote:
> On Tue, Feb 12, 2019 at 11:18 AM Rich Felker <dalias@libc.org> wrote:
> >
> > On Tue, Feb 12, 2019 at 09:35:22AM -0600, A. Wilcox wrote:
> > > GCC Go dereferences `regs` for `nip`.  Without this change, compilation
> > > fails with the following message:
> > >
> > > .../../../libgo/runtime/go-signal.c: In function ‘getSiginfo’:
> > > .../../../libgo/runtime/go-signal.c:225:56: warning: dereferencing ‘void *’ pointer
> > >   ret.sigpc = ((ucontext_t*)(context))->uc_mcontext.regs->nip;
> > >                                                         ^~
> > > .../../../libgo/runtime/go-signal.c:225:56: error: request for member ‘nip’ in something not a structure or union
> > > ---
> > >  arch/powerpc64/bits/signal.h | 4 +++-
> > >  1 file changed, 3 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/arch/powerpc64/bits/signal.h b/arch/powerpc64/bits/signal.h
> > > index 34693a68..6736c69a 100644
> > > --- a/arch/powerpc64/bits/signal.h
> > > +++ b/arch/powerpc64/bits/signal.h
> > > @@ -8,6 +8,8 @@
> > >
> > >  #if defined(_GNU_SOURCE) || defined(_BSD_SOURCE)
> > >
> > > +#include <bits/user.h>
> > > +
> >
> > This is almost surely not right. You can never use a bits/* header
> > except from the corresponding public header that includes it. They do
> > not have multiple-inclusion guards and are not designed to be usable
> > independently. And including sys/user.h is also almost surely wrong
> > since it's a problematic header you don't want getting included
> > (conflicts with linux/*.h stuff).
> >
> > >  typedef unsigned long greg_t, gregset_t[48];
> > >
> > >  typedef struct {
> > > @@ -29,7 +31,7 @@ typedef struct sigcontext {
> > >       int _pad0;
> > >       unsigned long handler;
> > >       unsigned long oldmask;
> > > -     void *regs;
> > > +     struct pt_regs *regs;
> > >       gregset_t gp_regs;
> > >       fpregset_t fp_regs;
> > >       vrregset_t *v_regs;
> > > --
> > > 2.19.2
> >
> > Do you know if there's a reason Go is trying to use this regs member
> > rather than gp_regs? I think that's the real issue here.
> 
> Apparently GCCGo runtime has dependencies on GLibc and has not been
> ported to Musl Libc.  The GCCGo runtime is trying to obtain the PC for
> a signal.
> 
> #ifdef __PPC__
>  #ifdef __linux__
>        ret.sigpc = ((ucontext_t*)(context))->uc_mcontext.regs->nip;
>  #endif
>  #ifdef _AIX
>        ret.sigpc = ((ucontext_t*)(context))->uc_mcontext.jmp_context.iar;
>  #endif
> #endif
> 
> The program counter is not part of gp_regs.

Yes it is, as musl already depends on having access to the program
counter for thread cancellation. See arch/powerpc64/pthread_arch.h:

// the kernel calls the ip "nip", it's the first saved value after the 32
// GPRs.
#define MC_PC gp_regs[32]

Rich