Re: [Qemu-devel] [PATCH] linux-user: fill target sigcontext struct accordingly

[joserz@linux.vnet.ibm.com]

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On Wed, Feb 01, 2017 at 09:43:57PM +0100, Laurent Vivier wrote:
> Le 31/01/2017 à 23:05, Jose Ricardo Ziviani a écrit :
> > A segfault is noticed when an emulated program uses any of ucontext
> > regs fields. Risu detected this issue in the following operation when
> > handling a signal:
> >   ucontext_t *uc = (ucontext_t*)uc;
> >   uc->uc_mcontext.regs->nip += 4;
> > 
> > but this works fine:
> >   uc->uc_mcontext.gp_regs[PT_NIP] += 4;
> > 
> > This patch set regs to a valid location as well as other sigcontext
> > fields.
> > 
> > Signed-off-by: Jose Ricardo Ziviani <joserz@linux.vnet.ibm.com>
> > ---
> >  linux-user/signal.c | 5 +++++
> >  1 file changed, 5 insertions(+)
> > 
> > diff --git a/linux-user/signal.c b/linux-user/signal.c
> > index 5064de0..8209539 100644
> > --- a/linux-user/signal.c
> > +++ b/linux-user/signal.c
> > @@ -5155,6 +5155,7 @@ static void setup_rt_frame(int sig, struct target_sigaction *ka,
> >      target_ulong rt_sf_addr, newsp = 0;
> >      int i, err = 0;
> >  #if defined(TARGET_PPC64)
> > +    struct target_sigcontext *sc = 0;
> >      struct image_info *image = ((TaskState *)thread_cpu->opaque)->info;
> >  #endif
> >  
> > @@ -5183,6 +5184,10 @@ static void setup_rt_frame(int sig, struct target_sigaction *ka,
> >  #if defined(TARGET_PPC64)
> >      mctx = &rt_sf->uc.tuc_sigcontext.mcontext;
> >      trampptr = &rt_sf->trampoline[0];
> > +
> > +    sc = &rt_sf->uc.tuc_sigcontext;
> > +    __put_user(h2g(mctx), &sc->regs);
> > +    __put_user(sig, &sc->signal);
> >  #else
> >      mctx = &rt_sf->uc.tuc_mcontext;
> >      trampptr = (uint32_t *)&rt_sf->uc.tuc_mcontext.tramp;
> > 
> 
> Reviewed-by: Laurent Vivier <laurent@vivier.eu>
> 
> This is correct, but QEMU and kernel implementation are really different.
> 
> In the kernel:
> 
> handle_rt_signal64()
> ...
>     frame = get_sigframe(ksig, get_tm_stackpointer(tsk),
>                          sizeof(*frame), 0);
> ...
>     err |= setup_sigcontext(&frame->uc.uc_mcontext, tsk, ksig->sig,
>                             NULL,
>                             (unsigned long)ksig->ka.sa.sa_handler,
>                             1);
> 
> static long setup_sigcontext(struct sigcontext __user *sc,
>                 struct task_struct *tsk, int signr, sigset_t *set,
>                 unsigned long handler, int ctx_has_vsx_region)
> 
>         err |= __put_user(&sc->gp_regs, &sc->regs);
> ...
>         err |= __put_user(signr, &sc->signal);
> ...
> 
> According to kernel definition of ucontext:
> 
> struct ucontext {
> ...
> #ifdef __powerpc64__
>         sigset_t        __unused[15];   /* Allow for uc_sigmask growth */
>         struct sigcontext uc_mcontext;  /* last for extensibility */
> #else
> ...
> }
> 
> kernel &frame->uc.uc_mcontext is qemu &rt_sf->uc.tuc_sigcontext
> 
> uc_sigcontext.mcontext doesn't exit in the kernel.
> 
> But QEMU code works because tuc_sigcontext.mcontext is where we have the
> CPU registers in sigcontext:
> 
> kernel:
> 
> struct sigcontext {
>         unsigned long   _unused[4];
>         int             signal;
> #ifdef __powerpc64__
>         int             _pad0;
> #endif
>         unsigned long   handler;
>         unsigned long   oldmask;
>         struct pt_regs  __user *regs;
> #ifdef __powerpc64__
>         elf_gregset_t   gp_regs;
>         elf_fpregset_t  fp_regs;
> ...
> 
> Qemu:
> 
> struct target_sigcontext {
>     target_ulong _unused[4];
>     int32_t signal;
> #if defined(TARGET_PPC64)
>     int32_t pad0;
> #endif
>     target_ulong handler;
>     target_ulong oldmask;
>     target_ulong regs;      /* struct pt_regs __user * */
> #if defined(TARGET_PPC64)
>     struct target_mcontext mcontext;
> #endif
> };
> 
> struct target_mcontext {
>     target_ulong mc_gregs[48];
>     /* Includes fpscr.  */
>     uint64_t mc_fregs[33];
> ...
> 
> I think we do like that to use the same
> save_user_regs()/save_user_regs() functions with PPC and PPC64... but
> comparison with kernel becomes harder.
> 
> Laurent
>