Re: [PATCH 1/2] sigaltstack: implement SS_AUTODISARM flag

[Stas Sergeev]

06.03.2016 23:10, Andy Lutomirski ÐÐÑÐÑ:
> On Sun, Mar 6, 2016 at 12:07 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
> > On Mon, Feb 29, 2016 at 1:29 PM, Stas Sergeev <stsp@xxxxxxx> wrote:
> > > This patch implements the SS_AUTODISARM flag that can be ORed with
> > > SS_ONSTACK when forming ss_flags.
> > > When this flag is set, sigaltstack will be disabled when entering
> > > the signal handler; more precisely, after saving sas to uc_stack.
> > > When leaving the signal handler, the sigaltstack is restored by
> > > uc_stack.
> > > When this flag is used, it is safe to switch from sighandler with
> > > swapcontext(). Without this flag, the subsequent signal will corrupt
> > > the state of the switched-away sighandler.
> > >
> > > CC: Ingo Molnar <mingo@xxxxxxxxxx>
> > > CC: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> > > CC: Richard Weinberger <richard@xxxxxx>
> > > CC: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> > > CC: Oleg Nesterov <oleg@xxxxxxxxxx>
> > > CC: Tejun Heo <tj@xxxxxxxxxx>
> > > CC: Heinrich Schuchardt <xypron.glpk@xxxxxx>
> > > CC: Jason Low <jason.low2@xxxxxx>
> > > CC: Andrea Arcangeli <aarcange@xxxxxxxxxx>
> > > CC: Frederic Weisbecker <fweisbec@xxxxxxxxx>
> > > CC: Konstantin Khlebnikov <khlebnikov@xxxxxxxxxxxxxx>
> > > CC: Josh Triplett <josh@xxxxxxxxxxxxxxxx>
> > > CC: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>
> > > CC: Aleksa Sarai <cyphar@xxxxxxxxxx>
> > > CC: "Amanieu d'Antras" <amanieu@xxxxxxxxx>
> > > CC: Paul Moore <pmoore@xxxxxxxxxx>
> > > CC: Sasha Levin <sasha.levin@xxxxxxxxxx>
> > > CC: Palmer Dabbelt <palmer@xxxxxxxxxxx>
> > > CC: Vladimir Davydov <vdavydov@xxxxxxxxxxxxx>
> > > CC: linux-kernel@xxxxxxxxxxxxxxx
> > > CC: linux-api@xxxxxxxxxxxxxxx
> > > CC: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
> > >
> > > Signed-off-by: Stas Sergeev <stsp@xxxxxxxxxxxxxxxxxxxxx>
> > > ---
> > >   include/linux/sched.h       |  8 ++++++++
> > >   include/linux/signal.h      |  4 +++-
> > >   include/uapi/linux/signal.h |  3 +++
> > >   kernel/fork.c               |  2 +-
> > >   kernel/signal.c             | 23 ++++++++++++-----------
> > >   5 files changed, 27 insertions(+), 13 deletions(-)
> > >
> > > diff --git a/include/linux/sched.h b/include/linux/sched.h
> > > index a10494a..26201cd 100644
> > > --- a/include/linux/sched.h
> > > +++ b/include/linux/sched.h
> > > @@ -1587,6 +1587,7 @@ struct task_struct {
> > >
> > >          unsigned long sas_ss_sp;
> > >          size_t sas_ss_size;
> > > +       unsigned sas_ss_flags;
> > >
> > >          struct callback_head *task_works;
> > >
> > > @@ -2573,6 +2574,13 @@ static inline int sas_ss_flags(unsigned long sp)
> > >          return on_sig_stack(sp) ? SS_ONSTACK : 0;
> > >   }
> > >
> > > +static inline void sas_ss_reset(struct task_struct *p)
> > > +{
> > > +       p->sas_ss_sp = 0;
> > > +       p->sas_ss_size = 0;
> > > +       p->sas_ss_flags = SS_DISABLE;
> > > +}
> > > +
> > >   static inline unsigned long sigsp(unsigned long sp, struct ksignal *ksig)
> > >   {
> > >          if (unlikely((ksig->ka.sa.sa_flags & SA_ONSTACK)) && ! sas_ss_flags(sp))
> > > diff --git a/include/linux/signal.h b/include/linux/signal.h
> > > index 92557bb..3fbe814 100644
> > > --- a/include/linux/signal.h
> > > +++ b/include/linux/signal.h
> > > @@ -432,8 +432,10 @@ int __save_altstack(stack_t __user *, unsigned long);
> > >          stack_t __user *__uss = uss; \
> > >          struct task_struct *t = current; \
> > >          put_user_ex((void __user *)t->sas_ss_sp, &__uss->ss_sp); \
> > > -       put_user_ex(sas_ss_flags(sp), &__uss->ss_flags); \
> > > +       put_user_ex(t->sas_ss_flags, &__uss->ss_flags); \
> > >          put_user_ex(t->sas_ss_size, &__uss->ss_size); \
> > > +       if (t->sas_ss_flags & SS_AUTODISARM) \
> > > +               sas_ss_reset(t); \
> > >   } while (0);
> > >
> > >   #ifdef CONFIG_PROC_FS
> > > diff --git a/include/uapi/linux/signal.h b/include/uapi/linux/signal.h
> > > index e1bd50c2..4691bc5 100644
> > > --- a/include/uapi/linux/signal.h
> > > +++ b/include/uapi/linux/signal.h
> > > @@ -6,5 +6,8 @@
> > >
> > >   #define SS_ONSTACK     1
> > >   #define SS_DISABLE     2
> > > +#define SS_VALMASK     0xf
> > SS_MODE_MASK, perhaps?
> Actually, let's invert that.
>
> #define SS_AUTODISARM (1U << 31)
> #define SS_FLAG_BITS SS_AUTODISARM
>
> ss_mode = ss_flags & ~SS_FLAG_BITS;
>
> this way flag bits that are currently undefined will continue to trigger EINVAL.
OK, will do.
Though I am still going to keep SS_AUTODISARM =(1 << 4) -
it doesn't matter from what side we add new flags, does it?