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

[Andy Lutomirski]

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?

> diff --git a/kernel/signal.c b/kernel/signal.c
> index 0508544..fab6bea 100644
> --- a/kernel/signal.c
> +++ b/kernel/signal.c
> @@ -3100,13 +3100,14 @@ do_sigaltstack (const stack_t __user *uss, stack_t __user *uoss, unsigned long s
>         if (uss) {
>                 void __user *ss_sp;
>                 size_t ss_size;
> -               int ss_flags;
> +               unsigned ss_flags;
> +               unsigned ss_xflags;
>
>                 error = -EFAULT;
>                 if (!access_ok(VERIFY_READ, uss, sizeof(*uss)))
>                         goto out;
>                 error = __get_user(ss_sp, &uss->ss_sp) |
> -                       __get_user(ss_flags, &uss->ss_flags) |
> +                       __get_user(ss_xflags, &uss->ss_flags) |
>                         __get_user(ss_size, &uss->ss_size);
>                 if (error)
>                         goto out;
> @@ -3115,14 +3116,8 @@ do_sigaltstack (const stack_t __user *uss, stack_t __user *uoss, unsigned long s
>                 if (on_sig_stack(sp))
>                         goto out;
>
> +               ss_flags = ss_xflags & SS_VALMASK;

This is poorly named.  How about "ss_mode = ss_flags & SS_MODE_MASK";

--Andy