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

[Andy Lutomirski]

On Sun, Mar 6, 2016 at 12:17 PM, Stas Sergeev <stsp@xxxxxxx> wrote:
> 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?

Adding them starting with a high bit means that more low bits are
available for use as mode numbers.  Admittedly, this barely matters.

--Andy

-- 
Andy Lutomirski
AMA Capital Management, LLC