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Date: Sun, 6 Mar 2016 12:55:19 -0800
From: Andy Lutomirski <luto@...capital.net>
To: Stas Sergeev <stsp@...t.ru>
Cc: Ingo Molnar <mingo@...hat.com>,
Peter Zijlstra <peterz@...radead.org>,
Richard Weinberger <richard@....at>,
Andrew Morton <akpm@...ux-foundation.org>,
Oleg Nesterov <oleg@...hat.com>, Tejun Heo <tj@...nel.org>,
Heinrich Schuchardt <xypron.glpk@....de>,
Jason Low <jason.low2@...com>,
Andrea Arcangeli <aarcange@...hat.com>,
Frederic Weisbecker <fweisbec@...il.com>,
Konstantin Khlebnikov <khlebnikov@...dex-team.ru>,
Josh Triplett <josh@...htriplett.org>,
"Eric W. Biederman" <ebiederm@...ssion.com>,
Aleksa Sarai <cyphar@...har.com>,
"Amanieu d'Antras" <amanieu@...il.com>,
Paul Moore <pmoore@...hat.com>,
Sasha Levin <sasha.levin@...cle.com>,
Palmer Dabbelt <palmer@...belt.com>,
Vladimir Davydov <vdavydov@...allels.com>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
Linux API <linux-api@...r.kernel.org>,
Stas Sergeev <stsp@...rs.sourceforge.net>
Subject: Re: [PATCH 1/2] sigaltstack: implement SS_AUTODISARM flag
On Sun, Mar 6, 2016 at 12:17 PM, Stas Sergeev <stsp@...t.ru> wrote:
> 06.03.2016 23:10, Andy Lutomirski пишет:
>
>> On Sun, Mar 6, 2016 at 12:07 PM, Andy Lutomirski <luto@...capital.net>
>> wrote:
>>>
>>> On Mon, Feb 29, 2016 at 1:29 PM, Stas Sergeev <stsp@...t.ru> 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@...hat.com>
>>>> CC: Peter Zijlstra <peterz@...radead.org>
>>>> CC: Richard Weinberger <richard@....at>
>>>> CC: Andrew Morton <akpm@...ux-foundation.org>
>>>> CC: Oleg Nesterov <oleg@...hat.com>
>>>> CC: Tejun Heo <tj@...nel.org>
>>>> CC: Heinrich Schuchardt <xypron.glpk@....de>
>>>> CC: Jason Low <jason.low2@...com>
>>>> CC: Andrea Arcangeli <aarcange@...hat.com>
>>>> CC: Frederic Weisbecker <fweisbec@...il.com>
>>>> CC: Konstantin Khlebnikov <khlebnikov@...dex-team.ru>
>>>> CC: Josh Triplett <josh@...htriplett.org>
>>>> CC: "Eric W. Biederman" <ebiederm@...ssion.com>
>>>> CC: Aleksa Sarai <cyphar@...har.com>
>>>> CC: "Amanieu d'Antras" <amanieu@...il.com>
>>>> CC: Paul Moore <pmoore@...hat.com>
>>>> CC: Sasha Levin <sasha.levin@...cle.com>
>>>> CC: Palmer Dabbelt <palmer@...belt.com>
>>>> CC: Vladimir Davydov <vdavydov@...allels.com>
>>>> CC: linux-kernel@...r.kernel.org
>>>> CC: linux-api@...r.kernel.org
>>>> CC: Andy Lutomirski <luto@...capital.net>
>>>>
>>>> Signed-off-by: Stas Sergeev <stsp@...rs.sourceforge.net>
>>>> ---
>>>> 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
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