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[tip:core/signals] signals/sigaltstack: Implement SS_AUTODISARM flag

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  [tip:core/signals] signals/sigaltstack: Implement SS_AUTODISARM  flag tip-bot for Stas Sergeev <tipbot@zytor.com> - 2016-05-03 10:00 +0200
    Re: [tip:core/signals] signals/sigaltstack: Implement SS_AUTODISARM flag Andy Lutomirski <luto@amacapital.net> - 2016-05-03 18:30 +0200

#1393133 — [tip:core/signals] signals/sigaltstack: Implement SS_AUTODISARM flag

Commit-ID:  2a74213838104a41588d86fd5e8d344972891ace
Gitweb:     http://git.kernel.org/tip/2a74213838104a41588d86fd5e8d344972891ace
Author:     Stas Sergeev <stsp@list.ru>
AuthorDate: Thu, 14 Apr 2016 23:20:04 +0300
Committer:  Ingo Molnar <mingo@kernel.org>
CommitDate: Tue, 3 May 2016 08:37:59 +0200

signals/sigaltstack: Implement SS_AUTODISARM flag

This patch implements the SS_AUTODISARM flag that can be OR-ed 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.

To detect the support of this functionality, one can do:

  err = sigaltstack(SS_DISABLE | SS_AUTODISARM);
  if (err && errno == EINVAL)
	unsupported();

Signed-off-by: Stas Sergeev <stsp@list.ru>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Aleksa Sarai <cyphar@cyphar.com>
Cc: Amanieu d'Antras <amanieu@gmail.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: Eric W. Biederman <ebiederm@xmission.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Heinrich Schuchardt <xypron.glpk@gmx.de>
Cc: Jason Low <jason.low2@hp.com>
Cc: Josh Triplett <josh@joshtriplett.org>
Cc: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Palmer Dabbelt <palmer@dabbelt.com>
Cc: Paul Moore <pmoore@redhat.com>
Cc: Pavel Emelyanov <xemul@parallels.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Richard Weinberger <richard@nod.at>
Cc: Sasha Levin <sasha.levin@oracle.com>
Cc: Shuah Khan <shuahkh@osg.samsung.com>
Cc: Tejun Heo <tj@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Vladimir Davydov <vdavydov@parallels.com>
Cc: linux-api@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Link: http://lkml.kernel.org/r/1460665206-13646-4-git-send-email-stsp@list.ru
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
 include/linux/sched.h       |  8 ++++++++
 include/linux/signal.h      |  4 +++-
 include/uapi/linux/signal.h |  4 +++-
 kernel/fork.c               |  2 +-
 kernel/signal.c             | 10 ++++++++--
 5 files changed, 23 insertions(+), 5 deletions(-)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 52c4847..2950c5c 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1596,6 +1596,7 @@ struct task_struct {
 
 	unsigned long sas_ss_sp;
 	size_t sas_ss_size;
+	unsigned sas_ss_flags;
 
 	struct callback_head *task_works;
 
@@ -2592,6 +2593,13 @@ 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 7c73165..7388260 100644
--- a/include/uapi/linux/signal.h
+++ b/include/uapi/linux/signal.h
@@ -7,7 +7,9 @@
 #define SS_ONSTACK	1
 #define SS_DISABLE	2
 
+/* bit-flags */
+#define SS_AUTODISARM	(1 << 4)	/* disable sas during sighandling */
 /* mask for all SS_xxx flags */
-#define SS_FLAG_BITS	0
+#define SS_FLAG_BITS	SS_AUTODISARM
 
 #endif /* _UAPI_LINUX_SIGNAL_H */
diff --git a/kernel/fork.c b/kernel/fork.c
index d277e83..3e84515 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1494,7 +1494,7 @@ static struct task_struct *copy_process(unsigned long clone_flags,
 	 * sigaltstack should be cleared when sharing the same VM
 	 */
 	if ((clone_flags & (CLONE_VM|CLONE_VFORK)) == CLONE_VM)
-		p->sas_ss_sp = p->sas_ss_size = 0;
+		sas_ss_reset(p);
 
 	/*
 	 * Syscall tracing and stepping should be turned off in the
diff --git a/kernel/signal.c b/kernel/signal.c
index b1c6eb4..bf97ea5 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -3137,6 +3137,7 @@ do_sigaltstack (const stack_t __user *uss, stack_t __user *uoss, unsigned long s
 
 		current->sas_ss_sp = (unsigned long) ss_sp;
 		current->sas_ss_size = ss_size;
+		current->sas_ss_flags = ss_flags;
 	}
 
 	error = 0;
@@ -3167,9 +3168,14 @@ int restore_altstack(const stack_t __user *uss)
 int __save_altstack(stack_t __user *uss, unsigned long sp)
 {
 	struct task_struct *t = current;
-	return  __put_user((void __user *)t->sas_ss_sp, &uss->ss_sp) |
-		__put_user(sas_ss_flags(sp), &uss->ss_flags) |
+	int err = __put_user((void __user *)t->sas_ss_sp, &uss->ss_sp) |
+		__put_user(t->sas_ss_flags, &uss->ss_flags) |
 		__put_user(t->sas_ss_size, &uss->ss_size);
+	if (err)
+		return err;
+	if (t->sas_ss_flags & SS_AUTODISARM)
+		sas_ss_reset(t);
+	return 0;
 }
 
 #ifdef CONFIG_COMPAT

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#1393594 — Re: [tip:core/signals] signals/sigaltstack: Implement SS_AUTODISARM flag

On Tue, May 3, 2016 at 12:50 AM, tip-bot for Stas Sergeev
<tipbot@zytor.com> wrote:
> Commit-ID:  2a74213838104a41588d86fd5e8d344972891ace
> Gitweb:     http://git.kernel.org/tip/2a74213838104a41588d86fd5e8d344972891ace
> Author:     Stas Sergeev <stsp@list.ru>
> AuthorDate: Thu, 14 Apr 2016 23:20:04 +0300
> Committer:  Ingo Molnar <mingo@kernel.org>
> CommitDate: Tue, 3 May 2016 08:37:59 +0200
>
> signals/sigaltstack: Implement SS_AUTODISARM flag
>
> This patch implements the SS_AUTODISARM flag that can be OR-ed 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.
>
> To detect the support of this functionality, one can do:
>
>   err = sigaltstack(SS_DISABLE | SS_AUTODISARM);
>   if (err && errno == EINVAL)
>         unsupported();

One of my review comments from last time got lost, I think.  I'll send
a followup patch.

> +/* bit-flags */
> +#define SS_AUTODISARM  (1 << 4)        /* disable sas during sighandling */

Before this gets enshrined as ABI, could we perhaps change this to (1
<< 31)?  I don't see why we should pick a bit in the middle of the
field as our first flag bit.

--Andy

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