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Commit 2a742138 authored by Stas Sergeev's avatar Stas Sergeev Committed by Ingo Molnar
Browse files

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: default avatarStas 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: default avatarIngo Molnar <mingo@kernel.org>
parent 407bc16a

include/linux/sched.h

+8 −0
Original line number Diff line number Diff line
@@ -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))

include/linux/signal.h

+3 −1
Original line number Diff line number Diff line
@@ -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

include/uapi/linux/signal.h

+3 −1
Original line number Diff line number Diff line
@@ -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 */

kernel/fork.c

+1 −1
Original line number Diff line number Diff line
@@ -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

kernel/signal.c

+8 −2
Original line number Diff line number Diff line
@@ -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