Re: [PATCH v4 0/2] make sigaltstack() compatible with swapcontext()

* Stas Sergeev [off-list ref] [2016-03-01 00:29:03 +0300]:

The following patches make it possible to use swapcontext()
in a sighandler that works on sigaltstack.

i don't think that's possible, the (obsolete) userspace
*context functions cannot operate on kernel provided
ucontext_t structs, so they are not usable in signal
handlers.

at least not on most target archs.

so i don't understand how the tests can work portably.

The approach is inspired by Andy Lutomirski's suggestion that
sigaltstack should disarm itself after saving into uc_stack:
https://lkml.org/lkml/2016/2/1/594

I add the SS_AUTODISARM flag that does exactly that.
On sighandler exit, the sigaltstack is restored from uc_stack.
Another possible name could be SS_ONESHOT, but, since it gets
always re-enabled, I choose SS_AUTODISARM.

[PATCH 1/2] sigaltstack: implement SS_AUTODISARM flag
	This patch implements SS_AUTODISARM flag
[PATCH 2/2] selftests: Add test for sigaltstack(SS_AUTODISARM)
	This patch adds the selftest code for new functionality

CC: linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org
CC: linux-api-u79uwXL29TY76Z2rM5mHXA@public.gmane.org
CC: Andy Lutomirski <redacted>
CC: Oleg Nesterov <redacted>
CC: Shuah Khan <redacted>

Diffstat:
 include/linux/sched.h                        |    8 +
 include/linux/signal.h                       |    4 
 include/uapi/linux/signal.h                  |    3 
 kernel/fork.c                                |    2 
 kernel/signal.c                              |   23 ++--
 tools/testing/selftests/Makefile             |    1 
 tools/testing/selftests/sigaltstack/Makefile |    8 +
 tools/testing/selftests/sigaltstack/sas.c    |  151 +++++++++++++++++++++++++++
 8 files changed, 187 insertions(+), 13 deletions(-)

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