Re: [PATCH 4/4] sigaltstack: allow disabling and re-enabling sas within sighandler

From	Andy Lutomirski <luto@amacapital.net>
Newsgroups	linux.kernel
Subject	Re: [PATCH 4/4] sigaltstack: allow disabling and re-enabling sas within sighandler
Date	2016-01-31 18:10 +0100
Message-ID	<qX3FE-6q4-9@gated-at.bofh.it> (permalink)
References	<qX2Tf-5OH-3@gated-at.bofh.it> <qX32W-5Tv-11@gated-at.bofh.it>
Organization	linux.* mail to news gateway

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On Sun, Jan 31, 2016 at 8:28 AM, Stas Sergeev <stsp@list.ru> wrote:
>
> linux implements the sigaltstack() in a way that makes it impossible to
> use with swapcontext(). Per the man page, sigaltstack is allowed to return
> EPERM if the process is altering its sigaltstack while running on
> sigaltstack.
> This is likely needed to consistently return oss->ss_flags, that indicates
> whether the process is being on sigaltstack or not.
> Unfortunately, linux takes that permission to return EPERM too literally:
> it returns EPERM even if you don't want to change to another sigaltstack,
> but only want to temporarily disable sigaltstack with SS_DISABLE.
> You can't use swapcontext() without disabling sigaltstack first, or the
> stack will be re-used and overwritten by a subsequent signal.
>
> With this patch, disabling sigaltstack inside a signal handler became
> possible, and the swapcontext() can then be used safely. After switching
> back to the sighandler, the app can re-enable the sigatlstack.
> The oss->ss_flags will correctly indicate the current use of sigaltstack,
> even if it is temporarily disabled. Any attempt to modify the sigaltstack
> (rather than to disable or re-enable it) within the sighandler, will still
> be punished with EPERM as suggested by POSIX.

This seems considerably more complicated than my previous proposal to
add an SS_FORCE flag to say "I know what I'm doing.  Ignore POSIX and
let me change the sigaltstack configuration even if it's in use".
What's the advantage?

--Andy

Thread

[PATCH 0/4] make sigaltstack() compatible with swapcontext() Stas Sergeev <stsp@list.ru> - 2016-01-31 17:20 +0100
  [PATCH 1/4] selftests: Add test for sigaltstack(SS_DISABLE) inside  sighandler Stas Sergeev <stsp@list.ru> - 2016-01-31 17:20 +0100
  [PATCH 2/4] score: signal: fix sigaltstack check Stas Sergeev <stsp@list.ru> - 2016-01-31 17:30 +0100
    Re: [PATCH 2/4] score: signal: fix sigaltstack check Lennox Wu <lennox.wu@gmail.com> - 2016-02-02 20:10 +0100
  [PATCH 4/4] sigaltstack: allow disabling and re-enabling sas within  sighandler Stas Sergeev <stsp@list.ru> - 2016-01-31 17:30 +0100
    Re: [PATCH 4/4] sigaltstack: allow disabling and re-enabling sas  within sighandler Andy Lutomirski <luto@amacapital.net> - 2016-01-31 18:10 +0100
      Re: [PATCH 4/4] sigaltstack: allow disabling and re-enabling sas  within sighandler Stas Sergeev <stsp@list.ru> - 2016-01-31 18:40 +0100
        Re: [PATCH 4/4] sigaltstack: allow disabling and re-enabling sas  within sighandler Andy Lutomirski <luto@amacapital.net> - 2016-01-31 20:10 +0100
          Re: [PATCH 4/4] sigaltstack: allow disabling and re-enabling sas  within sighandler Stas Sergeev <stsp@list.ru> - 2016-01-31 21:00 +0100
            Re: [PATCH 4/4] sigaltstack: allow disabling and re-enabling sas  within sighandler Andy Lutomirski <luto@amacapital.net> - 2016-01-31 21:20 +0100
              Re: [PATCH 4/4] sigaltstack: allow disabling and re-enabling sas  within sighandler Stas Sergeev <stsp@list.ru> - 2016-01-31 23:40 +0100
                Re: [PATCH 4/4] sigaltstack: allow disabling and re-enabling sas  within sighandler Andy Lutomirski <luto@amacapital.net> - 2016-01-31 23:50 +0100
                Re: [PATCH 4/4] sigaltstack: allow disabling and re-enabling sas  within sighandler Stas Sergeev <stsp@list.ru> - 2016-02-01 00:50 +0100
    Re: [PATCH 4/4] sigaltstack: allow disabling and re-enabling sas  within sighandler Oleg Nesterov <oleg@redhat.com> - 2016-02-01 17:10 +0100
      Re: [PATCH 4/4] sigaltstack: allow disabling and re-enabling sas  within sighandler Stas Sergeev <stsp@list.ru> - 2016-02-01 18:00 +0100
        Re: [PATCH 4/4] sigaltstack: allow disabling and re-enabling sas  within sighandler Oleg Nesterov <oleg@redhat.com> - 2016-02-01 18:30 +0100
      Re: [PATCH 4/4] sigaltstack: allow disabling and re-enabling sas  within sighandler Oleg Nesterov <oleg@redhat.com> - 2016-02-01 18:20 +0100
        Re: [PATCH 4/4] sigaltstack: allow disabling and re-enabling sas  within sighandler Stas Sergeev <stsp@list.ru> - 2016-02-01 18:30 +0100
          Re: [PATCH 4/4] sigaltstack: allow disabling and re-enabling sas  within sighandler Oleg Nesterov <oleg@redhat.com> - 2016-02-01 19:10 +0100
            Re: [PATCH 4/4] sigaltstack: allow disabling and re-enabling sas  within sighandler Stas Sergeev <stsp@list.ru> - 2016-02-01 19:20 +0100
              Re: [PATCH 4/4] sigaltstack: allow disabling and re-enabling sas  within sighandler Andy Lutomirski <luto@amacapital.net> - 2016-02-01 19:30 +0100
                Re: [PATCH 4/4] sigaltstack: allow disabling and re-enabling sas  within sighandler Stas Sergeev <stsp@list.ru> - 2016-02-01 19:50 +0100
              Re: [PATCH 4/4] sigaltstack: allow disabling and re-enabling sas  within sighandler Oleg Nesterov <oleg@redhat.com> - 2016-02-01 20:00 +0100
                Re: [PATCH 4/4] sigaltstack: allow disabling and re-enabling sas  within sighandler Stas Sergeev <stsp@list.ru> - 2016-02-01 20:10 +0100
                Re: [PATCH 4/4] sigaltstack: allow disabling and re-enabling sas  within sighandler Oleg Nesterov <oleg@redhat.com> - 2016-02-01 20:30 +0100
                Re: [PATCH 4/4] sigaltstack: allow disabling and re-enabling sas  within sighandler Stas Sergeev <stsp@list.ru> - 2016-02-01 20:50 +0100
                Re: [PATCH 4/4] sigaltstack: allow disabling and re-enabling sas  within sighandler Oleg Nesterov <oleg@redhat.com> - 2016-02-01 21:50 +0100
                Re: [PATCH 4/4] sigaltstack: allow disabling and re-enabling sas  within sighandler Stas Sergeev <stsp@list.ru> - 2016-02-02 00:10 +0100
  [PATCH 3/4] x86: signal: unify the sigaltstack check with other  arches Stas Sergeev <stsp@list.ru> - 2016-01-31 17:30 +0100
    Re: [PATCH 3/4] x86: signal: unify the sigaltstack check with other arches Andy Lutomirski <luto@amacapital.net> - 2016-01-31 18:00 +0100
      Re: [PATCH 3/4] x86: signal: unify the sigaltstack check with other  arches Stas Sergeev <stsp@list.ru> - 2016-01-31 19:10 +0100

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