linux-kernel - Re: Documenting sigaltstack SS

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Date:   Mon, 30 Oct 2017 11:04:58 +0100
From:   "Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>
To:     Stas Sergeev <stsp@...t.ru>
Cc:     mtk.manpages@...il.com, linux-man <linux-man@...r.kernel.org>,
        Andy Lutomirski <luto@...capital.net>,
        Oleg Nesterov <oleg@...hat.com>,
        lkml <linux-kernel@...r.kernel.org>
Subject: Re: Documenting sigaltstack SS_AUTODISRM

[So, things fell on the floor, a while back.]

On 05/25/2017 11:17 AM, Stas Sergeev wrote:
> 24.05.2017 14:09, Michael Kerrisk (man-pages) пишет:
>> One could do this I suppose, but I read POSIX differently from
>> you and, more importantly, SS_ONSTACK breaks portability on
>> numerous other systems and is a no-op on Linux. So, the Linux man
>> page really should warn against its use in the strongest terms.
> So how about instead of the strongest terms towards
> the code's author, just explain that SS_ONSTACK is a
> bit-value on some/many OSes, and as such, 0 is a
> valid value to enable sas on them, plus all the other
> values would give EINVAL?
> No strongest terms will help w/o an explanation,
> because people will keep looking for something that
> suits as a missing SS_ENABLE.

Fair enough. I've removed the statement in the manual page
about "confusion". By now the page says:

    BUGS
       In the lead up to the release of the Linux 2.4  kernel,  a  change
       was   made   to   allow  sigaltstack()  to  accept  SS_ONSTACK  in
       ss.ss_flags, which results in behavior that is the  same  as  when
       ss_flags is 0 (i.e., the inclusion of SS_ONSTACK in ss.ss_flags is
       a no-op).  On other implementations,  and  according  to  POSIX.1,
       SS_ONSTACK appears only as a reported flag in old_ss.ss_flags.  On
       Linux, there is no need ever to specify this flag in  ss.ss_flags,
       and indeed doing so should be avoided on portability grounds: var‐
       ious other systems give an error if  SS_ONSTACK  is  specified  in
       ss.ss_flags.

Cheers,

Michael

-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/