linux-kernel - Re: Documenting sigaltstack SS

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Date:   Mon, 30 Oct 2017 13:54:53 +0100
From:   walter harms <wharms@....de>
To:     "Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>
CC:     Stas Sergeev <stsp@...t.ru>, 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



Am 30.10.2017 11:50, schrieb Michael Kerrisk (man-pages):
> Hi Walter,
> 
> On 10/30/2017 11:21 AM, walter harms wrote:
>>
>>
>> Am 30.10.2017 11:04, schrieb Michael Kerrisk (man-pages):
>>> [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,
>>
>> i am confused, i understand that:
>>            ss.ss_sp = malloc(SIGSTKSZ);
>>
>>            ss.ss_size = SIGSTKSZ;
>>            ss.ss_flags = 0;
>>            if (sigaltstack(&ss, NULL) == -1)
>>
>> is equivalent to:
>>            ss.ss_sp = malloc(SIGSTKSZ);
>>
>>            ss.ss_size = SIGSTKSZ;
>>            ss.ss_flags = SS_ONSTACK ;
>>            if (sigaltstack(&ss, NULL) == -1)
>>
>> but also to
>>            ss.ss_sp = malloc(SIGSTKSZ);
>>
>>            ss.ss_size = SIGSTKSZ;
>>            ss.ss_flags = SS_ONSTACK | SOMETHING_FLAG ;
>>            if (sigaltstack(&ss, NULL) == -1)
>>
>> so the use of SS_ONSTACK would result in ss.ss_flags = 0 no matter what.
>> OR
>> SS_ONSTACK is a no-op in Linux
> 
> I see what you mean. The point is back then that SS_ONSTACK was
> the only flag that could (on Linux) be specified in ss.ss_flags,
> so that "SS_ONSTACK | SOMETHING_FLAG" was a nonexistent case.
> These days, it's possible to specify the new SS_AUTODISARM
> flag in ss.ss_flags, which I think is why you are doubtful
> about the new page text. How about this, as a tightened-up 
> version:
> 
>     BUGS
>        In Linux 2.2 and earlier, the only flag that could be specified in
>        ss.sa_flags  was SS_DISABLE.  In the lead up to the release of the
>        Linux 2.4 kernel, a change was  made  to  allow  sigaltstack()  to
>        allow   ss.ss_flags==SS_ONSTACK   with   the   same   meaning   as
>        ss.ss_flags==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 SS_ONSTACK 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.
> 
> ?

what about the other way around (general to special) ....

 the inclusion of SS_ONSTACK in ss.ss_flags is a no-op (setting ss.ss_flags=SS_ONSTACK
 will result in ss.ss_flags=0).

The details about older release will be helpful for upgrading pruposes.
So we can say:

Since Linux 2.4 the inclusion ....

does this help ?

re,
 wh


> 
> Thanks,
> 
> Michael
>