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
re,
wh
> 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
>
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