Re: For review: nptl(7) man page

["Michael Kerrisk (man-pages)" <mtk dot manpages at gmail dot com>]

On 08/05/2015 08:23 PM, Rich Felker wrote:
> On Wed, Aug 05, 2015 at 11:14:32AM -0700, Nicholas Miell wrote:
>>
>>
>>> On Aug 5, 2015, at 2:36 AM, Torvald Riegel <triegel@redhat.com> wrote:
>>>
>>>> On Tue, 2015-08-04 at 11:50 -0700, Nicholas Miell wrote:
>>>>> On Aug 4, 2015, at 8:06 AM, Roland McGrath <roland@hack.frob.com> wrote:
>>>>>
>>>>> I think we are asserting that they are exactly that by dint of the confstr
>>>>> results for _CS_POSIX_V7_ILP32_OFF32_CFLAGS et al.  So the question is what
>>>>> POSIX actually does or doesn't say about process-shared synchronization
>>>>> objects being shared between processes running programs built in different
>>>>> POSIX compilation environments.
>>>>>
>>>>> The other relevant question is whether 32/64 sharing of each particular
>>>>> pshared object has in fact worked reliably under glibc in the past.  Since
>>>>> we haven't been clear and explicit about the subject before AFAIK, then if
>>>>> in fact it worked before then people might well have inferred that we made
>>>>> such an ABI guarantee.  (I hope not, since if so we just broke it.)
>>>>
>>>> The relevant questions aren't what's the least useful behavior
>>>> that POSIX lets us get away with or can we leave it broken
>>>> because it never worked;'the questions are what do the other
>>>> operating systems do and what do the users want.
>>>
>>> Those are relevant questions, but they are not the only relevant ones.
>>> Some users would also like to share data structures between processes
>>> using different glibc builds (eg, different versions), and we won't
>>> promise that this works for obvioius reasons.
>>
>> The reasonable scenario is what we have right now -- 32 and 64-bit
>> versions of glibc built from the exact same git commit. And I'd
>> exclude static versions of glibc even if they're the same version.
> 
> Why would you exclude static? IMO two static-linked executables using
> the same version of glibc should be compatible with each other even if
> you don't want to guarantee they'll be compatible with the
> same-version shared glibc.
> 
>>> The semaphore example shows that there can be a disadvantage to
>>> guaranteeing 32/64b interoperability (specifically, the 64b code is more
>>> efficient).  For mutex, I *currently* don't see a reason why we couldn't
>>> get away with just doing 32b stuff for the pshared case, but there's no
>>> guarantee that I can foresee all future needs either.
>>>
>>> Thus, if we would decide to guarantee 32/64b interoperability, we'd need
>>> to have at least strong use cases for that and a decent amount of
>>> confidence that making such a guarantee is unlikely to constrain the
>>> implementation in the future.
>>
>> Well, POSIX semaphores are supposed to be a replacement for System V
>> semaphores (and this extends to the rest of the POSIX IPC
>> primitives); right now they aren't.
> 
> Only for some usage cases. As far as I can tell, POSIX semaphores are
> not intended to be required to be implemented as a kernel resource.

That last is also true of SysV semaphores, surely?

> They don't have permissions enforcement/safety against malicious
> processes, 

(I'm a little lost here. POSIX semaphores do have a permissions mask.)

>  backout on async process termination, etc.

Actually, System V semaphores don't reliably have this either (see BUGS
in semop(2)).

Cheers,

Michael

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