On 19.08.2020 20:02, Roy Marples wrote:
> On 18/08/2020 20:52, Mouse wrote:
Perhaps it would need a new flavour of file descriptor, [...]
>>> Linux has apparently done this: pidfd (file descriptors representing
>>> a process). The idea is that you can pass them to various system
>>> call variants that otherwise take pids, without the risk that the
>>> process has exited in the mean time and the pid re-used.
>>
>> I've been thinking about something like that myself, starting with
>> AF_PID sockets, then deciding they wouldn't/couldn't work (as think I
>> mentioned in this thread, the socket infrastructure really wants the
>> contents of a socket to be independent of who's accessing it).
>> Personally, I've wanted it as a way to provide an out-of-band channel
>> to userland programs (like a control command channel for various
>> daemons), but...hmm.
>>
>> Feels strange to find an idea I like coming from Linux.
>
> You can relax.
> FreeBSD did it first for Capsicum.
>
> https://www.freebsd.org/cgi/man.cgi?query=pdfork=2
>
> Roy
Hi,
I have got a draft work on this.
It's composed of:
1. New id_t type in P_PIDFD.
2. sigsendset(2) + sigsend(3), picked from SVID (Solaris, etc) It's a
generalized version of kill(2). It allows specifying P_PIDFD.
3. waitid supporting new id type: P_PIDFD.
Linux is close to the above, except reinventing the wheel instead of
picking sigsendset(2) + integration with /proc.
FreeBSD went with a distinct direction and invented new semantics of
process file descriptors, that differs to references over pid_t. For
example whenever you close(2) a file descriptor, it kills the process.
You also must wait for process events in FreeBSD using kevent(2). This
introduced incompatibilities with the established UNIX semantics. Linux
went a different path and whenever a process dies, we get ESRCH, which
is sensible as it avoids e.g. killing a random process with recycled pid_t.
We want process file descriptors to reference process handles through
PID namespaces, for hopefully upcoming support of containers.
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