linux-kernel - Re: [RFC][v8][PATCH 10/10]: Document clone3() syscall

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Message-Id: <200910211037.38413.arnd@arndb.de>
Date:	Wed, 21 Oct 2009 10:37:38 +0200
From:	Arnd Bergmann <arndbergmann@...glemail.com>
To:	Pavel Machek <pavel@....cz>
Cc:	Sukadev Bhattiprolu <sukadev@...ux.vnet.ibm.com>,
	linux-kernel@...r.kernel.org, Oren Laadan <orenl@...columbia.edu>,
	serue@...ibm.com, "Eric W. Biederman" <ebiederm@...ssion.com>,
	Alexey Dobriyan <adobriyan@...il.com>,
	Pavel Emelyanov <xemul@...nvz.org>,
	Andrew Morton <akpm@...l.org>, torvalds@...ux-foundation.org,
	mikew@...gle.com, mingo@...e.hu, hpa@...or.com,
	Nathan Lynch <nathanl@...tin.ibm.com>, peterz@...radead.org,
	Louis.Rilling@...labs.com, roland@...hat.com,
	kosaki.motohiro@...fujitsu.com, randy.dunlap@...cle.com,
	linux-api@...r.kernel.org,
	Containers <containers@...ts.linux-foundation.org>,
	sukadev@...ibm.com
Subject: Re: [RFC][v8][PATCH 10/10]: Document clone3() syscall

> 
> > +struct clone_struct {
> > +	u64 flags;
> > +	u64 child_stack;
> 
> u64 seems wrong  on 32 bit platforms. ulong?

That would make it incompatible between 64 bit kernels and
32 bit user space, requiring a wrapper. Better leave it at u64.

> > +	If a pid in the @pids list is 0, the kernel will assign the next
> > +	available pid in the pid namespace, for the process.
> > +
> > +	If a pid in the @pids list is non-zero, the kernel tries to assign
> > +	the specified pid in that namespace.  If that pid is already in use
> > +	by another process, the system call fails with -EBUSY.
> ...
> > +	On failure, clone3() returns -1 and sets 'errno' to one of following
> > +	values (the child process is not created).
> 
> Inconsistent with above. Syscalls really return -ERRCODE, errno is
> glibc magic.

Quite the opposite is true.

The man page describes what the user space sees, which is errno. Returning
-ERRCODE to libc from the kernel is part of the architecture specific
kernel ABI and should not be documented in this place but in the architecture
documentation.

> > +	pid_t pids[] = { 77, 99 };
> > +	struct clone_struct cs;
> > +
> > +	cs.flags = (u64) SIGCHLD;
> > +	cs.child_stack = (u64) setup_child_stack();
> > +	cs.nr_pids = 2;
> > +	cs.parent_tid = 0LL;
> > +	cs.child_tid = 0LL;
> > +
> > +	rc = syscall(__NR_clone3, &cs, pids);
> 
> Hmm, is there reason why pids are not at the end of struct
> clone_struct? Passing most parameters in special structure, then pids
> separately is strange...

I suggested doing that, it's a lot easier to handle fixed length data
structures than an array at the end.

	Arnd <><
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