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

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Date:	Wed, 21 Oct 2009 11:33:38 +0200
From:	Pavel Machek <pavel@....cz>
To:	Arnd Bergmann <arndbergmann@...glemail.com>
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

On Wed 2009-10-21 10:37:38, Arnd Bergmann wrote:
> > 
> > > +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.

Ok.

> > > +	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.

Well, it is still inconsistent. Half the docs talks -ERRCODE, half
talks -1/errno=ERRCODE.

> > > +	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.

You could still put the something * pids at the end of (fixed length)
structure.

(Not that I agree with the argument, pid array is variable-length,
anyway, and inlining it at the end of structure should not make code
more complex).

									Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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