linux-kernel - Re: [RFC][v8][PATCH 0/10] Implement clone3() system call

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Date:	Wed, 14 Oct 2009 15:36:34 -0700
From:	Sukadev Bhattiprolu <sukadev@...ux.vnet.ibm.com>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Roland McGrath <roland@...hat.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,
	Nathan Lynch <nathanl@...tin.ibm.com>, arnd@...db.de,
	peterz@...radead.org, Louis.Rilling@...labs.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 0/10] Implement clone3() system call

H. Peter Anvin [hpa@...or.com] wrote:
| On 10/13/2009 04:53 PM, Roland McGrath wrote:
| >> My only concern is the support of 64-bit clone flags on 32-bit architectures.
| > 
| > Oy.  I didn't realize there was serious consideration of having more than
| > 32 flags.  IMHO it would be a bad choice, since they could only be used via
| > clone3.  Having high-bit flags work in clone on 64-bit machines but not on
| > 32-bit machines just seems like a wrongly confusing way for things to be.
| > If any high-bits flags are constrained even on 64-bit machines to uses in
| > clone3 calls for sanity purposes, then it seems questionable IMHO to have
| > them be more flags in the same u64 at all.
| > 
| > Since all new features will be via this struct, various new kinds of things
| > could potentially be done by other new struct fields independent of flags.
| > But that would of course require putting enough reserved fields in now and
| > requiring that they be zero-filled now in anticipation of such future uses,
| > which is not very pleasant either.
| > 
| > In short, I guess I really am saying that "clone_flags_high" (or
| > "more_flags" or something) does seem better to me than any of the
| > possibilities for having more than 32 CLONE_* in the current flags word.
| > 
| 
| Overall it seems sane to:
| 
| a) make it an actual 3-argument call;
| b) make the existing flags a u32 forever, and make it a separate
|    argument;
| c) any new expansion can be via the struct, which may want to have
|    an "c3_flags" field first in the structure.

Ok, So will this work ?

	struct clone_args {
		u32 flags_high;		/* new clone flags (higher bits) */ 
		u32 reserved1;
		u32 nr_pids;
		u32 reserved2;
		u64 child_stack_base;
		u64 child_stack_size;
		u64 parent_tid_ptr;
		u64 child_tid_ptr;
		u64 reserved3;
	};

	sys_clone3(u32 flags_low, struct clone_args *args, pid_t *pid_list)

Even on 64bit architectures the applications have to use sys_clone3() for
the extended features.
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