[PATCH 2/2] extend clone_flags using parent_tidptr argument

[Cedric Le Goater <legoater-GANU6spQydw@public.gmane.org>]

From: Cedric Le Goater <clg-NmTC/0ZBporQT0dZR+AlfA@public.gmane.org>

We have at least 2 patchsets requiring each a new clone flag and 
there it is, we've reached the limit, none are left.  

This patch uses the CLONE_DETACHED flag (unused) as a marker to 
extend the clone flags through the parent_tidptr argument.

Initially, we thought on using the last bit but it has recently 
been taken by CLONE_IO.

Obviously, this hack doesn't work for unshare() for which I don't 
see any other solution than to add a new syscall : 

	long sys_unshare64(unsigned long clone_flags_high, 
			unsigned long clone_flags_low);



Is this the right path to extend the clone flags ? should we add a 
clone64() rather than hack the extending clone() ? 

Thanks for any comments !

C.

Signed-off-by: Cedric Le Goater <clg-NmTC/0ZBporQT0dZR+AlfA@public.gmane.org>
---
 include/linux/sched.h |    1 +
 kernel/fork.c         |   14 +++++++++++++-
 2 files changed, 14 insertions(+), 1 deletion(-)

Index: 2.6.24-mm1/include/linux/sched.h
===================================================================
--- 2.6.24-mm1.orig/include/linux/sched.h
+++ 2.6.24-mm1/include/linux/sched.h
@@ -28,6 +28,7 @@
 #define CLONE_NEWPID		0x20000000	/* New pid namespace */
 #define CLONE_NEWNET		0x40000000	/* New network namespace */
 #define CLONE_IO		0x80000000	/* Clone io context */
+#define CLONE_EXTFLAGS		CLONE_DETACHED	/* use parent_tidptr as an extended set of flags */
 
 /*
  * Scheduling policies
Index: 2.6.24-mm1/kernel/fork.c
===================================================================
--- 2.6.24-mm1.orig/kernel/fork.c
+++ 2.6.24-mm1/kernel/fork.c
@@ -1012,6 +1012,14 @@ static struct task_struct *copy_process(
 	struct task_struct *p;
 	int cgroup_callbacks_done = 0;
 
+	/*
+	 * It is not permitted to specify both CLONE_EXTFLAGS and
+	 * CLONE_PARENT_SETTID
+	 */
+	if ((clone_flags & (CLONE_EXTFLAGS|CLONE_PARENT_SETTID)) ==
+	    (CLONE_EXTFLAGS|CLONE_PARENT_SETTID))
+		return ERR_PTR(-EINVAL);
+
 	if ((clone_flags & (CLONE_NEWNS|CLONE_FS)) == (CLONE_NEWNS|CLONE_FS))
 		return ERR_PTR(-EINVAL);
 
@@ -1455,6 +1463,7 @@ long do_fork(unsigned long clone_flags,
 	struct task_struct *p;
 	int trace = 0;
 	long nr;
+	u64 clone_flags64 = clone_flags;
 
 	/*
 	 * We hope to recycle these flags after 2.6.26
@@ -1479,7 +1488,10 @@ long do_fork(unsigned long clone_flags,
 			clone_flags |= CLONE_PTRACE;
 	}
 
-	p = copy_process(clone_flags, stack_start, regs, stack_size,
+	if (clone_flags & CLONE_EXTFLAGS)
+		clone_flags64 = ((u64) (uintptr_t) parent_tidptr << 32) | clone_flags;
+
+	p = copy_process(clone_flags64, stack_start, regs, stack_size,
 			child_tidptr, NULL);
 	/*
 	 * Do this prior waking up the new thread - the thread pointer