[PATCH 2/2] extend clone_flags using parent_tidptr argument

[Serge E. Hallyn]

Quoting Cedric Le Goater (legoater at free.fr):
> From: Cedric Le Goater <clg at fr.ibm.com>
>
> 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 

Are we pretty sure that there is no legacy software out there which has
continued to specify CLONE_DETACHED since the kernel ignores it?

> 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 at fr.ibm.com>
> ---
> 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
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