Re: [RFC][v7][PATCH 0/9] Implement clone2() system call

[Oren Laadan <orenl@librato.com>]

Arnd Bergmann wrote:
> On Friday 25 September 2009, Oren Laadan wrote:
>> In this form, you place a constraints on where userspace may
>> place the {parent,child}_tid variable, and require that this
>> particular clone_struct remain valid memory in the parent until
>> the child terminates.  This may break existing programs that
>> use this (threads libraries ?)
> 
> No existing program uses sys_clone2, and the kernel function
> may well differ from the user space calling conventions, which
> are not bound by the six-argument limitation.
> 
> So a clone2 library call could set up the structure with the
> arguments to the real syscall, call into the kernel and
> copy the output data back into the pointers it was given by
> the user.

That may work well for parent_tid, however child_tid is also
kept on the task_struct and written to when the child exits,
and there is no explicit user-space wrapper on that.

Also, I may be mistaken, but I thought that the idea of these
was that the kernel writes them to user space, so other threads
may see them quickly, _before_ the parent returns to userspace;
otherwise, the parent (at the library) could himself copy the
return value.

Oren.