Re: [v11][PATCH 8/9] Define clone_with_pids() syscall

From: Sukadev Bhattiprolu
Date: Sat Nov 07 2009 - 15:17:36 EST

Serge E. Hallyn [serue@xxxxxxxxxx] wrote:
| Quoting Sukadev Bhattiprolu (sukadev@xxxxxxxxxx):
| > + stack_size = (unsigned long)kca.child_stack_size;
| > + child_stack = (unsigned long)kca.child_stack_base + stack_size;
| > +
| > + if (!child_stack)
| > + child_stack = regs->sp;
| I'm hooking up the s390 version right now. Do you think you should
| make this
| if (!kca.child_stack_base)
| child_stack = regs->sp;
| ?
| I suppose that in general if I pass in a NULL kca.child_stack_base
| I'll also pass in a 0 stacksize, but as a user I'd expect that if
| I pass in NULL, the size gets ignored. Instead, if I pass in NULL
| plus a size, then the kernel will take (void *)size as the stacktop.

Good point. Like copy_thread() on IA64, how about ignoring 'stack_size'
if base is NULL ?

child_stack = 0UL;
if (kca.child_stack_base)
child_stack = (unsigned long)kca.child_stack_base + stack_size;

if (!child_stack)
child_stack = regs->sp;

The other question is whether we should force all architectures to pass in
the stack *base* ? clone(2) man page says:

Stacks grow downwards on all processors that run Linux (except
the HP PA processors), so child_stack usually points to the topmost
address of the memory space set up for the child stack.

To be compatibile with clone() on most architectures, should we rename
'clone_args.child_stack_base' to 'clone_args.child_stack' and let
architectures use this field like they currently use the 'child_stack'
parameter to clone(2) ?

So x86 would pass in address of top-of-stack while HP-PA can pass in address
of base-of-stack.

Arnd, Roland, Peter please let me know if you have any inputs on this.

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