[Devel] Re: [PATCH 1/1] implement s390 clone_with_pids syscall

[Nathan Lynch]

On Wed, 2009-11-11 at 08:46 -0600, Serge E. Hallyn wrote:
> Quoting Nathan Lynch (nathanl at austin.ibm.com):
> > 
> > > +	parent_tid_ptr = (int *)kca.parent_tid_ptr;
> > > +	child_tid_ptr =  (int *)kca.child_tid_ptr;
> > > +
> > > +	stack_size = (unsigned long)kca.child_stack_size;
> > > +	child_stack = (unsigned long)kca.child_stack_base;
> > > +	if (child_stack)
> > > +		child_stack += stack_size;
> > 
> > Should this calculation not be of the form:
> > child_stack = arch_dependent_alignment(child_stack + stack_size - 1)
> > ?
> > 
> > Is overflow a concern?
> > 
> > Same questions apply to the x86 version.
> 
> Hmm...  if the stack isn't valid, the task will just segfault, so
> it's not dangerous for the kernel, right?  Note that for instance
> arch/s390/kernel/process.c:SYS_clone() doesn't check the validity
> of the new stack pointer passed in either.

clone expects the stack argument to be the desired value of the stack
pointer in the child.  cwp is different in that the clone_args struct
specifies the base and size of the region the child is to use for stack,
meaning that the kernel must derive from these a sane value for the
child's stack pointer (on every arch where the stack grows down).

Your current calculation results in an unaligned SP outside of the
region that the caller has presumably allocated for the child stack.
How is that useful behavior?


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