On Fri, 2023-04-14 at 13:29 -0400, Liam R. Howlett wrote:
> * Liam R. Howlett <Liam.Howlett@xxxxxxxxxx> [230414 13:26]:
> > * Edgecombe, Rick P <rick.p.edgecombe@xxxxxxxxx> [230414 12:27]:
> > > On Fri, 2023-04-14 at 10:57 -0400, Liam R. Howlett wrote:<br>
> > > > + tmp = mas_next(&mas, ULONG_MAX);
> > > > + if (tmp && (tmp->vm_flags & VM_GROWSDOWN)) {
> > >
> > > Why also check VM_GROWSDOWN here (and VM_GROWSUP below)?
> > > vm_start/end_gap() already have checks inside.
> >
> > An artifact of a plan that was later abandoned.
> >
> > >
> > > > + if (vm_start_gap(tmp) < gap + length - 1) {
> > > > + low_limit = tmp->vm_end;
> > > > + mas_reset(&mas);
> > > > + goto retry;
> > > > + }
> > > > + } else {
> > > > + tmp = mas_prev(&mas, 0);
> > > > + if (tmp && (tmp->vm_flags & VM_GROWSUP) &&
> > > > + vm_end_gap(tmp) > gap) {
> > > > + low_limit = vm_end_gap(tmp);
> > > > + mas_reset(&mas);
> > > > + goto retry;
> > > > + }
> > > > + }
> > > > +
> > >
> > > Could it be like this?
> >
> > Yes, I'll make this change. Thanks for the suggestion.
>
>
> Wait, I like how it is.
>
> In my version, if there is a stack that is VM_GROWSDOWN there, but
> does
> not intercept the gap, then I won't check the prev.. in yours, we
> will
> never avoid checking prev.
Hmm, I see. I guess I'm thinking ahead a bit to adding the shadow stack
guard gap, but I can always add to these vm_flags checks.
But are you sure this optimization is even possible? The old
vma_compute_gap() had this comment:
/*
* Note: in the rare case of a VM_GROWSDOWN above a VM_GROWSUP, we
* allow two stack_guard_gaps between them here, and when choosing
* an unmapped area; whereas when expanding we only require one.
* That's a little inconsistent, but keeps the code here simpler.
*/
Assuming this is a real scenario, if you have VM_GROWSDOWN above and
VM_GROWSUP below, don't you need to check the gaps for above and below?
Again thinking about adding shadow stack guard pages, something like
that could be a more common scenario. Not that you need to fix my out
of tree issues, but I would probably need to adjust it to check both
directions.
I guess there is no way to embed this inside maple tree search so we
don't need to retry? (sorry if this is a dumb question, it's an opaque
box to me).
