Re: [PATCH] xenbus: avoid stack overflow warning

[JÃrgen GroÃ]

On 05.05.20 17:01, Arnd Bergmann wrote:
> On Tue, May 5, 2020 at 4:34 PM JÃrgen Groà <jgross@xxxxxxxx> wrote:
> > On 05.05.20 16:15, Arnd Bergmann wrote:
> > > The __xenbus_map_ring() function has two large arrays, 'map' and
> > > 'unmap' on its stack. When clang decides to inline it into its caller,
> > > xenbus_map_ring_valloc_hvm(), the total stack usage exceeds the warning
> > > limit for stack size on 32-bit architectures.
> > >
> > > drivers/xen/xenbus/xenbus_client.c:592:12: error: stack frame size of 1104 bytes in function 'xenbus_map_ring_valloc_hvm' [-Werror,-Wframe-larger-than=]
> > >
> > > As far as I can tell, other compilers don't inline it here, so we get
> > > no warning, but the stack usage is actually the same. It is possible
> > > for both arrays to use the same location on the stack, but the compiler
> > > cannot prove that this is safe because they get passed to external
> > > functions that may end up using them until they go out of scope.
> > >
> > > Move the two arrays into separate basic blocks to limit the scope
> > > and force them to occupy less stack in total, regardless of the
> > > inlining decision.
> >
> > Why don't you put both arrays into a union?
>
> I considered that as well, and don't really mind either way. I think it does
> get a bit ugly whatever we do. If you prefer the union, I can respin the
> patch that way.

Hmm, thinking more about it I think the real clean solution would be to
extend struct map_ring_valloc_hvm to cover the pv case, too, to add the
map and unmap arrays (possibly as a union) to it and to allocate it
dynamically instead of having it on the stack.

Would you be fine doing this?


Juergen