Re: [PATCH] xenbus: avoid stack overflow warning

[Jürgen Groß <jgross@xxxxxxxx>]

On 05.05.20 18:12, Boris Ostrovsky wrote:
> On 5/5/20 12:02 PM, Jürgen Groß wrote:
> > On 05.05.20 17:01, Arnd Bergmann wrote:
> > > On Tue, May 5, 2020 at 4:34 PM Jürgen Groß <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?
>
>
> Another option might be to factor out/modify code from
> xenbus_unmap_ring() and call the resulting code from
> __xenbus_map_ring()'s fail path.
This will still allocate large arrays on the stack. If we ever increase
the max ring page order it will explode.


Juergen