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Date:   Tue, 5 May 2020 17:01:18 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Jürgen Groß <jgross@...e.com>
Cc:     Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Stefano Stabellini <sstabellini@...nel.org>,
        Yan Yankovskyi <yyankovskyi@...il.com>, Wei Liu <wl@....org>,
        xen-devel <xen-devel@...ts.xenproject.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>
Subject: Re: [PATCH] xenbus: avoid stack overflow warning

On Tue, May 5, 2020 at 4:34 PM Jürgen Groß <jgross@...e.com> 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.

      Arnd

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