Re: [PATCH] structleak: disable BYREF_ALL in combination with KASAN_STACK

[Arnd Bergmann <arnd@arndb.de>]

On Fri, Jun 21, 2019 at 3:32 PM Ard Biesheuvel
<ard.biesheuvel@linaro.org> wrote:
> On Fri, 21 Jun 2019 at 11:44, Arnd Bergmann <arnd@arndb.de> wrote:
> > On Thu, Jun 20, 2019 at 7:36 PM Kees Cook <keescook@chromium.org> wrote:
> > > On Tue, Jun 18, 2019 at 11:47:13AM +0200, Arnd Bergmann wrote:
> > > > The combination of KASAN_STACK and GCC_PLUGIN_STRUCTLEAK_BYREF_ALL
> > > > leads to much larger kernel stack usage, as seen from the warnings
> > > > about functions that now exceed the 2048 byte limit:
> > >
> > > Is the preference that this go into v5.2 (there's not much time left),
> > > or should this be v5.3? (You didn't mark it as Cc: stable?)
> >
> > Having it in 5.2 would be great. I had not done much build testing in the last
> > months, so I didn't actually realize that your patch was merged a while ago
> > rather than only in linux-next.
> >
> > BTW, I have now run into a small number of files that are still affected
> > by a stack overflow warning from STRUCTLEAK_BYREF_ALL. I'm trying
> > to come up with patches for those as well, we can probably do it in a way
> > that also improves the affected drivers. I'll put you on Cc when I
> > find another one.
> >
>
> There is something fundamentally wrong here, though. BYREF_ALL only
> initializes variables that have their address taken, which does not
> explain why the size of the stack frame should increase (since in
> order to have an address in the first place, the variable must already
> have a stack slot assigned)
>
> So I suspect that BYREF_ALL is defeating some optimizations where.
> e.g., the call involving the address of the variable is optimized
> away, but the the initialization remains, thus forcing the variable to
> be allocated in the stack frame even though the initializer is the
> only thing that references it.

One pattern I have seen here is temporary variables from macros or
inline functions whose lifetime now extends over the entire function
rather than just the basic block in which they are defined, see e.g.
lpfc_debug_dump_qe() being inlined multiple times into
lpfc_debug_dump_all_queues(). Each instance of the local
"char line_buf[LPFC_LBUF_SZ];" seems to add on to the previous
one now, where the behavior without the structleak plugin is that
they don't.

      Arnd