Re: [PATCH] Set DEMANGLE_RECURSION_LIMIT to 1536

[Jason Merrill <jason@redhat.com>]

On Mon, Dec 10, 2018 at 10:10 AM Jakub Jelinek <jakub@redhat.com> wrote:
> On Mon, Dec 10, 2018 at 02:52:39PM +0000, Michael Matz wrote:
> > On Fri, 7 Dec 2018, H.J. Lu wrote:
> >
> > > > > On Thu, Dec 6, 2018 at 3:12 AM Nick Clifton <nickc@redhat.com> wrote:
> > > > > >
> > > > > >   Is the patch OK with you ?
> > > > >
> > > >
> > > > This caused:
> > > >
> > > > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88409
> > > >
> > >
> > > Here is the fix.   OK for trunk?
> >
> > I think this points toward the limit being _much_ too low.  With template
> > meta programming you easily get these mangled names, it's not even a
> > particularly long one.  But I'm wondering a bit, without tracing the
> > demangler, just looking at the symbol name and demangled result I don't
> > readily see where the depth of recursion really is more than 1024, are
> > there perhaps some recursion_level-- statements skipped?
>
> That is because the recursion_level limit isn't hit in this case at all (far
> from it).
>
> What breaks it is this:
>
>   /* PR 87675 - Check for a mangled string that is so long
>      that we do not have enough stack space to demangle it.  */
>   if (((options & DMGL_NO_RECURSE_LIMIT) == 0)
>       /* This check is a bit arbitrary, since what we really want to do is to
>          compare the sizes of the di.comps and di.subs arrays against the
>          amount of stack space remaining.  But there is no portable way to do
>          this, so instead we use the recursion limit as a guide to the maximum
>          size of the arrays.  */
>       && (unsigned long) di.num_comps > DEMANGLE_RECURSION_LIMIT)
>     {
>       /* FIXME: We need a way to indicate that a stack limit has been reached.  */
>       return 0;
>     }

> where di.num_comps is just strlen (mangled) * 2.  Without any analysis
> whatsoever, bumping the "recursion" limit will just mean we can process 1.5
> times long names.  Either we need more precise analysis on what we are
> looking for (how big arrays we'll need) or it needs to be an independent
> limit and certainly should allow say 10KB symbols too if they are
> reasonable.

If the problem is alloca, we could avoid using alloca if the size
passes a threshold.  Perhaps even use a better data structure than a
preallocated array based on a guess about the number of components...

Jason