[cfe-users] Clang9 UBSan and GMP

[David Blaikie via cfe-users]

UBSan doesn't cover this sort of thing - MSan is the sanitizer for catching
uninitialized values. (MSan is, unfortunately, a bit more finicky to use
because the whole program (including the standard library) must be compiled
with the feature enabled for it to work correctly - I don't have an MSan
enabled environment to test whether it would've caught this bug or not)

On Thu, Nov 7, 2019 at 9:15 AM Hans Åberg <haberg-1 at telia.com> wrote:

>
> > On 7 Nov 2019, at 15:24, Hans Wennborg <hans at chromium.org> wrote:
> >
> > Looking at LLVM's -print-after-all shows a diff after GVN which seems
> > to come from this if-statement in tests/mpn/t-sqrlo.c:
> >
> >      if (pp[-1] != p_before || pp[n] != p_after
> >  || scratch[-1] != s_before || scratch[itch] != s_after
> >  || mpn_cmp (refp, pp, n) != 0)
> > {
> >
> > It looks like the "scratch[-1] != s_before" expression was previously
> > folded to false, but now it's folded to undef (and the branch ends up
> > going the other way). That matches the commit message from the
> > bisection.
>
> GMP bugs may be reported to their bug list, and the UBSan might be
> strengthened to capture this.
>
>
>
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