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From:   Dmitry Vyukov <dvyukov@google.com>
Date:   Thu, 7 Jan 2021 11:51:24 +0100
Message-ID: <CACT4Y+bL3sEE6edPmFkW3zbac4D9+9c19jCNRysXohWx+7=Lzw@mail.gmail.com>
Subject: Re: [PATCH] mm/uaccess: Use 'unsigned long' to placate UBSAN
 warnings, again
To:     Kees Cook <keescook@chromium.org>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        LKML <linux-kernel@vger.kernel.org>,
        Randy Dunlap <rdunlap@infradead.org>,
        Andrey Ryabinin <aryabinin@virtuozzo.com>
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On Thu, Jan 7, 2021 at 12:37 AM Kees Cook <keescook@chromium.org> wrote:
>
> On Mon, Jan 04, 2021 at 04:13:17PM +0100, Peter Zijlstra wrote:
> > On Tue, Dec 22, 2020 at 11:04:54PM -0600, Josh Poimboeuf wrote:
> > > GCC 7 has a known bug where UBSAN ignores '-fwrapv' and generates false
> > > signed-overflow-UB warnings.  The type mismatch between 'i' and
> > > 'nr_segs' in copy_compat_iovec_from_user() is causing such a warning,
> > > which also happens to violate uaccess rules:
> > >
> > >   lib/iov_iter.o: warning: objtool: iovec_from_user()+0x22d: call to __ubsan_handle_add_overflow() with UACCESS enabled
> > >
> > > Fix it by making the variable types match.
> > >
> > > This is similar to a previous commit:
> > >
> > >   29da93fea3ea ("mm/uaccess: Use 'unsigned long' to placate UBSAN warnings on older GCC versions")
> >
> > Maybe it's time we make UBSAN builds depend on GCC-8+ ?
>
> I would be totally fine with that. The only thing I can think of that
> might care is syzbot. Dmitry, does syzbot use anything older than gcc 8?

syzbot should use gcc 10 everywhere by now (and if not, we will update).
And in general I think it's a reasonable requirement today.