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Date:   Thu, 7 Jan 2021 13:07:21 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Randy Dunlap <rdunlap@...radead.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        linux-kernel@...r.kernel.org, aryabinin@...tuozzo.com,
        dvyukov@...gle.com
Subject: Re: [PATCH] mm/uaccess: Use 'unsigned long' to placate UBSAN
 warnings, again

On Wed, Jan 06, 2021 at 04:06:57PM -0800, Randy Dunlap wrote:
> On 1/6/21 3:37 PM, Kees Cook 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?
> 
> I use UBSAN successfully with GCC 7.5.0.
> However, I can revert whatever future patch someone adds for this...

Peter, which GCC version specifically are you seeing this on? (i.e. can
I just make in 7.5+ instead of 8+ to make Randy's life easier?)

-- 
Kees Cook

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