linux-kernel - Re: [PATCH] ubsan: Require GCC-8+ or Clang to use UBSAN

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Date:   Mon, 18 Jan 2021 11:53:37 -0600
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Andrey Ryabinin <aryabinin@...tuozzo.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        linux-kernel@...r.kernel.org, Randy Dunlap <rdunlap@...radead.org>,
        dvyukov@...gle.com, keescook@...omium.org
Subject: Re: [PATCH] ubsan: Require GCC-8+ or Clang to use UBSAN

On Thu, Jan 14, 2021 at 02:09:28PM +0300, Andrey Ryabinin wrote:
> 
> 
> On 1/14/21 1:59 PM, Peter Zijlstra 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+ ?
> > 
> > ---
> > Subject: ubsan: Require GCC-8+ or Clang to use UBSAN
> > 
> > Just like how we require GCC-8.2 for KASAN due to compiler bugs, require
> > a sane version of GCC for UBSAN.
> > 
> > Specifically, before GCC-8 UBSAN doesn't respect -fwrapv and thinks
> > signed arithmetic is buggered.
> > 
> 
> Actually removing CONFIG_UBSAN_SIGNED_OVERFLOW would give us the same
> effect without restricting GCC versions.

Is that preferable?  Always happy to remove code, just need some
justification behind it.

-- 
Josh