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Subject: Re: [PATCH] ubsan: Require GCC-8+ or Clang to use UBSAN
To:     Peter Zijlstra <peterz@infradead.org>,
        Josh Poimboeuf <jpoimboe@redhat.com>
Cc:     Alexander Viro <viro@zeniv.linux.org.uk>,
        linux-kernel@vger.kernel.org, Randy Dunlap <rdunlap@infradead.org>,
        dvyukov@google.com, keescook@chromium.org
References: <590998aa9cc50f431343f76cae72b2abf8ac1fdd.1608699683.git.jpoimboe@redhat.com>
 <20210104151317.GR3021@hirez.programming.kicks-ass.net>
 <YAAj9aAcPsV9I6UL@hirez.programming.kicks-ass.net>
From:   Andrey Ryabinin <aryabinin@virtuozzo.com>
Message-ID: <e291008b-6b4d-9da4-1353-0762bc68e8ea@virtuozzo.com>
Date:   Thu, 14 Jan 2021 14:09:28 +0300
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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.