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

[Andrey Rybainin <ryabinin.a.a@gmail.com>]

On 2/9/21 9:24 PM, Josh Poimboeuf wrote:
> On Mon, Jan 18, 2021 at 11:53:37AM -0600, Josh Poimboeuf wrote:
>> 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.
> 
> Andrey,
> 
> Is Peter's patch acceptable or do you want to do something else?
> 

I do prefer to just remove the code, I'll send the patch shortly.