linux-kernel - Re: [PATCH] ubsan: remove overflow checks

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Date:   Wed, 10 Feb 2021 14:55:20 +0300
From:   Andrey Rybainin <ryabinin.a.a@...il.com>
To:     Kees Cook <keescook@...omium.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, linux-next@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Randy Dunlap <rdunlap@...radead.org>,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Alexander Viro <viro@...iv.linux.org.uk>
Subject: Re: [PATCH] ubsan: remove overflow checks



On 2/10/21 3:19 AM, Kees Cook wrote:
> On Wed, Feb 10, 2021 at 02:23:48AM +0300, Andrey Ryabinin wrote:
>> Since GCC 8.0 -fsanitize=signed-integer-overflow doesn't work with -fwrapv.
>> -fwrapv makes signed overflows defines and GCC essentially disables
>> ubsan checks. On GCC < 8.0 -fwrapv doesn't have influence on
>> -fsanitize=signed-integer-overflow setting, so it kinda works
>> but generates false-positves and violates uaccess rules:
>>
>> lib/iov_iter.o: warning: objtool: iovec_from_user()+0x22d: call to __ubsan_handle_add_overflow() with UACCESS enabled
>>
>> Disable signed overflow checks to avoid these problems.
>> Remove unsigned overflow checks as well.
>> Unsigned overflow appeared as side effect of the commit
>>  cdf8a76fda4a ("ubsan: move cc-option tests into Kconfig"),
>> but it never worked (kernel doesn't boot). And unsigned overflows
>> are allowed by C standard, so it just pointless.
>>
>> Signed-off-by: Andrey Ryabinin <ryabinin.a.a@...il.com>
> 
> NAK, please don't remove the entire thing. I want this to work again
> with -fwrapv, and it's not entirely broken under Clang. But the feature
> shouldn't be removed from the kernel.
> 

This is dead code. How exactly it's useful to keep it? You can always resurrect it later when you'll need it.
Clang has the same behavior as GCC > 8, signed-integer-overflow does nothing when compiled -fwrapv or -fno-strict-overflow