linux-kernel - Re: [PATCH] mm/uaccess: Use 'unsigned long' to placate UBSAN warnings, again

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Message-ID: <20210104151317.GR3021@hirez.programming.kicks-ass.net>
Date:   Mon, 4 Jan 2021 16:13:17 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     Alexander Viro <viro@...iv.linux.org.uk>,
        linux-kernel@...r.kernel.org, Randy Dunlap <rdunlap@...radead.org>,
        aryabinin@...tuozzo.com, dvyukov@...gle.com, keescook@...omium.org
Subject: Re: [PATCH] mm/uaccess: Use 'unsigned long' to placate UBSAN
 warnings, again

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+ ?