linux-kernel - Re: [PATCH] mips: avoid explicit UB in assignment of mips_io_port

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Message-ID: <20190729211511.GA74577@archlinux-threadripper>
Date:   Mon, 29 Jul 2019 14:15:11 -0700
From:   Nathan Chancellor <natechancellor@...il.com>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     ralf@...ux-mips.org, paul.burton@...s.com, jhogan@...nel.org,
        Eli Friedman <efriedma@...cinc.com>,
        "Maciej W. Rozycki" <macro@...ux-mips.org>,
        Hassan Naveed <hnaveed@...ecomp.com>,
        Stephen Kitt <steve@....org>,
        Serge Semin <fancer.lancer@...il.com>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...e.com>, linux-mips@...r.kernel.org,
        linux-kernel@...r.kernel.org, clang-built-linux@...glegroups.com
Subject: Re: [PATCH] mips: avoid explicit UB in assignment of
 mips_io_port_base

On Mon, Jul 29, 2019 at 02:10:12PM -0700, Nick Desaulniers wrote:
> The code in question is modifying a variable declared const through
> pointer manipulation.  Such code is explicitly undefined behavior, and
> is the lone issue preventing malta_defconfig from booting when built
> with Clang:
> 
> If an attempt is made to modify an object defined with a const-qualified
> type through use of an lvalue with non-const-qualified type, the
> behavior is undefined.
> 
> LLVM is removing such assignments. A simple fix is to not declare
> variables const that you plan on modifying.  Limiting the scope would be
> a better method of preventing unwanted writes to such a variable.
> 
> Further, the code in question mentions "compiler bugs" without any links
> to bug reports, so it is difficult to know if the issue is resolved in
> GCC. The patch was authored in 2006, which would have been GCC 4.0.3 or
> 4.1.1. The minimal supported version of GCC in the Linux kernel is
> currently 4.6.
> 
> For what its worth, there was UB before the commit in question, it just
> added a barrier and got lucky IRT codegen. I don't think there's any
> actual compiler bugs related, just runtime bugs due to UB.
> 
> Link: https://github.com/ClangBuiltLinux/linux/issues/610
> Fixes: 966f4406d903 ("[MIPS] Work around bad code generation for <asm/io.h>.")
> Reported-by: Nathan Chancellor <natechancellor@...il.com>
> Debugged-by: Nathan Chancellor <natechancellor@...il.com>
> Suggested-by: Eli Friedman <efriedma@...cinc.com>
> Signed-off-by: Nick Desaulniers <ndesaulniers@...gle.com>

Reviewed-by: Nathan Chancellor <natechancellor@...il.com>
Tested-by: Nathan Chancellor <natechancellor@...il.com>