Re: [PATCH] mips: avoid explicit UB in assignment of mips_io_port_base

[Nathan Chancellor <natechancellor@gmail.com>]

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@gmail.com>
> Debugged-by: Nathan Chancellor <natechancellor@gmail.com>
> Suggested-by: Eli Friedman <efriedma@quicinc.com>
> Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>

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