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    Subject[PATCH 4.19 522/639] mips: avoid explicit UB in assignment of mips_io_port_base
    Date
    From: Nick Desaulniers <ndesaulniers@google.com>

    [ Upstream commit 12051b318bc3ce5b42d6d786191008284b067d83 ]

    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>
    Signed-off-by: Paul Burton <paul.burton@mips.com>
    Cc: ralf@linux-mips.org
    Cc: jhogan@kernel.org
    Cc: Maciej W. Rozycki <macro@linux-mips.org>
    Cc: Hassan Naveed <hnaveed@wavecomp.com>
    Cc: Stephen Kitt <steve@sk2.org>
    Cc: Serge Semin <fancer.lancer@gmail.com>
    Cc: Mike Rapoport <rppt@linux.ibm.com>
    Cc: Andrew Morton <akpm@linux-foundation.org>
    Cc: Michal Hocko <mhocko@suse.com>
    Cc: linux-mips@vger.kernel.org
    Cc: linux-kernel@vger.kernel.org
    Cc: clang-built-linux@googlegroups.com
    Signed-off-by: Sasha Levin <sashal@kernel.org>
    ---
    arch/mips/include/asm/io.h | 14 ++------------
    arch/mips/kernel/setup.c | 2 +-
    2 files changed, 3 insertions(+), 13 deletions(-)

    diff --git a/arch/mips/include/asm/io.h b/arch/mips/include/asm/io.h
    index 54c730aed3271..df1eaa3652794 100644
    --- a/arch/mips/include/asm/io.h
    +++ b/arch/mips/include/asm/io.h
    @@ -62,21 +62,11 @@
    * instruction, so the lower 16 bits must be zero. Should be true on
    * on any sane architecture; generic code does not use this assumption.
    */
    -extern const unsigned long mips_io_port_base;
    +extern unsigned long mips_io_port_base;

    -/*
    - * Gcc will generate code to load the value of mips_io_port_base after each
    - * function call which may be fairly wasteful in some cases. So we don't
    - * play quite by the book. We tell gcc mips_io_port_base is a long variable
    - * which solves the code generation issue. Now we need to violate the
    - * aliasing rules a little to make initialization possible and finally we
    - * will need the barrier() to fight side effects of the aliasing chat.
    - * This trickery will eventually collapse under gcc's optimizer. Oh well.
    - */
    static inline void set_io_port_base(unsigned long base)
    {
    - * (unsigned long *) &mips_io_port_base = base;
    - barrier();
    + mips_io_port_base = base;
    }

    /*
    diff --git a/arch/mips/kernel/setup.c b/arch/mips/kernel/setup.c
    index 8aaaa42f91ed6..e87c98b8a72c0 100644
    --- a/arch/mips/kernel/setup.c
    +++ b/arch/mips/kernel/setup.c
    @@ -76,7 +76,7 @@ static char __initdata builtin_cmdline[COMMAND_LINE_SIZE] = CONFIG_CMDLINE;
    * mips_io_port_base is the begin of the address space to which x86 style
    * I/O ports are mapped.
    */
    -const unsigned long mips_io_port_base = -1;
    +unsigned long mips_io_port_base = -1;
    EXPORT_SYMBOL(mips_io_port_base);

    static struct resource code_resource = { .name = "Kernel code", };
    --
    2.20.1


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