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 <natechancel...@gmail.com> > Debugged-by: Nathan Chancellor <natechancel...@gmail.com> > Suggested-by: Eli Friedman <efrie...@quicinc.com> > Signed-off-by: Nick Desaulniers <ndesaulni...@google.com>
Reviewed-by: Nathan Chancellor <natechancel...@gmail.com> Tested-by: Nathan Chancellor <natechancel...@gmail.com>
