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 ab1df19b0957..60604b26fa72 100644 --- a/arch/mips/include/asm/io.h +++ b/arch/mips/include/asm/io.h @@ -60,21 +60,11 @@ * 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 4f9f1ae49213..fadc946b306d 100644 --- a/arch/mips/kernel/setup.c +++ b/arch/mips/kernel/setup.c @@ -80,7 +80,7 @@ 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", };