Re: [PATCH -next v2 0/2] Remove unused variables in x86/boot

[Nathan Chancellor <nathan@kernel.org>]

On Thu, Sep 29, 2022 at 10:16:11AM +0800, Li Zetao wrote:
> Hi Nathan,
> 
> On 2022/9/28 5:50, Nathan Chancellor wrote:
> > Hi Li,
> > 
> > On Tue, Sep 27, 2022 at 08:15:10AM +0000, Li Zetao wrote:
> > > This patch set removes some unused variables in x86/boot, and add the
> > > "-Wall" flag to Makefile, which is the old problem of x86 not sharing
> > > makefiles.
> > > 
> > > Changes since v1:
> > > - Add "-Wall" flag to x86/boot/compressed/Makefile
> > > - Remove unused variables "et" in efi_get_system_table() and "ret" in
> > >    efi_get_conf_table()
> > > - Remove unused variables "ret" in __efi_get_rsdp_addr() and
> > >    "nr_tables" in efi_get_rsdp_addr()
> > > 
> > > v1 at:
> > > https://lore.kernel.org/all/20220923113209.3046960-1-lizetao1@huawei.com/
> > > 
> > > Li Zetao (2):
> > >    x86/boot/compressed: Add "-Wall" flag to Makefile
> > >    x86/boot: Remove unused variables
> > > 
> > >   arch/x86/boot/compressed/Makefile | 2 +-
> > >   arch/x86/boot/compressed/acpi.c   | 2 --
> > >   arch/x86/boot/compressed/efi.c    | 2 --
> > >   arch/x86/boot/compressed/sev.c    | 1 -
> > >   4 files changed, 1 insertion(+), 6 deletions(-)
> > I took this series for a spin with clang and found a few extra warnings.
> > 
> > 1.
> > 
> >    In file included from arch/x86/boot/compressed/misc.c:15:
> >    In file included from arch/x86/boot/compressed/misc.h:24:
> >    In file included from ./include/linux/elf.h:6:
> >    In file included from ./arch/x86/include/asm/elf.h:8:
> >    In file included from ./include/linux/thread_info.h:60:
> >    ./arch/x86/include/asm/thread_info.h:175:13: warning: calling '__builtin_frame_address' with a nonzero argument is unsafe [-Wframe-address]
> >            oldframe = __builtin_frame_address(1);
> >                       ^~~~~~~~~~~~~~~~~~~~~~~~~~
> >    ./arch/x86/include/asm/thread_info.h:177:11: warning: calling '__builtin_frame_address' with a nonzero argument is unsafe [-Wframe-address]
> >                    frame = __builtin_frame_address(2);
> >                            ^~~~~~~~~~~~~~~~~~~~~~~~~~
> > 
> > This warning is disabled in the main Makefile for this reason so we
> > should just be able to disable it:
> > 
> > diff --git a/arch/x86/boot/compressed/Makefile b/arch/x86/boot/compressed/Makefile
> > index 10abb7c45d04..3f004567f3d5 100644
> > --- a/arch/x86/boot/compressed/Makefile
> > +++ b/arch/x86/boot/compressed/Makefile
> > @@ -43,6 +43,7 @@ KBUILD_CFLAGS += -mno-mmx -mno-sse
> >   KBUILD_CFLAGS += -ffreestanding -fshort-wchar
> >   KBUILD_CFLAGS += -fno-stack-protector
> >   KBUILD_CFLAGS += $(call cc-disable-warning, address-of-packed-member)
> > +KBUILD_CFLAGS += $(call cc-disable-warning, frame-address)
> >   KBUILD_CFLAGS += $(call cc-disable-warning, gnu)
> >   KBUILD_CFLAGS += -Wno-pointer-sign
> >   KBUILD_CFLAGS += $(call cc-option,-fmacro-prefix-map=$(srctree)/=)
> > 
> > 2.
> > 
> >    arch/x86/boot/compressed/kaslr.c:627:6: warning: unused variable 'i' [-Wunused-variable]
> >            int i;
> >                ^
> > 
> > This happens when CONFIG_MEMORY_HOTREMOVE or CONFIG_ACPI are 'n'. I
> > think it can just be fixed by aligning arch/x86/boot/compressed with the
> > rest of the kernel and explicitly compiling with '-std=gnu11', which
> > will allow us to declare the variable within the for loop, like so.
> > 
> > diff --git a/arch/x86/boot/compressed/Makefile b/arch/x86/boot/compressed/Makefile
> > index 3f004567f3d5..6c7e366a437b 100644
> > --- a/arch/x86/boot/compressed/Makefile
> > +++ b/arch/x86/boot/compressed/Makefile
> > @@ -34,7 +34,7 @@ targets := vmlinux vmlinux.bin vmlinux.bin.gz vmlinux.bin.bz2 vmlinux.bin.lzma \
> >   # be valid.
> >   KBUILD_CFLAGS := -m$(BITS) -O2 $(CLANG_FLAGS)
> >   KBUILD_CFLAGS += -fno-strict-aliasing -fPIE
> > -KBUILD_CFLAGS += -Wundef -Wall
> > +KBUILD_CFLAGS += -Wundef -Wall -std=gnu11
> >   KBUILD_CFLAGS += -DDISABLE_BRANCH_PROFILING
> >   cflags-$(CONFIG_X86_32) := -march=i386
> >   cflags-$(CONFIG_X86_64) := -mcmodel=small -mno-red-zone
> > diff --git a/arch/x86/boot/compressed/kaslr.c b/arch/x86/boot/compressed/kaslr.c
> > index 4a3f223973f4..be859c7e7f6b 100644
> > --- a/arch/x86/boot/compressed/kaslr.c
> > +++ b/arch/x86/boot/compressed/kaslr.c
> > @@ -624,7 +624,6 @@ static bool process_mem_region(struct mem_vector *region,
> >   			       unsigned long minimum,
> >   			       unsigned long image_size)
> >   {
> > -	int i;
> >   	/*
> >   	 * If no immovable memory found, or MEMORY_HOTREMOVE disabled,
> >   	 * use @region directly.
> > @@ -644,7 +643,7 @@ static bool process_mem_region(struct mem_vector *region,
> >   	 * If immovable memory found, filter the intersection between
> >   	 * immovable memory and @region.
> >   	 */
> > -	for (i = 0; i < num_immovable_mem; i++) {
> > +	for (int i = 0; i < num_immovable_mem; i++) {
> >   		u64 start, end, entry_end, region_end;
> >   		struct mem_vector entry;
> > 
> > Additionally, I think these two patches should be reordered so that the
> > warnings are fixed before they are enabled.
> > 
> > With those comments addressed, consider the series:
> > 
> > Reviewed-by: Nathan Chancellor <nathan@kernel.org>
> > 
> > Cheers,
> > Nathan
> 
> What is your clang compiler version and .config? My clang compiler is
> 14.0.0-1ubuntu1, and I can't reproduce
> 
> problem 1("calling '__builtin_frame_address' with a nonzero argument is
> unsafe"). Can you provide more
> 
> information.

Sure thing! My compiler is tip of tree LLVM, so clang 16.0.0. That
should not matter for this particular issue though. I can reproduce it
with my distribution's clang (14.0.6) with a configuration that has
CONFIG_FRAME_POINTER enabled; allnoconfig works for me here.

  $ clang --version
  clang version 14.0.6
  Target: x86_64-pc-linux-gnu
  Thread model: posix
  InstalledDir: /usr/bin

  $ make -skj"$(nproc)" ARCH=i386 LLVM=1 clean allnoconfig all
  ...
  In file included from arch/x86/boot/compressed/misc.c:15:
  In file included from arch/x86/boot/compressed/misc.h:24:
  In file included from ./include/linux/elf.h:6:
  In file included from ./arch/x86/include/asm/elf.h:8:
  In file included from ./include/linux/thread_info.h:60:
  ./arch/x86/include/asm/thread_info.h:175:13: warning: calling '__builtin_frame_address' with a nonzero argument is unsafe [-Wframe-address]
          oldframe = __builtin_frame_address(1);
                     ^~~~~~~~~~~~~~~~~~~~~~~~~~
  ./arch/x86/include/asm/thread_info.h:177:11: warning: calling '__builtin_frame_address' with a nonzero argument is unsafe [-Wframe-address]
                  frame = __builtin_frame_address(2);
                          ^~~~~~~~~~~~~~~~~~~~~~~~~~
  ...

  $ grep CONFIG_FRAME_POINTER .config
  CONFIG_FRAME_POINTER=y

If there is any other information I can provide, please let me know!

Cheers,
Nathan