Re: arch/x86/lib/copy_mc.c:30:1: warning: no previous prototype for 'copy_mc_fragile_handle_tail'

[Dan Williams <dan.j.williams@intel.com>]

On Wed, Jun 9, 2021 at 3:43 PM kernel test robot <lkp@intel.com> wrote:
>
> Hi Dan,
>
> FYI, the error/warning still remains.
>
> tree:   https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git master
> head:   368094df48e680fa51cedb68537408cfa64b788e
> commit: ec6347bb43395cb92126788a1a5b25302543f815 x86, powerpc: Rename memcpy_mcsafe() to copy_mc_to_{user, kernel}()
> date:   8 months ago
> config: x86_64-randconfig-r025-20210610 (attached as .config)
> compiler: gcc-9 (Debian 9.3.0-22) 9.3.0
> reproduce (this is a W=1 build):
>         # https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=ec6347bb43395cb92126788a1a5b25302543f815
>         git remote add linus https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
>         git fetch --no-tags linus master
>         git checkout ec6347bb43395cb92126788a1a5b25302543f815
>         # save the attached .config to linux build tree
>         make W=1 ARCH=x86_64
>
> If you fix the issue, kindly add following tag as appropriate
> Reported-by: kernel test robot <lkp@intel.com>
>
> All warnings (new ones prefixed by >>):
>
> >> arch/x86/lib/copy_mc.c:30:1: warning: no previous prototype for 'copy_mc_fragile_handle_tail' [-Wmissing-prototypes]
>       30 | copy_mc_fragile_handle_tail(char *to, char *from, unsigned len)
>          | ^~~~~~~~~~~~~~~~~~~~~~~~~~~

This routine is only ever called from ASM code, so the fact that there
is no C declaration is expected.

For W=1 builds we could do something like this:

diff --git a/arch/x86/lib/Makefile b/arch/x86/lib/Makefile
index bad4dee4f0e4..b87f40cbc3d5 100644
--- a/arch/x86/lib/Makefile
+++ b/arch/x86/lib/Makefile
@@ -44,6 +44,7 @@ obj-$(CONFIG_SMP) += msr-smp.o cache-smp.o
 lib-y := delay.o misc.o cmdline.o cpu.o
 lib-y += usercopy_$(BITS).o usercopy.o getuser.o putuser.o
 lib-y += memcpy_$(BITS).o
+CFLAGS_copy_mc.o = -Wno-strict-prototypes
 lib-$(CONFIG_ARCH_HAS_COPY_MC) += copy_mc.o copy_mc_64.o
 lib-$(CONFIG_INSTRUCTION_DECODER) += insn.o inat.o insn-eval.o
 lib-$(CONFIG_RANDOMIZE_BASE) += kaslr.o

...but I don't like it nor adding a dummy declaration.