[PATCH v9 02/10] um: use execveat to create userspace MMs

[Nathan Chancellor]

Hi Benjamin,

On Thu, Sep 19, 2024 at 02:45:03PM +0200, Benjamin Berg wrote:
...
> diff --git a/arch/um/kernel/skas/Makefile b/arch/um/kernel/skas/Makefile
> index 6f86d53e3d69..fbb61968055f 100644
> --- a/arch/um/kernel/skas/Makefile
> +++ b/arch/um/kernel/skas/Makefile
> @@ -3,14 +3,43 @@
>  # Copyright (C) 2002 - 2007 Jeff Dike (jdike@{addtoit,linux.intel}.com)
>  #
>  
> -obj-y := stub.o mmu.o process.o syscall.o uaccess.o
> +obj-y := stub.o mmu.o process.o syscall.o uaccess.o \
> +	 stub_exe_embed.o
> +
> +# Stub executable
> +
> +stub_exe_objs-y := stub_exe.o
> +
> +stub_exe_objs := $(foreach F,$(stub_exe_objs-y),$(obj)/$F)
> +
> +# Object file containing the ELF executable
> +$(obj)/stub_exe_embed.o: $(src)/stub_exe_embed.S $(obj)/stub_exe
> +
> +$(obj)/stub_exe.dbg: $(stub_exe_objs) FORCE
> +	$(call if_changed,stub_exe)
> +
> +$(obj)/stub_exe: OBJCOPYFLAGS := -S
> +$(obj)/stub_exe: $(obj)/stub_exe.dbg FORCE
> +	$(call if_changed,objcopy)
> +
> +quiet_cmd_stub_exe = STUB_EXE $@
> +      cmd_stub_exe = $(CC) -nostdlib -o $@ \
> +			   $(KBUILD_CFLAGS) $(STUB_EXE_LDFLAGS) \
> +			   $(filter %.o,$^)
> +
> +STUB_EXE_LDFLAGS = -n -static

After this change in -next as commit 32e8eaf263d9 ("um: use execveat to
create userspace MMs"), I am seeing an error when building ARCH=um
defconfig with clang.

  $ make -skj"$(nproc)" ARCH=um LLVM=1 mrproper defconfig linux
  clang: error: unknown argument: '-n'
  make[7]: *** [arch/um/kernel/skas/Makefile:19: arch/um/kernel/skas/stub_exe.dbg] Error 1
  ...

I have looked through the GCC manual and options index but I do not see
what '-n' is. Was this intended to be passed to the linker like -Wl,-n
or was there some other option that this should be? If I try dropping
it, I end up running into another issue, which could be entirely
unrelated.

  $ make -skj"$(nproc)" ARCH=um LLVM=1 mrproper defconfig arch/um/kernel/skas/
  /usr/bin/ld: arch/um/kernel/skas/stub_exe.o: in function `_start':
  arch/um/kernel/skas/stub_exe.c:83:(.ltext+0x15): undefined reference to `memset'
  clang: error: linker command failed with exit code 1 (use -v to see invocation)
  make[7]: *** [arch/um/kernel/skas/Makefile:19: arch/um/kernel/skas/stub_exe.dbg] Error 1

Cheers,
Nathan

> +targets += stub_exe.dbg stub_exe $(stub_exe_objs-y)
> +
> +# end
>  
>  # stub.o is in the stub, so it can't be built with profiling
>  # GCC hardened also auto-enables -fpic, but we need %ebx so it can't work ->
>  # disable it
>  
>  CFLAGS_stub.o := $(CFLAGS_NO_HARDENING)
> -UNPROFILE_OBJS := stub.o
> +CFLAGS_stub_exe.o := $(CFLAGS_NO_HARDENING)
> +UNPROFILE_OBJS := stub.o stub_exe.o
>  KCOV_INSTRUMENT := n
>  
>  include $(srctree)/arch/um/scripts/Makefile.rules