[PATCH v2] efistub: Only link libstub to final vmlinux

[Ard Biesheuvel]

On Thu, 9 Oct 2025 at 00:27, Tiezhu Yang <yangtiezhu at loongson.cn> wrote:
>
> On 2025/9/28 下午10:41, Ard Biesheuvel wrote:
> > On Sun, 28 Sept 2025 at 16:39, Ard Biesheuvel <ardb at kernel.org> wrote:
> >>
> >> On Sun, 28 Sept 2025 at 15:52, Huacai Chen <chenhuacai at kernel.org> wrote:
> >>>
> >>> Hi, Ard,
> >>>
> >>> On Sun, Sep 28, 2025 at 9:42 PM Ard Biesheuvel <ardb at kernel.org> wrote:
> >>>>
> >>>> On Sun, 28 Sept 2025 at 10:55, Tiezhu Yang <yangtiezhu at loongson.cn> wrote:
> >>>>>
> >>>>> When compiling with LLVM and CONFIG_LTO_CLANG is set, there exists
> >>>>> the following objtool warning on LoongArch:
> >>>>>
> >>>>>    vmlinux.o: warning: objtool: __efistub_efi_boot_kernel()
> >>>>>    falls through to next function __efistub_exit_boot_func()
> >>>>>
> >>>>> This is because efi_boot_kernel() doesn't end with a return instruction
> >>>>> or an unconditional jump, then objtool has determined that the function
> >>>>> can fall through into the next function.
> >>>>>
> >>>>> At the beginning, try to do something to make efi_boot_kernel() ends with
> >>>>> an unconditional jump instruction, but this modification seems not proper.
> >>>>>
> >>>>> Since the efistub functions are useless for stack unwinder, they can be
> >>>>> ignored by objtool. After many discussions, no need to link libstub to
> >>>>> the vmlinux.o, only link libstub to the final vmlinux.
> >>>>>
> >>>>
...
> Are you OK with the following changes?
>
> (1) libstub doesn't link to vmlinux.o, only link libstub with vmlinux.o
> during the final vmlinux link, keep the changes confined to LoongArch,
> no need to be something more generic.
>
> diff --git a/arch/loongarch/Makefile b/arch/loongarch/Makefile
> index dc5bd3f1b8d2..f34b416f5ca2 100644
> --- a/arch/loongarch/Makefile
> +++ b/arch/loongarch/Makefile
> @@ -169,7 +169,6 @@ CHECKFLAGS += $(shell $(CC) $(KBUILD_CPPFLAGS)
> $(KBUILD_CFLAGS) -dM -E -x c /dev
>   endif
>
>   libs-y += arch/loongarch/lib/
> -libs-$(CONFIG_EFI_STUB) += $(objtree)/drivers/firmware/efi/libstub/lib.a
>
>   drivers-y              += arch/loongarch/crypto/
>
> diff --git a/scripts/link-vmlinux.sh b/scripts/link-vmlinux.sh
> index 433849ff7529..ed94871c3606 100755
> --- a/scripts/link-vmlinux.sh
> +++ b/scripts/link-vmlinux.sh
> @@ -69,6 +69,12 @@ vmlinux_link()
>                  libs="${KBUILD_VMLINUX_LIBS}"
>          fi
>
> +       if [ "${SRCARCH}" = "loongarch" ]; then
> +               if is_enabled CONFIG_EFI_STUB; then
> +                       libs="${libs} drivers/firmware/efi/libstub/lib.a"
> +               fi
> +       fi
> +
>          if is_enabled CONFIG_GENERIC_BUILTIN_DTB; then
>                  objs="${objs} .builtin-dtbs.o"
>          fi
>
> (2) remove the attribute __noreturn for real_kernel_entry() and add
> "while (1);" at the end of efi_boot_kernel().
>
> diff --git a/drivers/firmware/efi/libstub/loongarch.c
> b/drivers/firmware/efi/libstub/loongarch.c
> index 3782d0a187d1..e309fd78fca7 100644
> --- a/drivers/firmware/efi/libstub/loongarch.c
> +++ b/drivers/firmware/efi/libstub/loongarch.c
> @@ -10,7 +10,7 @@
>   #include "efistub.h"
>   #include "loongarch-stub.h"
>
> -typedef void __noreturn (*kernel_entry_t)(bool efi, unsigned long cmdline,
> +typedef void (*kernel_entry_t)(bool efi, unsigned long cmdline,
>                                           unsigned long systab);
>
>   efi_status_t check_platform_features(void)
> @@ -81,4 +81,7 @@ efi_status_t efi_boot_kernel(void *handle,
> efi_loaded_image_t *image,
>
>         real_kernel_entry(true, (unsigned long)cmdline_ptr,
>                           (unsigned long)efi_system_table);
> +
> +       /* We should never get here */
> +       while (1);
>   }
>

Why do we need both (1) and (2)?