Re: [PATCH] riscv/efistub: Ensure GP-relative addressing is not used

[Palmer Dabbelt]

On Fri, 12 Jan 2024 10:51:16 PST (-0800), Ard Biesheuvel wrote:
> Hi Jan,
>
> On Fri, 12 Jan 2024 at 19:37, Jan Kiszka <jan.kiszka@xxxxxxxxxxx> wrote:
> > From: Jan Kiszka <jan.kiszka@xxxxxxxxxxx>
> >
> > The cflags for the RISC-V efistub were missing -mno-relax, thus were
> > under the risk that the compiler could use GP-relative addressing. That
> > happened for _edata with binutils-2.41 and kernel 6.1, causing the
> > relocation to fail due to an invalid kernel_size in handle_kernel_image.
> > It was not yet observed with newer versions, but that may just be luck.
> >
> > Signed-off-by: Jan Kiszka <jan.kiszka@xxxxxxxxxxx>
> > ---
> >
> > Something like this should go to stable as well, but we will need
> > rebased patches.
> >
> >  drivers/firmware/efi/libstub/Makefile | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/drivers/firmware/efi/libstub/Makefile b/drivers/firmware/efi/libstub/Makefile
> > index 06964a3c130f..d561d7de46a9 100644
> > --- a/drivers/firmware/efi/libstub/Makefile
> > +++ b/drivers/firmware/efi/libstub/Makefile
> > @@ -28,7 +28,7 @@ cflags-$(CONFIG_ARM)          += -DEFI_HAVE_STRLEN -DEFI_HAVE_STRNLEN \
> >                                    -DEFI_HAVE_MEMCHR -DEFI_HAVE_STRRCHR \
> >                                    -DEFI_HAVE_STRCMP -fno-builtin -fpic \
> >                                    $(call cc-option,-mno-single-pic-base)
> > -cflags-$(CONFIG_RISCV)         += -fpic -DNO_ALTERNATIVE
> > +cflags-$(CONFIG_RISCV)         += -fpic -DNO_ALTERNATIVE -mno-relax
>
> Can we detect the presence of these references (via the relocation
> type)? We already do something similar for ordinary absolute
> references too.

If there's no `__global_pointer$` symbol then the linker won't make 
GP-relative relaxations (because it doesn't know where GP is).  We 
usually define that symbol in the linker script, but I'm not entierly 
sure how libstub gets its linker script...

> >  cflags-$(CONFIG_LOONGARCH)     += -fpie
> >
> >  cflags-$(CONFIG_EFI_PARAMS_FROM_FDT)   += -I$(srctree)/scripts/dtc/libfdt
> > --
> > 2.35.3