[PATCH] efi_loader: Avoid emitting efi_var_buf to .GOT

[Ilias Apalodimas]

On Fri, Jan 15, 2021 at 11:33:40AM -0800, Atish Patra wrote:
> On Fri, Jan 15, 2021 at 8:00 AM Ilias Apalodimas
> <ilias.apalodimas at linaro.org> wrote:
> >
> > Atish reports than on RISC-V, accessing the EFI variables causes
> > a kernel panic. An objdump of the file verifies that, since the
> > global pointer for efi_var_buf ends up in .GOT section which is
> > not mapped in virtual address space for Linux.
> >
> > <snip of efi_var_mem_find>
> >
> > 0000000000000084 <efi_var_mem_find>:
> >   84:   715d                    addi    sp,sp,-80
> >
> > * objdump -dr
> > 0000000000000086 <.LCFI2>:
> >   86:   e0a2                    sd  s0,64(sp)
> >   88:   fc26                    sd  s1,56(sp)
> >   8a:   e486                    sd  ra,72(sp)
> >   8c:   f84a                    sd  s2,48(sp)
> >   8e:   f44e                    sd  s3,40(sp)
> >   90:   f052                    sd  s4,32(sp)
> >   92:   ec56                    sd  s5,24(sp)
> >   94:   00000497            auipc   s1,0x0
> >             94: R_RISCV_GOT_HI20    efi_var_buf
> >   98:   0004b483            ld  s1,0(s1) # 94 <.LCFI2+0xe>
> >             98: R_RISCV_PCREL_LO12_I    .L0
> >             98: R_RISCV_RELAX   *ABS*
> >
> > * objdump -t
> > 0000000000000084 g     F .text.efi_runtime  00000000000000b8 efi_var_mem_find
> >
> > With the patch applied:
> >
> > * objdump -dr
> > 0000000000000086 <.LCFI2>:
> >   86:   e0a2                    sd  s0,64(sp)
> >   88:   fc26                    sd  s1,56(sp)
> >   8a:   e486                    sd  ra,72(sp)
> >   8c:   f84a                    sd  s2,48(sp)
> >   8e:   f44e                    sd  s3,40(sp)
> >   90:   f052                    sd  s4,32(sp)
> >   92:   ec56                    sd  s5,24(sp)
> >   94:   00000497            auipc   s1,0x0
> >             94: R_RISCV_PCREL_HI20  .LANCHOR0
> >             94: R_RISCV_RELAX   *ABS*
> >   98:   00048493            mv  s1,s1
> >             98: R_RISCV_PCREL_LO12_I    .L0
> >             98: R_RISCV_RELAX   *ABS*
> >
> > * objdump -t
> > 0000000000000008 l     O .data.efi_runtime  0000000000000008 efi_var_buf
> >
> > On arm64 this works, because there's no .GOT entries for this
> > and everything is converted to relative references.
> >
> 
> Just curious to know: Is it because of linker script magic or compiler
> optimization ?
> I might have missed something but I did not find anything relevant in
> the arm64 linker scripts.
> 

I replied on the original thread regarding what happens in Arm and we get the 
feature working [1]

[1] https://lists.denx.de/pipermail/u-boot/2021-January/437484.html
Cheers
/Ilias