Re: [PATCH] BaseTools/GenFw AARCH64: fix up GOT based relative relocations

["Leif Lindholm" <leif.lindholm@linaro.org>]

On Tue, Sep 03, 2019 at 09:17:33PM -0700, Ard Biesheuvel wrote:
> We take great care to avoid GOT based relocations in EDK2 executables,
> primarily because they are pointless - we don't care about things like
> the CoW footprint or relocations that target read-only sections, and so
> GOT entries only bloat the binary.
> 
> However, in some cases (e.g., when building the relocatable PrePi SEC
> module in ArmVirtPkg with the CLANG38 toolchain), we may end up with
> some GOT based relocations nonetheless, which break the build since
> GenFw does not know how to deal with them.
> 
> The relocations emitted in this case are ADRP/LDR instruction pairs
> that are annotated as GOT based, which means that it is the linker's
> job to emit the GOT entry and tag it with an appropriate dynamic
> relocation that ensures that the correct absolute value is stored into
> the GOT entry when the executable is loaded. This dynamic relocation
> not visible to GenFw, and so populating the PE/COFF relocation section
> for these entries is non-trivial.
> 
> Since each ADRP/LDR pair refers to a single symbol that is local to the
> binary (given that shared libraries are not supported), we can actually
> convert the ADRP/LDR pair into an ADRP/ADD pair that produces the symbol
> address directly rather than loading it from memory. This leaves the
> GOT entry in the binary, but since it is now unused, it is no longer
> necessary to emit a PE/COFF relocation entry for it.
> 
> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>

This is a very neat fix. My only concern is that I am not able to
reproduce the issue on my local Buster with clang7 (default). Is it
reproducible with clang8?

/
    Leif

> ---
>  BaseTools/Source/C/GenFw/Elf64Convert.c | 28 +++++++++++++++++++-
>  1 file changed, 27 insertions(+), 1 deletion(-)
> 
> diff --git a/BaseTools/Source/C/GenFw/Elf64Convert.c b/BaseTools/Source/C/GenFw/Elf64Convert.c
> index 3d6319c821e9..d574300ac4fe 100644
> --- a/BaseTools/Source/C/GenFw/Elf64Convert.c
> +++ b/BaseTools/Source/C/GenFw/Elf64Convert.c
> @@ -1017,6 +1017,31 @@ WriteSections64 (
>          } else if (mEhdr->e_machine == EM_AARCH64) {
>  
>            switch (ELF_R_TYPE(Rel->r_info)) {
> +            INT64 Offset;
> +
> +          case R_AARCH64_LD64_GOT_LO12_NC:
> +            //
> +            // Convert into an ADD instruction - see R_AARCH64_ADR_GOT_PAGE below.
> +            //
> +            *(UINT32 *)Targ &= 0x3ff;
> +            *(UINT32 *)Targ |= 0x91000000 | ((Sym->st_value & 0xfff) << 10);
> +            break;
> +
> +          case R_AARCH64_ADR_GOT_PAGE:
> +            //
> +            // This relocation points to the GOT entry that contains the absolute
> +            // address of the symbol we are referring to. Since EDK2 only uses
> +            // fully linked binaries, we can avoid the indirection, and simply
> +            // refer to the symbol directly. This implies having to patch the
> +            // subsequent LDR instruction (covered by a R_AARCH64_LD64_GOT_LO12_NC
> +            // relocation) into an ADD instruction - this is handled above.
> +            //
> +            Offset = (Sym->st_value - (Rel->r_offset & ~0xfff)) >> 12;
> +
> +            *(UINT32 *)Targ &= 0x9000001f;
> +            *(UINT32 *)Targ |= ((Offset & 0x1ffffc) << (5 - 2)) | ((Offset & 0x3) << 29);
> +
> +            /* fall through */
>  
>            case R_AARCH64_ADR_PREL_PG_HI21:
>              //
> @@ -1037,7 +1062,6 @@ WriteSections64 (
>                // Attempt to convert the ADRP into an ADR instruction.
>                // This is only possible if the symbol is within +/- 1 MB.
>                //
> -              INT64 Offset;
>  
>                // Decode the ADRP instruction
>                Offset = (INT32)((*(UINT32 *)Targ & 0xffffe0) << 8);
> @@ -1212,6 +1236,8 @@ WriteRelocations64 (
>              case R_AARCH64_LDST32_ABS_LO12_NC:
>              case R_AARCH64_LDST64_ABS_LO12_NC:
>              case R_AARCH64_LDST128_ABS_LO12_NC:
> +            case R_AARCH64_ADR_GOT_PAGE:
> +            case R_AARCH64_LD64_GOT_LO12_NC:
>                //
>                // No fixups are required for relative relocations, provided that
>                // the relative offsets between sections have been preserved in
> -- 
> 2.17.1
>