Re: [PATCH 1/1] BaseTools/GenFw AARCH64: disregard ADRP instructions that are patched already

["Ard Biesheuvel" <ard.biesheuvel@linaro.org>]

(+ Leif, Liming)

On Thu, 7 Nov 2019 at 10:06, Ard Biesheuvel <ard.biesheuvel@linaro.org> wrote:
>
> In order to permit the use of compilers that only implement the small
> code model [which involves the use of ADRP instructions that require
> 4 KB segment alignment] for generating PE/COFF binaries with a small
> footprint, we patch ADRP instructions into ADR instructions while doing
> the ELF to PE/COFF conversion.
>
> As it turns out, the linker may be doing the same, but for different
> reasons: there is a silicon erratum #843419 for ARM Cortex-A53 which
> affects ADRP instructions appearing at a certain offset in memory, and
> one of the mitigations for this erratum is to patch them into ADR
> instructions at link time if the symbol reference is within -/+ 1 MB.
> However, the LD linker fails to update the static relocation tables, and
> so we end up with an ADR instruction in the fully linked binary, but
> with a relocation entry in the RELA section identifying it as an ADRP
> instruction.
>
> Since the linker has already updated the symbol reference, there is no
> handling needed in GenFw for such instructions, and we can simply treat
> it as an ordinary ADR. However, since it is guaranteed to be accompanied
> by an add or load instruction with a LO12 relocation referencing the same
> symbol, the section offset check we apply to ADR instructions is going to
> take place anyway, so we can just disregard the ADR instruction entirely.
>
> Reported-by: Eugene Cohen <eugene@hp.com>
> Suggested-by: Eugene Cohen <eugene@hp.com>
> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
> ---
>  BaseTools/Source/C/GenFw/Elf64Convert.c | 13 +++++++++++++
>  1 file changed, 13 insertions(+)
>
> diff --git a/BaseTools/Source/C/GenFw/Elf64Convert.c b/BaseTools/Source/C/GenFw/Elf64Convert.c
> index d574300ac4fe..d623dce1f9da 100644
> --- a/BaseTools/Source/C/GenFw/Elf64Convert.c
> +++ b/BaseTools/Source/C/GenFw/Elf64Convert.c
> @@ -1044,6 +1044,19 @@ WriteSections64 (
>              /* fall through */
>
>            case R_AARCH64_ADR_PREL_PG_HI21:
> +            //
> +            // In order to handle Cortex-A53 erratum #843419, the LD linker may
> +            // convert ADRP instructions into ADR instructions, but without
> +            // updating the static relocation type, and so we may end up here
> +            // while the instruction in question is actually ADR. So let's
> +            // just disregard it: the section offset check we apply below to
> +            // ADR instructions will trigger for its R_AARCH64_xxx_ABS_LO12_NC
> +            // companion instruction as well, so it is safe to omit it here.
> +            //
> +            if ((*(UINT32 *)Targ & BIT31) == 0) {
> +              break;
> +            }
> +
>              //
>              // AArch64 PG_H21 relocations are typically paired with ABS_LO12
>              // relocations, where a PC-relative reference with +/- 4 GB range is
> --
> 2.17.1
>