Re: [edk2-devel] [PATCH] BaseTools/GenFw AARCH64: Convert more types of explicit GOT references

["Leif Lindholm" <quic_llindhol@quicinc.com>]

On 2022-09-06 11:30, Ard Biesheuvel wrote:
> Bob, Liming, Leif: any thoughts?

I'm happy with this.
(Spotted one typo - "ofset".)

Acked-by: Leif Lindholm <quic_llindhol@quicinc.com>

> On Sun, 21 Aug 2022 at 16:33, Rebecca Cran <rebecca@bsdio.com> wrote:
>>
>> Tested-by: Rebecca Cran <rebecca@bsdio.com>
>>
>>
>> On 8/21/22 08:16, Ard Biesheuvel wrote:
>>> Rebecca reports that builds of AArch64 DSCs that involve PIE linking
>>> when using ELF based toolchains are failing in some cases, resulting in
>>> an error message like
>>>
>>>     bad definition for symbol '_GLOBAL_OFFSET_TABLE_'@0x72d8 or
>>>     unsupported symbol type.  For example, absolute and undefined symbols
>>>     are not supported.
>>>
>>> The reason turns out to be that, while GenFw does carry some logic to
>>> convert GOT based symbol references into direct ones (which is always
>>> possible given that our ELF to PE/COFF conversion only supports fully
>>> linked executables), it does not support all possible combinations of
>>> relocations that the linker may emit to load symbol addresses from the
>>> GOT.
>>>
>>> In particular, when performing a non-LTO link on object code built with
>>> GCC using -fpie, we may end up with GOT based references such as the one
>>> below, where the address of the GOT itself is taken, and the ofset of
>>> the symbol in the GOT is reflected in the immediate offset of the
>>> subsequent LDR instruction.
>>>
>>>     838:   adrp    x0, 16000
>>>     838: R_AARCH64_ADR_PREL_PG_HI21 _GLOBAL_OFFSET_TABLE_
>>>     83c:   ldr     x0, [x0, #2536]
>>>     83c: R_AARCH64_LD64_GOTPAGE_LO15        _gPcd_BinaryPatch_PcdFdBaseAddress
>>>
>>> The reason that we omit GOT based symbol references when performing ELF to
>>> PE/COFF conversion is that the GOT is not described by static ELF
>>> relocations, which means that the ELF file lacks the metadata to
>>> generate the PE/COFF relocations covering the GOT table in the PE/COFF
>>> executable. Given that none of the usual motivations for using a GOT
>>> (copy on write footprint, shared libraries) apply to EFI executables in
>>> the first place, the easiest way around this is to convert all GOT based
>>> symbol address loads to PC relative ADR/ADRP instructions.
>>>
>>> So implement this handling for R_AARCH64_LD64_GOTPAGE_LO15 and
>>> R_AARCH64_LD64_GOTOFF_LO15 relocations as well, and turn the LDR
>>> instructions in question into ADR instructions that generate the
>>> address immediately.
>>>
>>> This leaves the reference to _GLOBAL_OFFSET_TABLE_ itself, which is what
>>> generated the error to begin with. Considering that this symbol is never
>>> referenced (i.e., it doesn't appear anywhere in the code) and is only
>>> meaningful in combination with R_*_GOT_* based relocations that follow
>>> it, we can just disregard any references to it entirely, given that we
>>> convert all of those followup relocations into direct references.
>>>
>>> Cc: Rebecca Cran <rebecca@bsdio.com>
>>> Cc: Bob Feng <bob.c.feng@intel.com>
>>> Cc: Liming Gao <gaoliming@byosoft.com.cn>
>>> Cc: "Kinney, Michael D" <michael.d.kinney@intel.com>
>>> Cc: Leif Lindholm <quic_llindhol@quicinc.com>
>>> Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
>>> ---
>>>
>>> This patch can be tested using the following method:
>>>
>>> - add the following lines to ArmVirtPkg/PrePi/ArmVirtPrePiUniCoreRelocatable.inf
>>>
>>> [BuildOptions]
>>>     GCC:*_*_AARCH64_CC_FLAGS = -fpie
>>>     GCC:*_*_AARCH64_DLINK_FLAGS = -Wl,-Bsymbolic,-pie
>>>
>>> - build ArmVirtPkg/ArmVirtQemuKernel.dsc in DEBUG mode using GCC49
>>>     (other combos might work as well) and observe the build failure
>>>
>>> - apply this patch and observe that the build failure is gone
>>>
>>> - boot the resulting image in QEMU using the -kernel ../QEMU_EFI.fd
>>>     command line option and observe that the image boots as usual
>>>
>>>    BaseTools/Source/C/GenFw/Elf64Convert.c | 35 ++++++++++++++++++++
>>>    1 file changed, 35 insertions(+)
>>>
>>> diff --git a/BaseTools/Source/C/GenFw/Elf64Convert.c b/BaseTools/Source/C/GenFw/Elf64Convert.c
>>> index 35e96dd05bc2..3173ca9280f4 100644
>>> --- a/BaseTools/Source/C/GenFw/Elf64Convert.c
>>> +++ b/BaseTools/Source/C/GenFw/Elf64Convert.c
>>> @@ -1305,6 +1305,22 @@ WriteSections64 (
>>>            Elf_Shdr *SymShdr;
>>>            UINT8    *Targ;
>>>
>>> +        //
>>> +        // The _GLOBAL_OFFSET_TABLE_ symbol is not actually an absolute symbol,
>>> +        // but carries the SHN_ABS section index for historical reasons.
>>> +        // It must be accompanied by a R_*_GOT_* type relocation on a
>>> +        // subsequent instruction, which we handle below, specifically to avoid
>>> +        // the GOT indirection, and to refer to the symbol directly. This means
>>> +        // we can simply disregard direct references to the GOT symbol itself,
>>> +        // as the resulting value will never be used.
>>> +        //
>>> +        if (Sym->st_shndx == SHN_ABS) {
>>> +          const UINT8 *SymName = GetSymName (Sym);
>>> +          if (strcmp ((CHAR8 *)SymName, "_GLOBAL_OFFSET_TABLE_") == 0) {
>>> +            continue;
>>> +          }
>>> +        }
>>> +
>>>            //
>>>            // Check section header index found in symbol table and get the section
>>>            // header location.
>>> @@ -1448,6 +1464,23 @@ WriteSections64 (
>>>              switch (ELF_R_TYPE(Rel->r_info)) {
>>>                INT64 Offset;
>>>
>>> +          case R_AARCH64_LD64_GOTOFF_LO15:
>>> +          case R_AARCH64_LD64_GOTPAGE_LO15:
>>> +            //
>>> +            // Convert into an ADR instruction that refers to the symbol directly.
>>> +            //
>>> +            Offset = Sym->st_value - Rel->r_offset;
>>> +
>>> +            *(UINT32 *)Targ &= 0x1000001f;
>>> +            *(UINT32 *)Targ |= ((Offset & 0x1ffffc) << (5 - 2)) | ((Offset & 0x3) << 29);
>>> +
>>> +            if (Offset < -0x100000 || Offset > 0xfffff) {
>>> +              Error (NULL, 0, 3000, "Invalid", "WriteSections64(): %s failed to relax GOT based symbol reference - image is too big (>1 MiB).",
>>> +                mInImageName);
>>> +              break;
>>> +            }
>>> +            break;
>>> +
>>>              case R_AARCH64_LD64_GOT_LO12_NC:
>>>                //
>>>                // Convert into an ADD instruction - see R_AARCH64_ADR_GOT_PAGE below.
>>> @@ -1686,6 +1719,8 @@ WriteRelocations64 (
>>>                case R_AARCH64_LDST128_ABS_LO12_NC:
>>>                case R_AARCH64_ADR_GOT_PAGE:
>>>                case R_AARCH64_LD64_GOT_LO12_NC:
>>> +            case R_AARCH64_LD64_GOTOFF_LO15:
>>> +            case R_AARCH64_LD64_GOTPAGE_LO15:
>>>                  //
>>>                  // No fixups are required for relative relocations, provided that
>>>                  // the relative offsets between sections have been preserved in