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authorArd Biesheuvel <ardb@kernel.org>2022-08-21 15:31:21 +0200
committermergify[bot] <37929162+mergify[bot]@users.noreply.github.com>2022-09-08 16:46:11 +0000
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BaseTools/GenFw AARCH64: Convert more types of explicit GOT references
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 offset 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. Signed-off-by: Ard Biesheuvel <ardb@kernel.org> Tested-by: Rebecca Cran <rebecca@bsdio.com> Acked-by: Leif Lindholm <quic_llindhol@quicinc.com> Acked-by: Bob Feng <bob.c.feng@intel.com>
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