[PATCH] BaseTools/GenFw ARM: don't permit R_ARM_GOT_PREL relocations

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

We currently permit R_ARM_GOT_PREL relocations in the ELF32 conversion
routines, under the assumption that relative relocations are fine as
long as the section layout is the same between ELF and PE/COFF.

However, as is the case with any proxy generating relocation, it is
up to the linker to emit an entry in the GOT table and populate it
with the correct absolute address, which should also be fixed up at
PE/COFF load time. Unfortunately, the relocations covering the GOT
section are not emitted into the static relocation sections processed
by GenFw, but only in the dynamic relocation section as a R_ARM_RELATIVE
relocation, and so GenFw fails to emit the correct PE/COFF relocation
data for GOT entries.

Since GOT indirection is pointless anyway for PE/COFF modules running
in UEFI context, let's just drop the references to R_ARM_GOT_PREL from
GenFw, resulting in a build time failure rather than a runtime failure
if such relocations do occur.

Cc: Bob Feng <bob.c.feng@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Cc: Leif Lindholm <leif.lindholm@linaro.org>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
---

Without this patch, CLANG38 builds of ArmVirtQemuKernel-ARM (in LTO mode)
succeed, but the resulting binaries are broken. This is due to the fact that
the PIE linker running in LTO mode insists on emitting GOT based relocations,
while we don't have the code to fix up the contents of the GOT. This change
puts it in line with the AARCH64 build of the same platform/toolchains,
which chokes on GOT based relocations as well. Since the use of the PIE
linker is a peculiarity of ArmVirtQemuKernel/ArmVirtXen, and the fact that
it is impossible to prevent the linker from emitting GOT based relocations,
let's not go out of our way to fix it in the tools, but just drop CLANG38
support from those platforms.

 BaseTools/Source/C/GenFw/Elf32Convert.c | 2 --
 1 file changed, 2 deletions(-)

diff --git a/BaseTools/Source/C/GenFw/Elf32Convert.c b/BaseTools/Source/C/GenFw/Elf32Convert.c
index 3d7de6d5c123..23e8065756e6 100644
--- a/BaseTools/Source/C/GenFw/Elf32Convert.c
+++ b/BaseTools/Source/C/GenFw/Elf32Convert.c
@@ -837,7 +837,6 @@ WriteSections32 (
           case R_ARM_LDC_PC_G0:
           case R_ARM_LDC_PC_G1:
           case R_ARM_LDC_PC_G2:
-          case R_ARM_GOT_PREL:
           case R_ARM_THM_JUMP11:
           case R_ARM_THM_JUMP8:
           case R_ARM_TLS_GD32:
@@ -964,7 +963,6 @@ WriteRelocations32 (
             case R_ARM_LDC_PC_G0:
             case R_ARM_LDC_PC_G1:
             case R_ARM_LDC_PC_G2:
-            case R_ARM_GOT_PREL:
             case R_ARM_THM_JUMP11:
             case R_ARM_THM_JUMP8:
             case R_ARM_TLS_GD32:
-- 
2.19.2