[PATCH 2/2] ArmVirtPkg/PrePiUniCoreRelocatable CLANG38: work around build issues

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

The self-relocating PrePi module that is used by the ArmVirtQemuKernel
and ArmVirtXen targets runs the linker in PIE mode so that it emits
dynamic relocations into the final image in a way that permits the
module to relocate itself into place before calling into the C code.

When building these targets using the CLANG38 toolchain, we switch
from the BFD to the GOLD linker, which behaves a bit differently when
building PIE executables, and insists on emitting GOT indirected symbol
references throughout, which means a) that we end up with absolute
addresses (which need to be fixed up at load time) for no good reason,
and b) we have to add support for handling GOT entries to GenFw if we
want to convert them into PE/COFF.

So instead, let's emit a shared library. Since the ELF image only serves
as the input to GenFw, this does not lead to any loss of functionality,
although it does require the -Bsymbolic linker option to be added to
ensure that no symbol based dynamic relocations are emitted (which
would, e.g., permit lazy binding for shared libraries). So for all
other toolchains, the linker option changes are a no-op.

Then, we have to convince CLANG38/GOLD that there is no need to refer
to symbols via a GOT entry. This is done by forcing hidden visibility
for all symbols in all components that make up the PrePi SEC module:
this informs the linker that a symbol is never exported or preempted,
making it safe to refer to it directly from anywhere in the code,
rather than indirectly via a GOT entry.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
---
 ArmVirtPkg/ArmVirtQemuKernel.dsc                    | 12 ++++++++--
 ArmVirtPkg/ArmVirtXen.dsc                           | 12 ++++++++--
 ArmVirtPkg/PrePi/ArmVirtPrePiUniCoreRelocatable.inf |  2 +-
 ArmVirtPkg/Include/Platform/Hidden.h                | 23 ++++++++++++++++++++
 4 files changed, 44 insertions(+), 5 deletions(-)

diff --git a/ArmVirtPkg/ArmVirtQemuKernel.dsc b/ArmVirtPkg/ArmVirtQemuKernel.dsc
index 9928919bf5b0..c4324a9e264b 100644
--- a/ArmVirtPkg/ArmVirtQemuKernel.dsc
+++ b/ArmVirtPkg/ArmVirtQemuKernel.dsc
@@ -68,11 +68,19 @@ [LibraryClasses.common]
 [LibraryClasses.common.UEFI_DRIVER]
   UefiScsiLib|MdePkg/Library/UefiScsiLib/UefiScsiLib.inf
 
-[BuildOptions.ARM.EDKII.SEC, BuildOptions.ARM.EDKII.BASE]
+[BuildOptions.common.EDKII.SEC, BuildOptions.common.EDKII.BASE]
   # Avoid MOVT/MOVW instruction pairs in code that may end up in the PIE
   # executable we build for the relocatable PrePi. They are not runtime
   # relocatable in ELF.
-  *_CLANG35_*_CC_FLAGS = -mno-movt
+  *_CLANG35_ARM_CC_FLAGS = -mno-movt
+
+  #
+  # CLANG38 with LTO support enabled uses the GNU GOLD linker, which insists
+  # on emitting GOT based symbol references when running in shared mode, unless
+  # we override visibility to 'hidden' in all modules that make up the PrePi
+  # build.
+  #
+  GCC:*_CLANG38_*_CC_FLAGS = -include $(WORKSPACE)/ArmVirtPkg/Include/Platform/Hidden.h
 
 ################################################################################
 #
diff --git a/ArmVirtPkg/ArmVirtXen.dsc b/ArmVirtPkg/ArmVirtXen.dsc
index 20fae9e675bb..e083666f54ea 100644
--- a/ArmVirtPkg/ArmVirtXen.dsc
+++ b/ArmVirtPkg/ArmVirtXen.dsc
@@ -57,11 +57,19 @@ [LibraryClasses]
 [LibraryClasses.common.UEFI_DRIVER]
   UefiScsiLib|MdePkg/Library/UefiScsiLib/UefiScsiLib.inf
 
-[BuildOptions.ARM.EDKII.SEC, BuildOptions.ARM.EDKII.BASE]
+[BuildOptions.common.EDKII.SEC, BuildOptions.common.EDKII.BASE]
   # Avoid MOVT/MOVW instruction pairs in code that may end up in the PIE
   # executable we build for the relocatable PrePi. They are not runtime
   # relocatable in ELF.
-  *_CLANG35_*_CC_FLAGS = -mno-movt
+  *_CLANG35_ARM_CC_FLAGS = -mno-movt
+
+  #
+  # CLANG38 with LTO support enabled uses the GNU GOLD linker, which insists
+  # on emitting GOT based symbol references when running in shared mode, unless
+  # we override visibility to 'hidden' in all modules that make up the PrePi
+  # build.
+  #
+  GCC:*_CLANG38_*_CC_FLAGS = -include $(WORKSPACE)/ArmVirtPkg/Include/Platform/Hidden.h
 
 ################################################################################
 #
diff --git a/ArmVirtPkg/PrePi/ArmVirtPrePiUniCoreRelocatable.inf b/ArmVirtPkg/PrePi/ArmVirtPrePiUniCoreRelocatable.inf
index 034ddb41cb48..5fe6cd8eb481 100755
--- a/ArmVirtPkg/PrePi/ArmVirtPrePiUniCoreRelocatable.inf
+++ b/ArmVirtPkg/PrePi/ArmVirtPrePiUniCoreRelocatable.inf
@@ -105,4 +105,4 @@ [Pcd]
   gArmTokenSpaceGuid.PcdFvBaseAddress
 
 [BuildOptions]
-  GCC:*_*_*_DLINK_FLAGS = -pie -Wl,-T,$(MODULE_DIR)/Scripts/PrePi-PIE.lds
+  GCC:*_*_*_DLINK_FLAGS = -shared -Wl,-Bsymbolic -Wl,-T,$(MODULE_DIR)/Scripts/PrePi-PIE.lds
diff --git a/ArmVirtPkg/Include/Platform/Hidden.h b/ArmVirtPkg/Include/Platform/Hidden.h
new file mode 100644
index 000000000000..20e9f5d896b2
--- /dev/null
+++ b/ArmVirtPkg/Include/Platform/Hidden.h
@@ -0,0 +1,23 @@
+/** @file
+
+  Copyright (c) 2018, Linaro Limited. All rights reserved.
+
+  This program and the accompanying materials
+  are licensed and made available under the terms and conditions of the BSD License
+  which accompanies this distribution.  The full text of the license may be found at
+  http://opensource.org/licenses/bsd-license.php
+
+  THE PROGRAM IS DISTRIBUTED UNDER THE BSD LICENSE ON AN "AS IS" BASIS,
+  WITHOUT WARRANTIES OR REPRESENTATIONS OF ANY KIND, EITHER EXPRESS OR IMPLIED.
+
+**/
+
+//
+// Setting the GCC -fvisibility=hidden command line option is not quite the same
+// as setting the pragma below: the former only affects definitions, whereas the
+// latter affects extern declarations as well. So if we want to ensure that no
+// GOT indirected symbol references are emitted, we need to use the pragma, or
+// GOT based cross object references could be emitted, e.g., in libraries, and
+// these cannot be relaxed to ordinary symbol references at link time.
+//
+#pragma GCC visibility push (hidden)
-- 
2.19.2