[PATCH v5 7/8] MdePkg GCC/X64: avoid 'hidden' visibility for module entry points

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

When building with LTO enabled, the LTO routines infer from the 'hidden'
visibility of the _ModuleEntryPoint symbol that its code only needs to be
retained if it is referenced internally, and disregards the fact that
it is referenced by the entry point field in the ELF metadata.

This is arguably a bug in LTO, but we can work around it by ensuring that
the _ModuleEntryPoint symbol is emitted with protected visibility instead.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
---
 MdePkg/Include/X64/ProcessorBind.h                                     | 9 ++++++++-
 MdePkg/Library/DxeCoreEntryPoint/DxeCoreEntryPoint.inf                 | 2 ++
 MdePkg/Library/PeiCoreEntryPoint/PeiCoreEntryPoint.inf                 | 2 ++
 MdePkg/Library/PeimEntryPoint/PeimEntryPoint.inf                       | 2 ++
 MdePkg/Library/UefiApplicationEntryPoint/UefiApplicationEntryPoint.inf | 2 ++
 MdePkg/Library/UefiDriverEntryPoint/UefiDriverEntryPoint.inf           | 2 ++
 6 files changed, 18 insertions(+), 1 deletion(-)

diff --git a/MdePkg/Include/X64/ProcessorBind.h b/MdePkg/Include/X64/ProcessorBind.h
index 666cc8e8bd16..e45b9cba2bb3 100644
--- a/MdePkg/Include/X64/ProcessorBind.h
+++ b/MdePkg/Include/X64/ProcessorBind.h
@@ -29,13 +29,20 @@
 
 #if defined(__GNUC__) && defined(__pic__)
 //
-// Mark all symbol declarations and references as hidden, meaning they will
+// Mark all symbol declarations and references as hidden*, meaning they will
 // not be subject to symbol preemption. This allows the compiler to refer to
 // symbols directly using relative references rather than via the GOT, which
 // contains absolute symbol addresses that are subject to runtime relocation.
 //
+// * Under LTO, the entry point of a module must have protected or default
+//   visibility to prevent it from being pruned.
+//
+#ifdef GCC_VISIBILITY_PROTECTED
+#pragma GCC visibility push (protected)
+#else
 #pragma GCC visibility push (hidden)
 #endif
+#endif
 
 #if defined(__INTEL_COMPILER)
 //
diff --git a/MdePkg/Library/DxeCoreEntryPoint/DxeCoreEntryPoint.inf b/MdePkg/Library/DxeCoreEntryPoint/DxeCoreEntryPoint.inf
index 01f64c34c7a1..2d6f87ed062e 100644
--- a/MdePkg/Library/DxeCoreEntryPoint/DxeCoreEntryPoint.inf
+++ b/MdePkg/Library/DxeCoreEntryPoint/DxeCoreEntryPoint.inf
@@ -39,3 +39,5 @@ [LibraryClasses]
   BaseLib
   DebugLib
 
+[BuildOptions]
+  GCC:*_*_X64_CC_FLAGS = -DGCC_VISIBILITY_PROTECTED
diff --git a/MdePkg/Library/PeiCoreEntryPoint/PeiCoreEntryPoint.inf b/MdePkg/Library/PeiCoreEntryPoint/PeiCoreEntryPoint.inf
index d920306713c5..4e61783b3bd5 100644
--- a/MdePkg/Library/PeiCoreEntryPoint/PeiCoreEntryPoint.inf
+++ b/MdePkg/Library/PeiCoreEntryPoint/PeiCoreEntryPoint.inf
@@ -37,3 +37,5 @@ [LibraryClasses]
   BaseLib
   DebugLib
 
+[BuildOptions]
+  GCC:*_*_X64_CC_FLAGS = -DGCC_VISIBILITY_PROTECTED
diff --git a/MdePkg/Library/PeimEntryPoint/PeimEntryPoint.inf b/MdePkg/Library/PeimEntryPoint/PeimEntryPoint.inf
index a2db9e058bbe..adfd91bdc57e 100644
--- a/MdePkg/Library/PeimEntryPoint/PeimEntryPoint.inf
+++ b/MdePkg/Library/PeimEntryPoint/PeimEntryPoint.inf
@@ -37,3 +37,5 @@ [Packages]
 [LibraryClasses]
   DebugLib
 
+[BuildOptions]
+  GCC:*_*_X64_CC_FLAGS = -DGCC_VISIBILITY_PROTECTED
diff --git a/MdePkg/Library/UefiApplicationEntryPoint/UefiApplicationEntryPoint.inf b/MdePkg/Library/UefiApplicationEntryPoint/UefiApplicationEntryPoint.inf
index be92b3dc0760..9525c55c2051 100644
--- a/MdePkg/Library/UefiApplicationEntryPoint/UefiApplicationEntryPoint.inf
+++ b/MdePkg/Library/UefiApplicationEntryPoint/UefiApplicationEntryPoint.inf
@@ -38,3 +38,5 @@ [LibraryClasses]
   DebugLib
   BaseLib
 
+[BuildOptions]
+  GCC:*_*_X64_CC_FLAGS = -DGCC_VISIBILITY_PROTECTED
diff --git a/MdePkg/Library/UefiDriverEntryPoint/UefiDriverEntryPoint.inf b/MdePkg/Library/UefiDriverEntryPoint/UefiDriverEntryPoint.inf
index 7a9dcbcd4df2..8d30b1197850 100644
--- a/MdePkg/Library/UefiDriverEntryPoint/UefiDriverEntryPoint.inf
+++ b/MdePkg/Library/UefiDriverEntryPoint/UefiDriverEntryPoint.inf
@@ -65,3 +65,5 @@ [Depex.common.UEFI_DRIVER]
   gEfiVariableArchProtocolGuid AND
   gEfiWatchdogTimerArchProtocolGuid
 
+[BuildOptions]
+  GCC:*_*_X64_CC_FLAGS = -DGCC_VISIBILITY_PROTECTED
-- 
2.7.4