From a20f5fe70810e0a768c1814d69d10862965c21e4 Mon Sep 17 00:00:00 2001
From: Sterling Augustine <saugustine@google.com>
Date: Tue, 18 Aug 2020 12:05:07 -0700
Subject: [PATCH] Default to disabling the libunwind frameheader cache.

Although it works fine with glibc, as currently implemented the
frameheader cache is incompatible with certain platforms with
slightly different locking semantics inside dl_iterate_phdr.

Therefore only enable it when it is turned on explicitly with
a configure-time option.

Differential Revision: https://reviews.llvm.org/D86163
---
 libunwind/CMakeLists.txt                      | 5 +++++
 libunwind/src/AddressSpace.hpp                | 6 ++++++
 libunwind/test/frameheadercache_test.pass.cpp | 2 +-
 3 files changed, 12 insertions(+), 1 deletion(-)

diff --git a/libunwind/CMakeLists.txt b/libunwind/CMakeLists.txt
index 8419d851ab7f..f4243f3d2791 100644
--- a/libunwind/CMakeLists.txt
+++ b/libunwind/CMakeLists.txt
@@ -138,6 +138,7 @@ option(LIBUNWIND_WEAK_PTHREAD_LIB "Use weak references to refer to pthread funct
 option(LIBUNWIND_USE_COMPILER_RT "Use compiler-rt instead of libgcc" OFF)
 option(LIBUNWIND_INCLUDE_DOCS "Build the libunwind documentation." ${LLVM_INCLUDE_DOCS})
 option(LIBUNWIND_IS_BAREMETAL "Build libunwind for baremetal targets." OFF)
+option(LIBUNWIND_USE_FRAME_HEADER_CACHE "Cache frame headers for unwinding. Requires locking dl_iterate_phdr." OFF)
 
 set(LIBUNWIND_LIBDIR_SUFFIX "${LLVM_LIBDIR_SUFFIX}" CACHE STRING
     "Define suffix of library directory name (32/64)")
@@ -368,6 +369,10 @@ if(LIBUNWIND_IS_BAREMETAL)
   add_compile_definitions(_LIBUNWIND_IS_BAREMETAL)
 endif()
 
+if(LIBUNWIND_USE_FRAME_HEADER_CACHE)
+  add_compile_definitions(_LIBUNWIND_USE_FRAME_HEADER_CACHE)
+endif()
+
 # This is the _ONLY_ place where add_definitions is called.
 if (MSVC)
   add_definitions(-D_CRT_SECURE_NO_WARNINGS)
diff --git a/libunwind/src/AddressSpace.hpp b/libunwind/src/AddressSpace.hpp
index 78d2dd110865..2443bd761cc4 100644
--- a/libunwind/src/AddressSpace.hpp
+++ b/libunwind/src/AddressSpace.hpp
@@ -411,10 +411,12 @@ struct _LIBUNWIND_HIDDEN dl_iterate_cb_data {
     #error "_LIBUNWIND_SUPPORT_DWARF_UNWIND requires _LIBUNWIND_SUPPORT_DWARF_INDEX on this platform."
   #endif
 
+#if defined(_LIBUNWIND_USE_FRAME_HEADER_CACHE)
 #include "FrameHeaderCache.hpp"
 
 // There should be just one of these per process.
 static FrameHeaderCache ProcessFrameHeaderCache;
+#endif
 
 static bool checkAddrInSegment(const Elf_Phdr *phdr, size_t image_base,
                                dl_iterate_cb_data *cbdata) {
@@ -435,8 +437,10 @@ int findUnwindSectionsByPhdr(struct dl_phdr_info *pinfo, size_t pinfo_size,
   auto cbdata = static_cast<dl_iterate_cb_data *>(data);
   if (pinfo->dlpi_phnum == 0 || cbdata->targetAddr < pinfo->dlpi_addr)
     return 0;
+#if defined(_LIBUNWIND_USE_FRAME_HEADER_CACHE)
   if (ProcessFrameHeaderCache.find(pinfo, pinfo_size, data))
     return 1;
+#endif
 
   Elf_Addr image_base = calculateImageBase(pinfo);
   bool found_obj = false;
@@ -464,7 +468,9 @@ int findUnwindSectionsByPhdr(struct dl_phdr_info *pinfo, size_t pinfo_size,
       found_obj = checkAddrInSegment(phdr, image_base, cbdata);
     }
     if (found_obj && found_hdr) {
+#if defined(_LIBUNWIND_USE_FRAME_HEADER_CACHE)
       ProcessFrameHeaderCache.add(cbdata->sects);
+#endif
       return 1;
     }
   }
diff --git a/libunwind/test/frameheadercache_test.pass.cpp b/libunwind/test/frameheadercache_test.pass.cpp
index 9397e70d66cb..ebbc00464e07 100644
--- a/libunwind/test/frameheadercache_test.pass.cpp
+++ b/libunwind/test/frameheadercache_test.pass.cpp
@@ -6,7 +6,7 @@
 // The frame header cache should work fine for other architectures,
 // but the #ifdefs end up being even more complicated than this.
 
-#ifdef __x86_64__
+#if defined(__x86_64__) && defined(_LIBUNWIND_USE_FRAME_HEADER_CACHE)
 
 // This #if chain is ugly, but see the comments in AddressSpace.hpp for
 // the reasoning.