Temporarily disable libunwind's FrameHeaderCache, until there is a

resolution for <https://bugs.llvm.org/show_bug.cgi?id=47181>.

The cache implementation depends on dl_iterate_phdr(3) ensuring that its
callbacks are not called simultaneously for multiple threads, but that
is only the case for the dl_iterate_phdr() implementation in rtld.

In a statically linked executable, libc's dl_iterate_phdr() is used,
which does no such locking. If multiple threads then call into the
unwinder at the same time, it is possible to trigger a segfault.

In particular, the statically linked lld which is built during the
cross-tools stage can segfault in this way, because it starts multiple
worker threads that can exit in parallel. Since our pthread_exit(3)
invokes the unwinder, it will therefore call into it in parallel too.

1 files changed, 6 insertions, 0 deletions

diff --git a/contrib/llvm-project/libunwind/src/AddressSpace.hpp b/contrib/llvm-project/libunwind/src/AddressSpace.hpp
index a4564cb673286..d9d17ddb7bf49 100644
--- a/contrib/llvm-project/libunwind/src/AddressSpace.hpp
+++ b/contrib/llvm-project/libunwind/src/AddressSpace.hpp
@@ -452,10 +452,12 @@ 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 // _LIBUNWIND_USE_FRAME_HEADER_CACHE
 
 static bool checkAddrInSegment(const Elf_Phdr *phdr, size_t image_base,
                                dl_iterate_cb_data *cbdata) {
@@ -476,8 +478,10 @@ 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 // _LIBUNWIND_USE_FRAME_HEADER_CACHE
 
   Elf_Addr image_base = calculateImageBase(pinfo);
   bool found_obj = false;
@@ -505,7 +509,9 @@ 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 // _LIBUNWIND_USE_FRAME_HEADER_CACHE
       return 1;
     }
   }