[ndk][mips] writable .gcc_except_table for mips[64]

To match gcc behavior and avoid ld warning like this on project using
prebuilts/ndk 's libc++

   .../ld: warning: creating a DT_TEXTREL in a shared object.

(*1)
 switch_to_exception_section (const char * ARG_UNUSED (fnname))
 {
   ....
 	  if (EH_TABLES_CAN_BE_READ_ONLY)
 	    {
 	      int tt_format =
 		ASM_PREFERRED_EH_DATA_FORMAT (/*code=*/0, /*global=*/1);  # DW_EH_PE_indirect which is 0x80
 	      flags = ((! flag_pic
 			|| ((tt_format & 0x70) != DW_EH_PE_absptr/*0x00*/
 			    && (tt_format & 0x70) != DW_EH_PE_aligned/*0x50*/))
 		       ? 0 : SECTION_WRITE);
 	    }
 	  else
 	    flags = SECTION_WRITE;
   ...
 }

Note that PIC is the default for Android toolchain.  For MIPS,
ASM_PREFERRED_EH_DATA_FORMAT returns 0x80 and fails condition
about "DW_EH_PE_absptr"

The exact reason why .gcc_except_table contain reloc (and causes
ld warning turns in to error thanks to -Wl,--fatal-warnings when it
isn't writable thus requires load-time fixup in read-only section
and security concern) isn't well understood, though

Change-Id: I5e05ee052af48f06e8c4e3a01c317f4010a2ddaf

1 files changed, 26 insertions, 1 deletions

diff --git a/lib/MC/MCObjectFileInfo.cpp b/lib/MC/MCObjectFileInfo.cpp
index 858181dd4af..8015ebbfb32 100644
--- a/lib/MC/MCObjectFileInfo.cpp
+++ b/lib/MC/MCObjectFileInfo.cpp
@@ -264,6 +264,8 @@ void MCObjectFileInfo::InitMachOMCObjectFileInfo(Triple T) {
 }
 
 void MCObjectFileInfo::InitELFMCObjectFileInfo(Triple T) {
+  bool writable_gcc_except_table = false;
+
   switch (T.getArch()) {
   case Triple::mips:
   case Triple::mipsel:
@@ -350,6 +352,29 @@ void MCObjectFileInfo::InitELFMCObjectFileInfo(Triple T) {
     // eh_frame section can be read-only.  DW.ref.personality will be generated
     // for relocation.
     PersonalityEncoding = dwarf::DW_EH_PE_indirect;
+    if (TT.getEnvironment() == llvm::Triple::Android) {
+      // To match GCC/mips behavior in gcc/except.c as below, to avoid
+      // .../ld: warning: creating a DT_TEXTREL in a shared object.
+      //
+      // switch_to_exception_section (const char * ARG_UNUSED (fnname))
+      // {
+      //   ....
+      // 	  if (EH_TABLES_CAN_BE_READ_ONLY)
+      // 	    {
+      // 	      int tt_format =
+      // 		ASM_PREFERRED_EH_DATA_FORMAT (/*code=*/0, /*global=*/1);  # DW_EH_PE_indirect which is 0x80
+      // 	      flags = ((! flag_pic
+      // 			|| ((tt_format & 0x70) != DW_EH_PE_absptr/*0x00*/
+      // 			    && (tt_format & 0x70) != DW_EH_PE_aligned/*0x50*/))
+      // 		       ? 0 : SECTION_WRITE);
+      // 	    }
+      // 	  else
+      // 	    flags = SECTION_WRITE;
+      //
+      // FIXME: Check PIC (default in Android, though)
+      //
+      writable_gcc_except_table = true;
+    }
     break;
   case Triple::ppc64:
   case Triple::ppc64le:
@@ -502,7 +527,7 @@ void MCObjectFileInfo::InitELFMCObjectFileInfo(Triple T) {
   // adjusted or this should be a data section.
   LSDASection =
     Ctx->getELFSection(".gcc_except_table", ELF::SHT_PROGBITS,
-                       ELF::SHF_ALLOC,
+                       ELF::SHF_ALLOC | (writable_gcc_except_table? ELF::SHF_WRITE : 0),
                        SectionKind::getReadOnly());
 
   COFFDebugSymbolsSection = nullptr;