[MIPS] Mark .eh_frame as writable for PIC output

[arichardson]

Summary

This is needed since we have a R_MIPS_64 relocation that needs to be
processed at runtime. This allows running LLD without -z notext.

[joerg]

This is not acceptable. If anything, the encoding should be switched to indirect, but that should already be the case.

[arichardson]

The way it is currently it is emitting dynamic relocations into the .text segment. This needs -Wl,-z,notext with LLD but for BFD it will just emit those relocations anyway and (probably) crash at runtime.
At least now we are explicit that it requires a writable section (which will be in the relro section if you are using LLD).

I wonder if the following is the reason that we are emitting an absolute relocation for MIPS:

// We don't support PC-relative LSDA references in GAS so we use the default
// DW_EH_PE_absptr for those.

[atanasyan]

Probably we need to implement something like _bfd_elf_eh_frame_section_offset from BFD library to convert absolute relocations into relative ones.

BTW look at https://reviews.llvm.org/D13104

[arichardson]

Since this is not the correct solution I'll just keep this in our fork.

lib/MC/MCObjectFileInfo.cpp

unsigned EHSectionType = T.getArch() == Triple::x86_64
                                : ELF::SHT_PROGBITS;
 
   // Solaris requires different flags for .eh_frame to seemingly every other
   // platform.
   unsigned EHSectionFlags = ELF::SHF_ALLOC;
   if (T.isOSSolaris() && T.getArch() != Triple::x86_64)
     EHSectionFlags |= ELF::SHF_WRITE;
 
+  // This is (currently) also true for MIPS
+  // TODO: add a T.isMIPS()?
+  if (PositionIndependent &&
+      (T.getArch() == Triple::mips || T.getArch() == Triple::mipsel ||
+       T.getArch() == Triple::mips64 || T.getArch() == Triple::mips64el))
+    EHSectionFlags |= ELF::SHF_WRITE;
+
   // ELF
   BSSSection = Ctx->getELFSection(".bss", ELF::SHT_NOBITS,
                                   ELF::SHF_WRITE | ELF::SHF_ALLOC);
 
   TextSection = Ctx->getELFSection(".text", ELF::SHT_PROGBITS,
                                    ELF::SHF_EXECINSTR | ELF::SHF_ALLOC);
 
   DataSection = Ctx->getELFSection(".data", ELF::SHT_PROGBITS,

test/MC/Mips/eh_frame-section-attrs.s

+# RUN: llvm-mc -filetype=obj -triple=mips64-unknown-freebsd %s -o %t.o
+# RUN: llvm-readobj -r -s %t.o | FileCheck %s -check-prefix OBJECT
+# RUN: llvm-mc -filetype=obj -position-independent -triple=mips64-unknown-freebsd %s -o %t.o
+# RUN: llvm-readobj -r -s %t.o | FileCheck %s -check-prefixes OBJECT,PIC
+
+
+
+# OBJECT-LABEL:  Section {
+# OBJECT:         Name: .eh_frame
+# OBJECT-NEXT:    Type: SHT_PROGBITS (0x1)
+# OBJECT-NEXT:    Flags [
+# OBJECT-NEXT:      SHF_ALLOC (0x2)
+# For PIC we need to also emit SHF_WRITE since the relocation needs to be processed at runtime
+# PIC-NEXT:         SHF_WRITE (0x1)
+# OBJECT-NEXT:    ]
+
+# OBJECT-LABEL: Relocations [
+# OBJECT-NEXT:   Section ({{.+}}) .rela.eh_frame {
+# OBJECT-NEXT:     0x1C R_MIPS_64/R_MIPS_NONE/R_MIPS_NONE .text 0x0
+# OBJECT-NEXT:   }
+# OBJECT-NEXT: ]
+
+func:
+.cfi_startproc
+lui $3, 1
+.cfi_endproc