[ELF] Make .eh_frame relro if one of the inputs is SHF_WRITE

[arichardson]

Summary

Previously .eh_frame was always SHF_ALLOC and read-only. However, for
MIPS64 PIC code clang emits a R_MIPS_64 relocation that will be converted
to a dynamic relocation so .eh_frame can't be read-only in that case.
Since the input section has the SHF_WRITE flag set we were adding dynamic
relocations against even though the .eh_frame output section is read-only
without requiring -z notext.
Fix this by making .eh_frame writable if one of the input sections is has
the SHF_WRITE flag.

[atanasyan]

As I said in the D43185, probably we need to implement something like _bfd_elf_eh_frame_section_offset from BFD library to convert absolute relocations into relative ones and resolve them at the time of linking.

[arichardson]

In D43188#1005011, @atanasyan wrote:

As I said in the D43185, probably we need to implement something like _bfd_elf_eh_frame_section_offset from BFD library to convert absolute relocations into relative ones and resolve them at the time of linking.

I agree that this is the correct way forward but this at least allows me to create non-broken .so files.

This patch prevents LLD from creating dynamic relocations against a read-only section without -z notext so I believe it is a valid bugfix and is independent of a .eh_frame format for MIPS that can be read-only.

[joerg]

Please stop adding complexity to doctor around the symptoms. There are two real fixes here and this change doesn't help with either:
(1) Emit cross-section pointers as indirect. This increases the binary size, but otherwise ensures that any linker can create read-only .eh_frame on MIPS.
(2) Teach lld on MIPS to properly reassemble the DWARF instructions, similar to what GNU ld can do. The latter is a bit stupid and needs a good kick to work properly, but this is the correct approach forward.

ELF/SyntheticSections.cpp

void EhFrameSection::addSectionAux(EhInputSection *Sec, ArrayRef<RelTy> Rels) {
   }
 }
 
 template <class ELFT> void EhFrameSection::addSection(InputSectionBase *C) {
   auto *Sec = cast<EhInputSection>(C);
   Sec->Parent = this;
 
   Alignment = std::max(Alignment, Sec->Alignment);
+  // TODO: ideally we should only make .eh_frame writable if we have any dynamic
+  // relocations against it.
+  if (Sec->Flags & SHF_WRITE)
+    Flags |= SHF_WRITE; // TODO: Or should we just do Flags |= Sec->Flags?
   Sections.push_back(Sec);
 
   for (auto *DS : Sec->DependentSections)
     DependentSections.push_back(DS);
 
   // .eh_frame is a sequence of CIE or FDE records. This function
   // splits it into pieces so that we can call
   // SplitInputSection::getSectionPiece on the section.

test/ELF/eh-frame-writable.test

+# RUN: yaml2obj %s -o %t.o
+# RUN: llvm-readobj -r -s %t.o | FileCheck %s -check-prefix OBJECT
+# RUN: ld.lld --eh-frame-hdr --shared --fatal-warnings -o %t.so %t.o
+# RUN: llvm-readobj -elf-output-style=GNU -program-headers -relocations -sections %t.so | FileCheck %s
+
+# OBJECT-LABEL:  Section {
+# OBJECT:         Name: .eh_frame
+# OBJECT-NEXT:    Type: SHT_PROGBITS (0x1)
+# OBJECT-NEXT:    Flags [ (0x3)
+# OBJECT-NEXT:      SHF_ALLOC (0x2)
+# OBJECT-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: ]
+
+# CHECK-LABEL: Section Headers:
+# CHECK-NEXT: [Nr]     Name              Type            Address          Off    Size   ES Flg Lk Inf Al
+# CHECK:      [{{.+}}] .eh_frame         PROGBITS        0000000000030000 030000 000038 00  WA  0   0  8
+
+
+# CHECK-LABEL: Relocation section '.rel.dyn'
+# CHECK-SAME:  at offset 0x{{.+}} contains 1 entries:
+# CHECK-NEXT:    Offset             Info             Type               Symbol's Value  Symbol's Name
+# CHECK-NEXT: 0000000000030020  0000000000001203 R_MIPS_REL32/R_MIPS_64/R_MIPS_NONE
+
+# CHECK-LABEL: There are 8 program headers, starting at offset 64
+# CHECK:       Program Headers:
+# CHECK-NEXT:   Type           Offset   VirtAddr PhysAddr FileSiz  MemSiz   Flg Align
+# CHECK-NEXT:   PHDR           0x000040 0x{{.+}} 0x{{.+}} 0x{{.+}} 0x{{.+}} R   0x8
+# CHECK-NEXT:   LOAD           0x000000 0x{{.+}} 0x{{.+}} 0x{{.+}} 0x{{.+}} R   0x10000
+# CHECK-NEXT:   LOAD           0x010000 0x{{.+}} 0x{{.+}} 0x{{.+}} 0x{{.+}} R E 0x10000
+# CHECK-NEXT:   LOAD           0x020000 0x{{.+}} 0x{{.+}} 0x{{.+}} 0x{{.+}} RW  0x10000
+# CHECK-NEXT:   DYNAMIC        0x000248 0x{{.+}} 0x{{.+}} 0x{{.+}} 0x{{.+}} R   0x8
+# CHECK-NEXT:   GNU_RELRO      0x030000 0x{{.+}} 0x{{.+}} 0x{{.+}} 0x{{.+}} R   0x1
+# CHECK-NEXT:   GNU_EH_FRAME   0x000360 0x{{.+}} 0x{{.+}} 0x{{.+}} 0x{{.+}} R   0x4
+# CHECK-NEXT:   GNU_STACK      0x000000 0x{{.+}} 0x{{.+}} 0x{{.+}} 0x{{.+}} RW  0x0
+
+# CHECK-LABEL: Section to Segment mapping:
+# CHECK-NEXT:  Segment Sections...
+# CHECK-NEXT:   00
+# CHECK-NEXT:   01     .dynsym .hash .dynamic .dynstr .rel.dyn .eh_frame_hdr
+# CHECK-NEXT:   02     .text
+# CHECK-NEXT:   03     .data .got .eh_frame
+# CHECK-NEXT:   04     .dynamic
+# CHECK-NEXT:   05     .eh_frame
+# CHECK-NEXT:   06     .eh_frame_hdr
+# CHECK-NEXT:   07
+
+
+--- !ELF
+FileHeader:
+  Class:           ELFCLASS64
+  Data:            ELFDATA2MSB
+  OSABI:           ELFOSABI_FREEBSD
+  Type:            ET_REL
+  Machine:         EM_MIPS
+  Flags:           [ EF_MIPS_PIC, EF_MIPS_CPIC, EF_MIPS_ARCH_64 ]
+Sections:
+  - Name:            .text
+    Type:            SHT_PROGBITS
+    Flags:           [ SHF_ALLOC, SHF_EXECINSTR ]
+    AddressAlign:    0x0000000000000010
+    Content:         3C0B0001
+  - Name:            .data
+    Type:            SHT_PROGBITS
+    Flags:           [ SHF_WRITE, SHF_ALLOC ]
+    AddressAlign:    0x0000000000000010
+    Content:         ''
+  - Name:            .bss
+    Type:            SHT_NOBITS
+    Flags:           [ SHF_WRITE, SHF_ALLOC ]
+    AddressAlign:    0x0000000000000010
+  - Name:            .eh_frame
+    Type:            SHT_PROGBITS
+    Flags:           [ SHF_WRITE, SHF_ALLOC ]
+    AddressAlign:    0x0000000000000008
+    Content:         0000001000000000017A520001781F010C0C1D0000000018000000180000000000000000000000000000000400000000
+  - Name:            .rela.eh_frame
+    Type:            SHT_RELA
+    Link:            .symtab
+    AddressAlign:    0x0000000000000008
+    Info:            .eh_frame
+    Relocations:
+      - Offset:          0x000000000000001C
+        Symbol:          .text
+        Type:            R_MIPS_64
+Symbols:
+  Local:
+    - Name:            .text
+      Type:            STT_SECTION
+      Section:         .text
+  Global:
+    - Name:            foo
+      Section:         .text