[ELF][MIPS] Support linking of PIE for MIPS

[atanasyan]

Summary

MIPS ABI requires creation of the MIPS_RLD_MAP dynamic tag for non-PIE executables only and MIPS_RLD_MAP_REL tag for both PIE and non-PIE executables. The patch skips definition of the MIPS_RLD_MAP for PIE files and defines MIPS_RLD_MAP_REL.

The MIPS_RLD_MAP_REL tag stores the offset to the .rld_map section relative to the address of the tag itself.

[atanasyan]

Ping?

[grimar]

ELF/SyntheticSections.cpp

1027

I think new member (like addTagRelSec) would be better here.
Because logic controlled with parameter by default here looks too specific and
code looks like 2 methods were merged into one.

[atanasyan]

• Using a separate function to create a dynamic table entry with a relative address of a section

[arichardson]

I can verify that without this patch pie executables crash inside the FreeBSD (CheriBSD) RTLD when processing DT_MIPS_RLD_MAP and with it they work.

[atanasyan]

In D43347#1017220, @arichardson wrote:

I can verify that without this patch pie executables crash inside the FreeBSD (CheriBSD) RTLD when processing DT_MIPS_RLD_MAP and with it they work.

Thanks for the testing.

[atanasyan]

Ping?

[atanasyan]

Ping? Any objections to commit this patch?

[atanasyan]

Ping?

[atanasyan]

Ping? This patch got the last comment one month ago.

[atanasyan]

Ping?

[atanasyan]

Rebased against the trunk.

[grimar]

test/ELF/mips-dynamic.s

51–79

I think that this test case, for example, does not really check that
DT_MIPS_RLD_MAP is not emitted for PIE.

Problem is that *-NEXT and/or *-NOT are not used and check tags are
mixed, what is a bit hard to read.

I would suggest explicitly testing DynamicSection twice, one time with
use of PIE/PIE-NEXT and the second time with NOPIE/NOPIE-NEXT
to show which tags are produced for each case.

[atanasyan]

Explicitly testing DynamicSection content for PIE and no-PIE cases.

[atanasyan]

test/ELF/mips-dynamic.s

51–79

I would suggest explicitly testing DynamicSection twice

Good point. Thanks.

[grimar]

This LGTM. But please wait for the final accept from Rui/Rafael.

[atanasyan]

Thanks for review.

Rui, Rafael: I'm going to commit the patch by the end of the next week if I do not get any negative comments.

ELF/SyntheticSections.h

public:
   void finalizeContents() override;
   void writeTo(uint8_t *Buf) override;
   size_t getSize() const override { return Size; }
 
 private:
   void add(int32_t Tag, std::function<uint64_t()> Fn);
   void addInt(int32_t Tag, uint64_t Val);
   void addInSec(int32_t Tag, InputSection *Sec);
+  void addInSecRelative(int32_t Tag, InputSection *Sec);
   void addOutSec(int32_t Tag, OutputSection *Sec);
   void addSize(int32_t Tag, OutputSection *Sec);
   void addSym(int32_t Tag, Symbol *Sym);
 
   uint64_t Size = 0;
 };
 
 class RelocationBaseSection : public SyntheticSection {

ELF/SyntheticSections.cpp

 
 template <class ELFT>
 void DynamicSection<ELFT>::addInt(int32_t Tag, uint64_t Val) {
   Entries.push_back({Tag, [=] { return Val; }});
 }
 
 template <class ELFT>
 void DynamicSection<ELFT>::addInSec(int32_t Tag, InputSection *Sec) {
   Entries.push_back({Tag, [=] { return Sec->getVA(0); }});
 }
 
 template <class ELFT>
+void DynamicSection<ELFT>::addInSecRelative(int32_t Tag, InputSection *Sec) {
+  size_t TagOffset = Entries.size() * Entsize;
+  Entries.push_back(
+      {Tag, [=] { return Sec->getVA(0) - (getVA() + TagOffset); }});
+}
+
+template <class ELFT>
 void DynamicSection<ELFT>::addOutSec(int32_t Tag, OutputSection *Sec) {
   Entries.push_back({Tag, [=] { return Sec->Addr; }});
 }
 
 template <class ELFT>
 void DynamicSection<ELFT>::addSize(int32_t Tag, OutputSection *Sec) {
   Entries.push_back({Tag, [=] { return Sec->Size; }});
 }
if (Config->EMachine == EM_MIPS) {
 
     add(DT_MIPS_LOCAL_GOTNO, [] { return InX::MipsGot->getLocalEntriesNum(); });
 
     if (const Symbol *B = InX::MipsGot->getFirstGlobalEntry())
       addInt(DT_MIPS_GOTSYM, B->DynsymIndex);
     else
       addInt(DT_MIPS_GOTSYM, InX::DynSymTab->getNumSymbols());
     addInSec(DT_PLTGOT, InX::MipsGot);
-    if (InX::MipsRldMap)
+    if (InX::MipsRldMap) {
+      if (!Config->Pie)
         addInSec(DT_MIPS_RLD_MAP, InX::MipsRldMap);
+      // Store the offset to the .rld_map section
+      // relative to the address of the tag.
+      addInSecRelative(DT_MIPS_RLD_MAP_REL, InX::MipsRldMap);
+    }
   }
 
   addInt(DT_NULL, 0);
 
   getParent()->Link = this->Link;
   this->Size = Entries.size() * this->Entsize;
 }
 

test/ELF/mips-dynamic.s

 # Check MIPS specific .dynamic section entries.
 
 # RUN: llvm-mc -filetype=obj -triple=mips-unknown-linux %s -o %t.o
 # RUN: llvm-mc -filetype=obj -triple=mips-unknown-linux %p/Inputs/mips-dynamic.s -o %td.o
 # RUN: ld.lld -shared %td.o -o %td.so
 
 # RUN: ld.lld %t.o %td.so -o %t.exe
 # RUN: llvm-readobj -sections -dynamic-table %t.exe \
-# RUN:   | FileCheck -check-prefix=EXE %s
+# RUN:   | FileCheck -check-prefixes=EXE,NOPIE %s
+
+# RUN: ld.lld -pie %t.o %td.so -o %t.so
+# RUN: llvm-readobj -sections -dyn-symbols -dynamic-table %t.so \
+# RUN:   | FileCheck -check-prefixes=EXE,PIE %s
 
 # RUN: ld.lld %t.o --image-base=0x123000 %td.so -o %t.exe
 # RUN: llvm-readobj -sections -dynamic-table %t.exe \
 # RUN:   | FileCheck -check-prefix=IMAGE_BASE %s
 
 # RUN: ld.lld -shared %t.o %td.so -o %t.so
 # RUN: llvm-readobj -sections -dyn-symbols -dynamic-table %t.so \
 # RUN:   | FileCheck -check-prefix=DSO %s

 # EXE-NEXT:       SHF_ALLOC
 # EXE-NEXT:       SHF_MIPS_GPREL
 # EXE-NEXT:       SHF_WRITE
 # EXE-NEXT:     ]
 # EXE-NEXT:     Address: [[GOTADDR:0x[0-9a-f]+]]
 # EXE-NEXT:     Offset:
 # EXE-NEXT:     Size: 8
 # EXE:      ]
-# EXE:      DynamicSection [
-# EXE-NEXT:   Tag        Type                 Name/Value
-# EXE-DAG:    0x00000003 PLTGOT               [[GOTADDR]]
-# EXE-DAG:    0x70000001 MIPS_RLD_VERSION     1
-# EXE-DAG:    0x70000005 MIPS_FLAGS           NOTPOT
-# EXE-DAG:    0x70000006 MIPS_BASE_ADDRESS    0x10000
-# EXE-DAG:    0x7000000A MIPS_LOCAL_GOTNO     2
-# EXE-DAG:    0x70000011 MIPS_SYMTABNO        2
-# EXE-DAG:    0x70000013 MIPS_GOTSYM          0x2
-# EXE-DAG:    0x70000016 MIPS_RLD_MAP         [[RLDMAPADDR]]
-# EXE:      ]
+
+# PIE:      DynamicSection [
+# PIE-NEXT:   Tag        Type                 Name/Value
+# PIE:        0x00000004 HASH                 0x{{[0-9A-F]+}}
+# PIE-NEXT:   0x70000001 MIPS_RLD_VERSION     1
+# PIE-NEXT:   0x70000005 MIPS_FLAGS           NOTPOT
+# PIE-NEXT:   0x70000006 MIPS_BASE_ADDRESS    0x0
+# PIE-NEXT:   0x70000011 MIPS_SYMTABNO        2
+# PIE-NEXT:   0x7000000A MIPS_LOCAL_GOTNO     2
+# PIE-NEXT:   0x70000013 MIPS_GOTSYM          0x2
+# PIE-NEXT:   0x00000003 PLTGOT               [[GOTADDR]]
+# PIE-NEXT:   0x70000035 MIPS_RLD_MAP_REL     0x{{[0-9A-F]+}}
+# PIE-NEXT:   0x00000000 NULL                 0x0
+# PIE-NEXT: ]
+
+# NOPIE:      DynamicSection [
+# NOPIE-NEXT:   Tag        Type                 Name/Value
+# NOPIE:        0x00000004 HASH                 0x{{[0-9A-F]+}}
+# NOPIE-NEXT:   0x70000001 MIPS_RLD_VERSION     1
+# NOPIE-NEXT:   0x70000005 MIPS_FLAGS           NOTPOT
+# NOPIE-NEXT:   0x70000006 MIPS_BASE_ADDRESS    0x10000
+# NOPIE-NEXT:   0x70000011 MIPS_SYMTABNO        2
+# NOPIE-NEXT:   0x7000000A MIPS_LOCAL_GOTNO     2
+# NOPIE-NEXT:   0x70000013 MIPS_GOTSYM          0x2
+# NOPIE-NEXT:   0x00000003 PLTGOT               [[GOTADDR]]
+# NOPIE-NEXT:   0x70000016 MIPS_RLD_MAP         [[RLDMAPADDR]]
+# NOPIE-NEXT:   0x70000035 MIPS_RLD_MAP_REL     0x{{[0-9A-F]+}}
+# NOPIE-NEXT:   0x00000000 NULL                 0x0
+# NOPIE-NEXT: ]
 
 # IMAGE_BASE: 0x70000006 MIPS_BASE_ADDRESS    0x123000
 
 # DSO:      Sections [
 # DSO:          Name: .dynamic
 # DSO-NEXT:     Type: SHT_DYNAMIC
 # DSO-NEXT:     Flags [
 # DSO-NEXT:       SHF_ALLOC