[ELF][MIPS] Create GOT entries for R_MIPS_REL32 dynamic relocations

[atanasyan]

Summary

MIPS ABI turns using of GOT and dynamic relocations inside out. While regular ABI uses dynamic relocations to fill up GOT entries MIPS ABI requires dynamic linker to fills up GOT entries using specially sorted dynamic symbol table. This affects even dynamic relocations against symbols which do not require GOT entries creation explicitly, i.e. do not have any GOT-relocations. So if a preemptible symbol has a dynamic relocation we anyway have to create a GOT entry for it.

If a non-preemptible symbol has a dynamic relocation against it, dynamic linker takes it st_value, adds offset and writes down result of the dynamic relocation. In case of preemptible symbol dynamic linker performs symbol resolution, writes the symbol value to the GOT entry and reads the GOT entry when it needs to perform a dynamic relocation.

[atanasyan]

Rebase the patch.

[atanasyan]

Ping?

[• rafael]

sorry, this got buried on my email list. Taking a look now.

[• rafael]

This is really crazy, but LGTM.

lld/trunk/ELF/Writer.cpp

if (refersToGotEntry(Expr)) {
       }
       continue;
     }
 
     if (Preemptible) {
       // We don't know anything about the finaly symbol. Just ask the dynamic
       // linker to handle the relocation for us.
       AddDyn({Target->getDynRel(Type), C.OutSec, Offset, false, &Body, Addend});
+      // MIPS ABI turns using of GOT and dynamic relocations inside out.
+      // While regular ABI uses dynamic relocations to fill up GOT entries
+      // MIPS ABI requires dynamic linker to fills up GOT entries using
+      // specially sorted dynamic symbol table. This affects even dynamic
+      // relocations against symbols which do not require GOT entries
+      // creation explicitly, i.e. do not have any GOT-relocations. So if
+      // a preemptible symbol has a dynamic relocation we anyway have
+      // to create a GOT entry for it.
+      // If a non-preemptible symbol has a dynamic relocation against it,
+      // dynamic linker takes it st_value, adds offset and writes down
+      // result of the dynamic relocation. In case of preemptible symbol
+      // dynamic linker performs symbol resolution, writes the symbol value
+      // to the GOT entry and reads the GOT entry when it needs to perform
+      // a dynamic relocation.
+      // ftp://www.linux-mips.org/pub/linux/mips/doc/ABI/mipsabi.pdf p.4-19
+      if (Config->EMachine == EM_MIPS && !Body.isInGot())
+        Out<ELFT>::Got->addEntry(Body);
       continue;
     }
 
     if (Config->EMachine == EM_PPC64 && RI.getType(false) == R_PPC64_TOC) {
       C.Relocations.push_back({R_PPC_TOC, Type, Offset, Addend, &Body});
       AddDyn({R_PPC64_RELATIVE, C.OutSec, Offset, false, nullptr,
               (uintX_t)getPPC64TocBase() + Addend});
       continue;

lld/trunk/test/ELF/mips-32.s

 # Check R_MIPS_32 relocation calculation.
 
 # RUN: llvm-mc -filetype=obj -triple=mips-unknown-linux %s -o %t-be.o
 # RUN: ld.lld -shared %t-be.o -o %t-be.so
 # RUN: llvm-objdump -t -s %t-be.so \
 # RUN:   | FileCheck -check-prefix=SYM -check-prefix=BE %s
-# RUN: llvm-readobj -r -dynamic-table %t-be.so | FileCheck -check-prefix=REL %s
+# RUN: llvm-readobj -r -dynamic-table -mips-plt-got %t-be.so \
+# RUN:   | FileCheck -check-prefix=REL %s
 
 # RUN: llvm-mc -filetype=obj -triple=mipsel-unknown-linux %s -o %t-el.o
 # RUN: ld.lld -shared %t-el.o -o %t-el.so
 # RUN: llvm-objdump -t -s %t-el.so \
 # RUN:   | FileCheck -check-prefix=SYM -check-prefix=EL %s
-# RUN: llvm-readobj -r -dynamic-table %t-el.so | FileCheck -check-prefix=REL %s
+# RUN: llvm-readobj -r -dynamic-table -mips-plt-got %t-el.so \
+# RUN:   | FileCheck -check-prefix=REL %s
 
 # REQUIRES: mips
 
   .globl  __start
 __start:
   nop
 
   .data

 # REL-NEXT:     0x30008 R_MIPS_REL32 - 0x0
 # REL-NEXT:   }
 # REL-NEXT: ]
 
 # REL: DynamicSection [
 # REL:   Tag        Type                 Name/Value
 # REL:   0x00000012 RELSZ                16 (bytes)
 # REL:   0x00000013 RELENT               8 (bytes)
+
+# REL:      Primary GOT {
+# REL-NEXT:   Canonical gp value:
+# REL-NEXT:   Reserved entries [
+# REL:        ]
+# REL-NEXT:   Local entries [
+# REL-NEXT:   ]
+# REL-NEXT:   Global entries [
+# REL-NEXT:     Entry {
+# REL-NEXT:       Address:
+# REL-NEXT:       Access:
+# REL-NEXT:       Initial: 0x30004
+# REL-NEXT:       Value: 0x30004
+# REL-NEXT:       Type: Object
+# REL-NEXT:       Section: .data
+# REL-NEXT:       Name: v2
+# REL-NEXT:     }
+# REL-NEXT:   ]
+# REL-NEXT:   Number of TLS and multi-GOT entries: 0
+# REL-NEXT: }