[ELF] Fix writing the content of the .got section in a wrong place.

[ikudrin]

Summary

When filling the .got sections, InputSection::OutSecOff was added twice to find the position to apply a relocation: first time in InputSection::writeTo() and then in SectionBase::getOffset().

[ikudrin]

Ping.

[ruiu]

It looks like it's not the only problem for .got but for all sections. How does it work for other sections?

[ikudrin]

As far as I could check, other sections are not affected by the similar problem. Most of them don't call relocate*() methods. InputSection::writeTo uses unshifted Buf when calling relocate(). And EhInputSection avoids the problem because SectionBase::getOffset() doesn't add OutSecOff for them.

[ruiu]

LGTM

ELF/SyntheticSections.cpp

 void GotSection::finalizeContents() { Size = NumEntries * Config->Wordsize; }
 
 bool GotSection::empty() const {
   // If we have a relocation that is relative to GOT (such as GOTOFFREL),
   // we need to emit a GOT even if it's empty.
   return NumEntries == 0 && !HasGotOffRel;
 }
 
-void GotSection::writeTo(uint8_t *Buf) { relocateAlloc(Buf, Buf + Size); }
+void GotSection::writeTo(uint8_t *Buf) {
+  // Buf points to the start of this section's buffer,
+  // whereas InputSectionBase::relocateAlloc() expects its argument
+  // to point to the start of the output section.
+  relocateAlloc(Buf - OutSecOff, Buf - OutSecOff + Size);
+}
 
 MipsGotSection::MipsGotSection()
     : SyntheticSection(SHF_ALLOC | SHF_WRITE | SHF_MIPS_GPREL, SHT_PROGBITS, 16,
                        ".got") {}
 
 void MipsGotSection::addEntry(SymbolBody &Sym, int64_t Addend, RelExpr Expr) {
   // For "true" local symbols which can be referenced from the same module
   // only compiler creates two instructions for address loading:

test/ELF/linkerscript/got-write-offset.s

+# REQUIRES: x86
+# RUN: llvm-mc -filetype=obj -triple=i686-unknown-linux-gnu %s -o %t
+# RUN: echo "SECTIONS { \
+# RUN:   .data 0x1000 : { *(.data) } \
+# RUN:   .got 0x2000 : { \
+# RUN:     LONG(0) \
+# RUN:     *(.got) \
+# RUN:   } \
+# RUN:  };" > %t.script
+# RUN: ld.lld -shared -o %t.out --script %t.script %t
+# RUN: llvm-objdump -s %t.out | FileCheck %s
+.text
+.global foo
+foo:
+  movl bar@GOT, %eax
+.data
+.local bar
+bar:
+  .zero 4
+# CHECK:      Contents of section .data:
+# CHECK-NEXT:  1000 00000000
+# CHECK:      Contents of section .got:
+# CHECK-NEXT:  2000 00000000 00100000