[ELF] Reset OutputSection size prior to processing linker script commands

[jhenderson]

Summary

The size of an OutputSection is calculated early, to aid handling of compressed debug sections. However, subsequent to this point, unused synthetic sections are removed. In the event that an OutputSection, from which such an InputSection is removed, is still required (e.g. because it has a symbol assignment), and no longer has any InputSections, dot assignments, or BYTE()-family directives, the size member is never updated when processing the commands. If the removed InputSection had a non-zero size (such as a .got.plt section), the section ends up with the wrong size in the output.

The fix is to reset the OutputSection size prior to processing the linker script commands relating to that OutputSection. This ensures that the size is correct even in the above situation.

[ruiu]

ELF/LinkerScript.cpp

627–630

I think this kind of code is a sign that we compute a value too early. It seems that the real problem is that we compute output section size too early for non-compressed debug sections. We don't need to compute it at all if this function is executed, no?

[jhenderson]

Superseded by https://reviews.llvm.org/D38361.

ELF/LinkerScript.cpp

void LinkerScript::assignOffsets(OutputSection *Sec) {
 
   switchTo(Sec);
 
   // We do not support custom layout for compressed debug sectons.
   // At this point we already know their size and have compressed content.
   if (CurAddressState->OutSec->Flags & SHF_COMPRESSED)
     return;
 
+  // The Size was previously calculated for compressed debug sections. We now
+  // reset it, because unused synthetic sections may have since been removed,
+  // leaving the Size invalid.
+  Sec->Size = 0;
+
   for (BaseCommand *C : Sec->Commands)
     process(*C);
 }
 
 void LinkerScript::removeEmptyCommands() {
   // It is common practice to use very generic linker scripts. So for any
   // given run some of the output sections in the script will be empty.
   // We could create corresponding empty output sections, but that would

test/ELF/linkerscript/unused-synthetic.s

 # REQUIRES: x86
 # RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t.o
 # RUN: echo "SECTIONS { \
 # RUN:    .got  : { *(.got) } \
 # RUN:    .plt  : { *(.plt) } \
 # RUN:    .text : { *(.text) } \
 # RUN:  }" > %t.script
 # RUN: ld.lld -shared -o %t.so --script %t.script %t.o
 
 # RUN: llvm-objdump -section-headers %t.so | FileCheck %s
 # CHECK-NOT:  .got
 # CHECK-NOT:  .plt
 # CHECK:      .text
 # CHECK-NEXT: .dynsym
 
+# Test that the size of a removed unused synthetic input section does not get
+# added to the output section size, if the output section has symbol
+# assignments, preventing removal of the output section.
+# RUN: echo "SECTIONS { \
+# RUN:    .got.plt : { a_sym = .; *(.got.plt) } \
+# RUN:  }" > %t2.script
+# RUN: ld.lld -shared -o %t2.so --script %t2.script %t.o
+# RUN: llvm-objdump -section-headers %t2.so | FileCheck %s --check-prefix=CHECK2
+# CHECK2: .got.plt 00000000
+
 .global _start
 _start:
   nop