[ELF] Support discarding .got.plt

[MaskRay]

Summary

Fix a null pointer dereference when .got.plt is discarded.

This also adds a test for discarding .plt.

[ikudrin]

LGTM.

lld/ELF/SyntheticSections.cpp

void RelocationBaseSection::finalizeContents() {
   // When linking glibc statically, .rel{,a}.plt contains R_*_IRELATIVE
   // relocations due to IFUNC (e.g. strcpy). sh_link will be set to 0 in that
   // case.
   if (symTab && symTab->getParent())
     getParent()->link = symTab->getParent()->sectionIndex;
   else
     getParent()->link = 0;
 
-  if (in.relaPlt == this) {
+  if (in.relaPlt == this && in.gotPlt->getParent()) {
     getParent()->flags |= ELF::SHF_INFO_LINK;
     getParent()->info = in.gotPlt->getParent()->sectionIndex;
   }
-  if (in.relaIplt == this) {
+  if (in.relaIplt == this && in.igotPlt->getParent()) {
     getParent()->flags |= ELF::SHF_INFO_LINK;
     getParent()->info = in.igotPlt->getParent()->sectionIndex;
   }
 }
 
 RelrBaseSection::RelrBaseSection()
     : SyntheticSection(SHF_ALLOC,
                        config->useAndroidRelrTags ? SHT_ANDROID_RELR : SHT_RELR,

lld/test/ELF/linkerscript/discard-plt.s

+# REQUIRES: x86
+# RUN: split-file %s %t
+# RUN: llvm-mc -filetype=obj -triple=x86_64 %t/a.s -o %t/a.o
+
+## Discard .plt, .iplt, and .got.plt
+# RUN: ld.lld -shared -T %t/t %t/a.o -o %t/a
+# RUN: llvm-readelf -S -d %t/a > %t/readelf.txt
+# RUN: FileCheck %s --input-file %t/readelf.txt
+# RUN: FileCheck %s --input-file %t/readelf.txt --check-prefix=NEG
+
+# CHECK:      [Nr] Name      Type     Address  Off      Size   ES Flg Lk Inf Al
+# CHECK:      ] .rela.plt RELA     [[#%x,]] [[#%x,]] 000018 18   A  1   0  8
+
+# CHECK:      (PLTGOT)  0x0
+# CHECK:      (PLTREL)  RELA
+
+# NEG-NOT: ] .plt
+# NEG-NOT: ] .iplt
+# NEG-NOT: ] .got.plt
+
+#--- a.s
+  call foo
+  call ifunc
+
+.type ifunc, @gnu_indirect_function
+.hidden ifunc
+ifunc:
+  ret
+
+.data
+.quad ifunc
+
+#--- t
+SECTIONS {
+  .text : { *(.text) }
+  /DISCARD/ : { *(.plt .iplt .got.plt) }
+}