[ELF] Support input section description .rel[a].dyn in /DISCARD/

[MaskRay]

A gvisor developer reported that .rela.dyn cannot be discarded. I find that supporting .rela.dyn does not even need extra code, so let's do it. We can support .relr.dyn as well, but it may need more work, so I don't include it in this patch.

// SyntheticSections.cpp
    addSize(config->useAndroidRelrTags ? DT_ANDROID_RELRSZ : DT_RELRSZ,
            part.relrDyn->getParent());  // If .relr.dyn is allowed to be discarded, part.relrDyn->getParent() will be null and part.relrDyn->getParent()->size will dereference a null pointer.

Note, there is a difference lld and GNU ld. To suppress .rela.dyn with GNU ld, /DISCARD/ : { *(.rela.data) } should be used instead. I feel that the lld way is more consistent. Sections such as .dynsym, .dynstr and .dynamic are synthesized sections, and they are discarded with an input section description *(.dynsym). It is inconsistent for *(.rela.dyn) not to discard .rela.dyn in GNU ld. It may be unimplemented, though.

[ruiu]

LGTM

lld/ELF/LinkerScript.cpp

for (const SectionPattern &pat : cmd->sectionPatterns) {
 
     sortInputSections(
         MutableArrayRef<InputSectionBase *>(ret).slice(sizeBefore), pat);
   }
   return ret;
 }
 
 void LinkerScript::discard(InputSectionBase *s) {
-  if (s == in.shStrTab || s == mainPart->relaDyn || s == mainPart->relrDyn)
+  if (s == in.shStrTab || s == mainPart->relrDyn)
     error("discarding " + s->name + " section is not allowed");
 
   // You can discard .hash and .gnu.hash sections by linker scripts. Since
   // they are synthesized sections, we need to handle them differently than
   // other regular sections.
   if (s == mainPart->gnuHashTab)
     mainPart->gnuHashTab = nullptr;
   if (s == mainPart->hashTab)

lld/test/ELF/linkerscript/discard-section-err.s

 # RUN: echo "SECTIONS { /DISCARD/ : { *(.dynsym) } }" > %t.script
 # RUN: ld.lld -pie -o %t --script %t.script %t.o
 
 ## We allow discarding .dynstr, check we don't crash.
 # RUN: echo "SECTIONS { /DISCARD/ : { *(.dynstr) } }" > %t.script
 # RUN: ld.lld -pie -o %t --script %t.script %t.o
 
 # RUN: echo "SECTIONS { /DISCARD/ : { *(.rela.dyn) } }" > %t.script
-# RUN: not ld.lld -pie -o %t --script %t.script %t.o 2>&1 | \
-# RUN:   FileCheck -check-prefix=RELADYN %s
-# RELADYN: discarding .rela.dyn section is not allowed
+# RUN: ld.lld -pie -o %t %t.o
+# RUN: llvm-readobj -S %t | FileCheck --check-prefix=RELADYN %s
+# RELADYN: Name: .rela.dyn
+# RUN: ld.lld -pie -o %t --script %t.script %t.o
+# RUN: llvm-readobj -S %t | FileCheck /dev/null --implicit-check-not='Name: .rela.dyn'
 
-.comm foo,4,4
+# RUN: echo "SECTIONS { /DISCARD/ : { *(.relr.dyn) } }" > %t.script
+# RUN: not ld.lld -pie --pack-dyn-relocs=relr -o %t --script %t.script %t.o 2>&1 | \
+# RUN:   FileCheck -check-prefix=RELRDYN %s
+# RELRDYN: discarding .relr.dyn section is not allowed
+
+.data
+.align 8
+foo:
+## Emits an R_X86_64_RELATIVE in -pie mode.
+.quad foo