[ELF] - Allow discarding .hash and .gnu.hash from linker script.

[grimar]

Summary

Currently, LLD segfaults when linker script attempts to discard
one of the hash sections. This patch fixes that.

[ruiu]

Generally looking good, but ...

ELF/LinkerScript.cpp

363–364

Is that actually common? Could you give me an example?

[grimar]

ELF/LinkerScript.cpp

363–364

I am not saying it is common (I am fixing crash here first of all), but there is nothing wrong with that either.
We could disallow discarding of them instead probably, but there are no solid reasons for doing that I think,
as it would be an artificial limitation inconsistent with gnu linkers (they allow doing that).

[ruiu]

LGTM

ELF/LinkerScript.cpp

363–364

"normal" sounded like it was somewhat common. Maybe I'd say "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."

ELF/LinkerScript.cpp

 }
 
 void LinkerScript::discard(ArrayRef<InputSection *> V) {
   for (InputSection *S : V) {
     if (S == InX::ShStrTab || S == InX::Dynamic || S == InX::DynSymTab ||
         S == InX::DynStrTab)
       error("discarding " + S->Name + " section is not allowed");
 
+    // It's normal to discard the following sections. We drop the pointer to adjust
+    // the rest code behavior as there were no such sections from the begining.
+    if (S == InX::GnuHashTab)
+      InX::GnuHashTab = nullptr;
+    if (S == InX::HashTab)
+      InX::HashTab = nullptr;
+
     S->Assigned = false;
     S->Live = false;
     discard(S->DependentSections);
   }
 }
 
 std::vector<InputSection *>
 LinkerScript::createInputSectionList(OutputSection &OutCmd) {

test/ELF/linkerscript/discard-gnu-hash.s

+# REQUIRES: x86
+# RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t
+
+# RUN: ld.lld --hash-style both -shared -o %t1 %t
+# RUN: llvm-objdump -section-headers %t1 | FileCheck %s
+# CHECK: .gnu.hash
+# CHECK: .hash
+
+# RUN: echo "SECTIONS { /DISCARD/ : { *(.hash) } }" > %t.script
+# RUN: ld.lld --hash-style both -shared -o %t1 --script %t.script %t
+# RUN: llvm-objdump -section-headers %t1 \
+# RUN:   | FileCheck %s --check-prefix=HASH
+# HASH-NOT: .hash
+# HASH:     .gnu.hash
+# HASH-NOT: .hash
+
+# RUN: echo "SECTIONS { /DISCARD/ : { *(.gnu.hash) } }" > %t.script
+# RUN: ld.lld --hash-style both -shared -o %t1 --script %t.script %t
+# RUN: llvm-objdump -section-headers %t1 \
+# RUN:   | FileCheck %s --check-prefix=GNUHASH
+# GNUHASH-NOT: .gnu.hash
+# GNUHASH:     .hash
+# GNUHASH-NOT: .gnu.hash