[ELF] Allow KEEP within OVERLAY (#130661)

When attempting to add KEEP within an OVERLAY description, which the
Linux kernel would like to do for ARCH=arm to avoid dropping the
.vectors sections with '--gc-sections' [1], ld.lld errors with:

  ld.lld: error: ./arch/arm/kernel/vmlinux.lds:37: section pattern is expected
  >>>  __vectors_lma = .; OVERLAY 0xffff0000 : AT(__vectors_lma) { .vectors { KEEP(*(.vectors)) } ...
  >>>                                                                               ^

readOverlaySectionDescription() does not handle all input section
description keywords, despite GNU ld's documentation stating that "The
section definitions within the OVERLAY construct are identical to those
within the general SECTIONS construct, except that no addresses and no
memory regions may be defined for sections within an OVERLAY."

Reuse the existing parsing in readInputSectionDescription(), which
handles KEEP, allowing the Linux kernel's use case to work properly.

[1]: https://lore.kernel.org/[email protected]/

Author: nathanchance

lld/ELF/ScriptParser.cpp

@@ -988,20 +988,8 @@ OutputDesc *ScriptParser::readOverlaySectionDescription() {
       ctx.script->createOutputSection(readName(), getCurrentLocation());
   osd->osec.inOverlay = true;
   expect("{");
-  while (auto tok = till("}")) {
-    uint64_t withFlags = 0;
-    uint64_t withoutFlags = 0;
-    if (tok == "INPUT_SECTION_FLAGS") {
-      std::tie(withFlags, withoutFlags) = readInputSectionFlags();
-      tok = till("");
-    }
-    if (tok == "CLASS")
-      osd->osec.commands.push_back(make<InputSectionDescription>(
-          StringRef{}, withFlags, withoutFlags, readSectionClassName()));
-    else
-      osd->osec.commands.push_back(
-          readInputSectionRules(tok, withFlags, withoutFlags));
-  }
+  while (auto tok = till("}"))
+    osd->osec.commands.push_back(readInputSectionDescription(tok));
   osd->osec.phdrs = readOutputSectionPhdrs();
   return osd;
 }

lld/test/ELF/linkerscript/overlay-keep.test

@@ -0,0 +1,51 @@
+# REQUIRES: x86
+# RUN: rm -rf %t && split-file %s %t && cd %t
+# RUN: llvm-mc -filetype=obj -triple=x86_64 a.s -o a.o
+# RUN: ld.lld a.o --gc-sections --script nokeep.t -o a
+# RUN: llvm-objdump --section-headers a | FileCheck --check-prefix=NOKEEP %s
+# RUN: ld.lld a.o --gc-sections --script keep.t -o a
+# RUN: llvm-objdump --section-headers a | FileCheck --check-prefix=KEEP %s
+
+# NOKEEP:      Sections:
+# NOKEEP-NEXT: Idx Name          Size
+# NOKEEP-NEXT:   0               00000000
+# NOKEEP-NEXT:   1 .text         00000004
+# NOKEEP-NEXT:   2 .keep1        00000000
+
+# KEEP:      Sections:
+# KEEP-NEXT: Idx Name          Size
+# KEEP-NEXT:   0               00000000
+# KEEP-NEXT:   1 .text         00000004
+# KEEP-NEXT:   2 .keep1        00000004
+# KEEP-NEXT:   3 .keep2        00000004
+
+#--- a.s
+.global _start
+_start:
+ .long 1
+
+.section .keep1, "a"
+keep1:
+ .long 2
+
+.section .keep2, "a"
+keep2:
+ .long 3
+
+#--- nokeep.t
+SECTIONS {
+  .text : { *(.text) }
+  OVERLAY 0x1000 : AT ( 0x4000 ) {
+    .keep1 { *(.keep1) }
+    .keep2 { *(.keep2) }
+  }
+}
+
+#--- keep.t
+SECTIONS {
+  .text : { *(.text) }
+  OVERLAY 0x1000 : AT ( 0x4000 ) {
+    .keep1 { KEEP(*(.keep1)) }
+    .keep2 { KEEP(*(.keep2)) }
+  }
+}