[LLD][ELF] Support OVERLAY NOCROSSREFS (#133807)

This allows NOCROSSREFS to be specified in OVERLAY linker script
descriptions. This is a particularly useful part of the OVERLAY syntax,
since it's very rarely possible for one overlay section to sensibly
reference another.

Closes #128790

Author: mysterymath

lld/ELF/ScriptParser.cpp

@@ -565,6 +565,7 @@ SmallVector<SectionCommand *, 0> ScriptParser::readOverlay() {
     addrExpr = readExpr();
     expect(":");
   }
+  bool noCrossRefs = consume("NOCROSSREFS");
   Expr lmaExpr = consume("AT") ? readParenExpr() : Expr{};
   expect("{");
 
@@ -595,6 +596,12 @@ SmallVector<SectionCommand *, 0> ScriptParser::readOverlay() {
       static_cast<OutputDesc *>(od)->osec.memoryRegionName =
           std::string(regionName);
   }
+  if (noCrossRefs) {
+    NoCrossRefCommand cmd;
+    for (SectionCommand *od : v)
+      cmd.outputSections.push_back(static_cast<OutputDesc *>(od)->osec.name);
+    ctx.script->noCrossRefs.push_back(std::move(cmd));
+  }
 
   // According to the specification, at the end of the overlay, the location
   // counter should be equal to the overlay base address plus size of the

lld/docs/ReleaseNotes.rst

@@ -39,6 +39,9 @@ ELF Improvements
   items that kept it live during garbage collection. This is inspired by the
   Mach-O LLD feature of the same name.
 
+* Linker script ``OVERLAY`` descriptions now support virtual memory regions
+  (e.g. ``>region``) and ``NOCROSSREFS``.
+
 Breaking changes
 ----------------
 

lld/test/ELF/linkerscript/nocrossrefs.test

@@ -28,6 +28,10 @@
 # RUN: not ld.lld a.o data.o -T 3.t 2>&1 | FileCheck %s --check-prefix=CHECK3 --implicit-check-not=error:
 # CHECK3:      error: a.o:(.nonalloc+0x0): prohibited cross reference from '.nonalloc' to '.text' in '.text'
 
+# RUN: not ld.lld a.o data.o -T overlay.t 2>&1 | FileCheck %s --check-prefix=OVERLAY --implicit-check-not=error:
+# OVERLAY: error: a.o:(.text.start+0x11): prohibited cross reference from '.text' to 'data' in '.data'
+# OVERLAY-NEXT: error: a.o:(.text.start+0x17): prohibited cross reference from '.text' to 'str1' in '.rodata'
+
 #--- 0.t
 ## Some cases that do not cause errors.
 abs = 42;
@@ -50,6 +54,17 @@ NOCROSSREFS(.text .text1 .text2 .data .rodata .data .nonalloc)
 abs = 42;
 NOCROSSREFS_TO(.text .text .text1 .text2 .data .nonalloc)
 
+#--- overlay.t
+abs = 42;
+_edata = 43;
+SECTIONS {
+  OVERLAY : NOCROSSREFS {
+    .text { *(.text*) }
+    .data { *(.data*) }
+    .rodata { *(.rodata*) }
+  }
+}
+
 #--- err1.t
 NOCROSSREFS )
 

lld/test/ELF/linkerscript/overlay.test

@@ -133,3 +133,11 @@ SECTIONS {
 # RUN: not ld.lld a.o -T unclosed.lds 2>&1 | FileCheck %s --check-prefix=UNCLOSED
 # UNCLOSED:     error: unclosed.lds:2: unexpected EOF
 # UNCLOSED-NOT: {{.}}
+
+#--- at-nocrossrefs-order.lds
+SECTIONS { OVERLAY 0x1000 : AT(0x1234) NOCROSSREFS {} }
+
+# RUN: not ld.lld a.o -T at-nocrossrefs-order.lds 2>&1 | FileCheck %s --check-prefix=AT-NOCROSSREFS-ORDER --strict-whitespace
+# AT-NOCROSSREFS-ORDER:{{.*}}error: at-nocrossrefs-order.lds:1: { expected, but got NOCROSSREFS
+# AT-NOCROSSREFS-ORDER-NEXT:>>> SECTIONS { OVERLAY 0x1000 : AT(0x1234) NOCROSSREFS {} }
+# AT-NOCROSSREFS-ORDER-NEXT:>>>                                        ^