cmd/link: improve error message and debugging

Correct an error message to missing section, not unreachable
symbol.

Also, under -v >= 2, dump symbol info on error for debugging.

Updates #58966.

Change-Id: I0f832c517d64f4b672b313a8b9be2d028744f945
Reviewed-on: https://go-review.googlesource.com/c/go/+/476735
Run-TryBot: Cherry Mui <cherryyz@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>

src/cmd/go/testdata/script/link_external_undef.txt

@@ -7,7 +7,7 @@
 
 ! go build -ldflags='-linkmode=external' .
 ! stderr 'panic'
-stderr '^.*unreachable sym in relocation.*'
+stderr '^.*undefined symbol in relocation.*'
 
 -- go.mod --
 

src/cmd/link/internal/ld/data.go

@@ -444,14 +444,21 @@ func (st *relocSymState) relocsym(s loader.Sym, P []byte) {
 			if weak && !ldr.AttrReachable(rs) {
 				continue
 			}
-			if ldr.SymSect(rs) == nil {
-				st.err.Errorf(s, "unreachable sym in relocation: %s", ldr.SymName(rs))
+			sect := ldr.SymSect(rs)
+			if sect == nil {
+				if rst == sym.SDYNIMPORT {
+					st.err.Errorf(s, "cannot target DYNIMPORT sym in section-relative reloc: %s", ldr.SymName(rs))
+				} else if rst == sym.SUNDEFEXT {
+					st.err.Errorf(s, "undefined symbol in relocation: %s", ldr.SymName(rs))
+				} else {
+					st.err.Errorf(s, "missing section for relocation target %s", ldr.SymName(rs))
+				}
 				continue
 			}
 
 			// The method offset tables using this relocation expect the offset to be relative
 			// to the start of the first text section, even if there are multiple.
-			if ldr.SymSect(rs).Name == ".text" {
+			if sect.Name == ".text" {
 				o = ldr.SymValue(rs) - int64(Segtext.Sections[0].Vaddr) + r.Add()
 			} else {
 				o = ldr.SymValue(rs) - int64(ldr.SymSect(rs).Vaddr) + r.Add()

src/cmd/link/internal/ld/main.go

@@ -159,6 +159,14 @@ func Main(arch *sys.Arch, theArch Arch) {
 		// dump symbol info on crash
 		defer func() { ctxt.loader.Dump() }()
 	}
+	if ctxt.Debugvlog > 1 {
+		// dump symbol info on error
+		AtExit(func() {
+			if nerrors > 0 {
+				ctxt.loader.Dump()
+			}
+		})
+	}
 
 	switch *flagHeadType {
 	case "":