debug/gosym: read start line of a function from gopclntab

This field was introduced in go 1.20 therefore we need to safeguard it
to only be read for pclntabs of that version or higher, as otherwise
nonsensical bytes would be read. The fallback is now the default value
of 0.

Fixes golang#58474

src/debug/gosym/pclntab.go

@@ -301,6 +301,9 @@ func (t *LineTable) go12Funcs() []Func {
 		}()
 	}
 
+	// The start line only exists in pclntab starting at version 1.20.
+	readStartLine := t.version >= ver120
+
 	ft := t.funcTab()
 	funcs := make([]Func, ft.Count())
 	syms := make([]Sym, len(funcs))
@@ -309,6 +312,9 @@ func (t *LineTable) go12Funcs() []Func {
 		f.Entry = ft.pc(i)
 		f.End = ft.pc(i + 1)
 		info := t.funcData(uint32(i))
+		if readStartLine {
+			f.StartLine = int(info.startLine())
+		}
 		f.LineTable = t
 		f.FrameSize = int(info.deferreturn())
 		syms[i] = Sym{
@@ -461,6 +467,7 @@ func (f funcData) deferreturn() uint32 { return f.field(3) }
 func (f funcData) pcfile() uint32      { return f.field(5) }
 func (f funcData) pcln() uint32        { return f.field(6) }
 func (f funcData) cuOffset() uint32    { return f.field(8) }
+func (f funcData) startLine() uint32   { return f.field(9) }
 
 // field returns the nth field of the _func struct.
 // It panics if n == 0 or n > 9; for n == 0, call f.entryPC.

src/debug/gosym/pclntab_test.go

@@ -201,6 +201,24 @@ func TestLineAline(t *testing.T) {
 	}
 }
 
+func TestStartLine(t *testing.T) {
+	dotest(t)
+	defer endtest()
+
+	f, tab := crack(pclinetestBinary, t)
+	defer f.Close()
+
+	sym := tab.LookupFunc("main.main")
+	wantLine := 6
+	if sym == nil {
+		t.Fatal("no main.main")
+	}
+
+	if sym.StartLine != wantLine {
+		t.Fatalf("StartLine = %d, want %d", sym.StartLine, wantLine)
+	}
+}
+
 func TestPCLine(t *testing.T) {
 	dotest(t)
 	defer endtest()

src/debug/gosym/symtab.go

@@ -137,6 +137,7 @@ type Func struct {
 	FrameSize int
 	LineTable *LineTable
 	Obj       *Obj
+	StartLine int
 }
 
 // An Obj represents a collection of functions in a symbol table.