pkg/proc: merge register data before writing to register (#2699)

Right now, if (*compositeMemory).WriteMemory needs to write a value to
a register that's smaller than the full size of the register (say, a
uint32 being passed as an argument), then (*AMD64Registers).SetReg can
later fail a sanity check that ensures the passed DwarfRegister is a
full size register.

Fix this by reading the old value of the register and overwriting just
the relevant parts with the new register. For the purposes of an
argument, it would probably be fine to just pad with zeroes, but merging
with the existing value is what gdb does.

Fixes #2698

_fixtures/fncall.go

@@ -176,6 +176,17 @@ func regabistacktest2(n1, n2, n3, n4, n5, n6, n7, n8, n9, n10 int) (int, int, in
 	return n1 + n2, n2 + n3, n3 + n4, n4 + n5, n5 + n6, n6 + n7, n7 + n8, n8 + n9, n9 + n10, n10 + n1
 }
 
+type Issue2698 struct {
+	a uint32
+	b uint8
+	c uint8
+	d uintptr
+}
+
+func (i Issue2698) String() string {
+	return fmt.Sprintf("%d %d %d %d", i.a, i.b, i.c, i.d)
+}
+
 func main() {
 	one, two := 1, 2
 	intslice := []int{1, 2, 3}
@@ -192,6 +203,12 @@ func main() {
 	var pable_pa PRcvrable = pa
 	var x X = 2
 	var x2 X2 = 2
+	issue2698 := Issue2698{
+		a: 1,
+		b: 2,
+		c: 3,
+		d: 4,
+	}
 
 	fn2clos := makeclos(pa)
 	fn2glob := call1
@@ -208,5 +225,5 @@ func main() {
 	d.Method()
 	d.Base.Method()
 	x.CallMe()
-	fmt.Println(one, two, zero, call, call0, call2, callexit, callpanic, callbreak, callstacktrace, stringsJoin, intslice, stringslice, comma, a.VRcvr, a.PRcvr, pa, vable_a, vable_pa, pable_pa, fn2clos, fn2glob, fn2valmeth, fn2ptrmeth, fn2nil, ga, escapeArg, a2, square, intcallpanic, onetwothree, curriedAdd, getAStruct, getAStructPtr, getVRcvrableFromAStruct, getPRcvrableFromAStructPtr, getVRcvrableFromAStructPtr, pa2, noreturncall, str, d, x, x2.CallMe(5), longstrs, regabistacktest, regabistacktest2)
+	fmt.Println(one, two, zero, call, call0, call2, callexit, callpanic, callbreak, callstacktrace, stringsJoin, intslice, stringslice, comma, a.VRcvr, a.PRcvr, pa, vable_a, vable_pa, pable_pa, fn2clos, fn2glob, fn2valmeth, fn2ptrmeth, fn2nil, ga, escapeArg, a2, square, intcallpanic, onetwothree, curriedAdd, getAStruct, getAStructPtr, getVRcvrableFromAStruct, getPRcvrableFromAStructPtr, getVRcvrableFromAStructPtr, pa2, noreturncall, str, d, x, x2.CallMe(5), longstrs, regabistacktest, regabistacktest2, issue2698.String())
 }

pkg/dwarf/op/regs.go

@@ -164,3 +164,20 @@ func (reg *DwarfRegister) FillBytes() {
 	reg.Bytes = make([]byte, 8)
 	binary.LittleEndian.PutUint64(reg.Bytes, reg.Uint64Val)
 }
+
+// Overwrite takes the contents of reg1 and overwrites them with the contents
+// of reg2 in little-endian order, returning a new register. The new register
+// will always contain the complete contents of both registers, so if reg2 is
+// larger than reg1, the final register will be reg2's size.
+func (reg1 *DwarfRegister) Overwrite(reg2 *DwarfRegister) *DwarfRegister {
+	reg1.FillBytes()
+	reg2.FillBytes()
+	width := len(reg1.Bytes)
+	if len(reg2.Bytes) > len(reg1.Bytes) {
+		width = len(reg2.Bytes)
+	}
+	b := make([]byte, width)
+	copy(b, reg1.Bytes)
+	copy(b, reg2.Bytes)
+	return DwarfRegisterFromBytes(b)
+}

pkg/proc/dwarf_expr_test.go

@@ -245,7 +245,7 @@ func TestDwarfExprComposite(t *testing.T) {
 	if changeCalls[0] != "3 - 2f00000000000000" {
 		t.Errorf("wrong call to SetReg (Rbx)")
 	}
-	if changeCalls[1] != "2 - 0c00" {
+	if changeCalls[1] != "2 - 0c00000000000000" {
 		t.Errorf("wrong call to SetReg (Rcx)")
 	}
 	if mem.data[0x10] != 13 || mem.data[0x11] != 0x00 {

pkg/proc/mem.go

@@ -179,7 +179,9 @@ func (mem *compositeMemory) WriteMemory(addr uint64, data []byte) (int, error) {
 
 			switch piece.Kind {
 			case op.RegPiece:
-				err := mem.regs.ChangeFunc(piece.Val, op.DwarfRegisterFromBytes(pieceMem))
+				oldReg := mem.regs.Reg(piece.Val)
+				newReg := op.DwarfRegisterFromBytes(pieceMem)
+				err := mem.regs.ChangeFunc(piece.Val, oldReg.Overwrite(newReg))
 				if err != nil {
 					return donesz, err
 				}

service/test/variables_test.go

@@ -1286,6 +1286,7 @@ func TestCallFunction(t *testing.T) {
 	var testcases117 = []testCaseCallFunction{
 		{`regabistacktest("one", "two", "three", "four", "five", 4)`, []string{`:string:"onetwo"`, `:string:"twothree"`, `:string:"threefour"`, `:string:"fourfive"`, `:string:"fiveone"`, ":uint8:8"}, nil},
 		{`regabistacktest2(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)`, []string{":int:3", ":int:5", ":int:7", ":int:9", ":int:11", ":int:13", ":int:15", ":int:17", ":int:19", ":int:11"}, nil},
+		{`issue2698.String()`, []string{`:string:"1 2 3 4"`}, nil},
 	}
 
 	withTestProcessArgs("fncall", t, ".", nil, protest.AllNonOptimized, func(p *proc.Target, fixture protest.Fixture) {