pkg/compiler: fix alignment corner case

Fix alignemnt calculation for packed structs with alignment and bitfields.
Amusingly this affected only a single real struct -- ipv6_fragment_ext_header.

2 files changed, 19 insertions, 2 deletions

diff --git a/prog/analysis.go b/prog/analysis.go
index c93a13e6c..6a5cd03d9 100644
--- a/prog/analysis.go
+++ b/prog/analysis.go
@@ -120,9 +120,11 @@ func foreachArgImpl(arg Arg, ctx ArgCtx, f func(Arg, *ArgCtx)) {
 				totalSize += size
 			}
 		}
-		if totalSize > a.Size() {
+		claimedSize := a.Size()
+		varlen := a.Type().Varlen()
+		if varlen && totalSize > claimedSize || !varlen && totalSize != claimedSize {
 			panic(fmt.Sprintf("bad group arg size %v, should be <= %v for %+v",
-				totalSize, a.Size(), a))
+				totalSize, claimedSize, a))
 		}
 	case *PointerArg:
 		if a.Res != nil {
diff --git a/prog/encodingexec_test.go b/prog/encodingexec_test.go
index 92c36cad4..2ad19598e 100644
--- a/prog/encodingexec_test.go
+++ b/prog/encodingexec_test.go
@@ -390,6 +390,21 @@ func TestSerializeForExec(t *testing.T) {
 			},
 			nil,
 		},
+		{
+			"syz_test$align7(&(0x7f0000000000)={{0x1, 0x2, 0x3, 0x4, 0x5, 0x6}, 0x42})",
+			[]uint64{
+				execInstrCopyin, dataOffset + 0, execArgConst, 1 | 0<<16 | 1<<24, 0x1,
+				execInstrCopyin, dataOffset + 0, execArgConst, 1 | 1<<16 | 1<<24, 0x2,
+				execInstrCopyin, dataOffset + 0, execArgConst, 1 | 2<<16 | 1<<24, 0x3,
+				execInstrCopyin, dataOffset + 1, execArgConst, 2 | 0<<16 | 1<<24, 0x4,
+				execInstrCopyin, dataOffset + 1, execArgConst, 2 | 1<<16 | 1<<24, 0x5,
+				execInstrCopyin, dataOffset + 1, execArgConst, 2 | 2<<16 | 1<<24, 0x6,
+				execInstrCopyin, dataOffset + 8, execArgConst, 1, 0x42,
+				callID("syz_test$align7"), ExecNoCopyout, 1, execArgConst, ptrSize, dataOffset,
+				execInstrEOF,
+			},
+			nil,
+		},
 	}
 
 	buf := make([]byte, ExecBufferSize)