fix(gosym): prevent 32-bit truncation panic in PCIndex

ebpf/symtab/gosym/pcindex.go

@@ -69,8 +69,18 @@ func (it *PCIndex) First() uint64 {
 
 func (it *PCIndex) FindIndex(addr uint64) int {
 	if it.i32 != nil {
-
-		if addr < uint64(it.i32[0]) {
+		// The upper bound is load-bearing, not defensive. Set() only stores
+		// into i32 when the value fits in 32 bits (it promotes to i64
+		// otherwise), so an address above MaxUint32 cannot be covered by this
+		// index. Meanwhile the BinarySearch below truncates addr to uint32.
+		//
+		// Without this check the two disagree, and a high address takes one of
+		// two bad paths depending on its low word:
+		//   - low word below i32[0]  -> BinarySearch returns (0,false), i--
+		//     makes it -1, and it.i32[i] panics with index out of range [-1]
+		//   - low word inside range  -> a wrong entry matches, and the profiler
+		//     silently attributes samples to the wrong symbol
+		if addr > math.MaxUint32 || addr < uint64(it.i32[0]) {
 			return -1
 		}
 		i, found := slices.BinarySearch(it.i32, uint32(addr))

ebpf/symtab/gosym/pcindex_panic_test.go

@@ -0,0 +1,89 @@
+package gosym
+
+import "testing"
+
+// TestFindIndexTruncationPanic reproduces the production panic
+//
+//	panic: runtime error: index out of range [-1]
+//	  gosym.(*PCIndex).FindIndex  pcindex.go:81
+//
+// Root cause: the bounds guard compares the FULL 64-bit addr,
+//
+//	if addr < uint64(it.i32[0]) { return -1 }
+//
+// but the search TRUNCATES it,
+//
+//	slices.BinarySearch(it.i32, uint32(addr))
+//
+// For any addr >= 2^32 whose low 32 bits fall below i32[0], the guard passes
+// while the search looks for a value smaller than every element. BinarySearch
+// then returns (0, false), `i--` makes it -1, and `it.i32[i]` panics.
+//
+// FindIndex returning -1 is an expected outcome — elf.SymbolTable.Resolve
+// explicitly checks `if i == -1 { return "" }` (symbol_table.go:126). So the
+// correct behaviour here is to return -1, not to panic.
+func TestFindIndexTruncationPanic(t *testing.T) {
+	idx := NewPCIndex(2)
+	idx.Set(0, 0x1000)
+	idx.Set(1, 0x2000)
+
+	// 0x1_0000_0500: above 2^32, low word 0x500 is below i32[0] (0x1000).
+	const addr = uint64(0x100000500)
+
+	if got := idx.FindIndex(addr); got != -1 {
+		t.Fatalf("FindIndex(%#x) = %d, want -1 (address is not covered by this index)", addr, got)
+	}
+}
+
+// TestFindIndexBelowFirstStillReturnsMinusOne pins the behaviour the guard
+// already had, so a fix cannot regress it.
+func TestFindIndexBelowFirstStillReturnsMinusOne(t *testing.T) {
+	idx := NewPCIndex(2)
+	idx.Set(0, 0x1000)
+	idx.Set(1, 0x2000)
+
+	if got := idx.FindIndex(0x500); got != -1 {
+		t.Fatalf("FindIndex(0x500) = %d, want -1", got)
+	}
+}
+
+// TestFindIndexNormalLookupsUnaffected pins the happy paths.
+func TestFindIndexNormalLookupsUnaffected(t *testing.T) {
+	idx := NewPCIndex(3)
+	idx.Set(0, 0x1000)
+	idx.Set(1, 0x2000)
+	idx.Set(2, 0x3000)
+
+	for _, tc := range []struct {
+		addr uint64
+		want int
+	}{
+		{0x1000, 0}, // exact first
+		{0x1500, 0}, // between first and second
+		{0x2000, 1}, // exact middle
+		{0x3000, 2}, // exact last
+		{0x9999, 2}, // past the end still maps to the last entry
+	} {
+		if got := idx.FindIndex(tc.addr); got != tc.want {
+			t.Fatalf("FindIndex(%#x) = %d, want %d", tc.addr, got, tc.want)
+		}
+	}
+}
+
+// TestFindIndexTruncationWrongSymbol is the reason a bare `if i < 0` guard is
+// the wrong fix. Here the truncated low word lands INSIDE the index range, so
+// there is no negative index and no panic — FindIndex silently returns a match
+// for an address the index does not cover, and the profiler attributes samples
+// to the wrong symbol. Silent wrong data is worse than a crash.
+func TestFindIndexTruncationWrongSymbol(t *testing.T) {
+	idx := NewPCIndex(2)
+	idx.Set(0, 0x1000)
+	idx.Set(1, 0x2000)
+
+	// low word 0x1500 sits between the two entries, so BinarySearch "succeeds"
+	const addr = uint64(0x100001500)
+
+	if got := idx.FindIndex(addr); got != -1 {
+		t.Fatalf("FindIndex(%#x) = %d, want -1 — a 32-bit index cannot cover an address above MaxUint32", addr, got)
+	}
+}