internal: fix reading auxv on 32-bit platforms

It turns out that the auxiliary vector has a platform specific size.
Adjust the code to use uintptr to approximate "unsigned long" from C.

Fixes cilium#1133

Signed-off-by: Lorenz Bauer <lmb@isovalent.com>

internal/vdso.go

@@ -52,26 +52,35 @@ func vdsoVersion() (uint32, error) {
 	return c, nil
 }
 
+// auxvPair represents a tag and value pair in the auxiliary vector.
+//
+// The size depends on the width of a pointer. In C, each entry is of type
+// "unsigned long".
+type auxvPair struct {
+	Tag, Value uintptr
+}
+
 // vdsoMemoryAddress returns the memory address of the vDSO library
 // linked into the current process image. r is an io.Reader into an auxv blob.
-func vdsoMemoryAddress(r io.Reader) (uint64, error) {
+func vdsoMemoryAddress(r io.Reader) (uintptr, error) {
+	// See https://elixir.bootlin.com/linux/v6.5.5/source/include/uapi/linux/auxvec.h
 	const (
 		_AT_NULL         = 0  // End of vector
 		_AT_SYSINFO_EHDR = 33 // Offset to vDSO blob in process image
 	)
 
 	// Loop through all tag/value pairs in auxv until we find `AT_SYSINFO_EHDR`,
 	// the address of a page containing the virtual Dynamic Shared Object (vDSO).
-	aux := struct{ Tag, Val uint64 }{}
+	var aux auxvPair
 	for {
 		if err := binary.Read(r, NativeEndian, &aux); err != nil {
 			return 0, fmt.Errorf("reading auxv entry: %w", err)
 		}
 
 		switch aux.Tag {
 		case _AT_SYSINFO_EHDR:
-			if aux.Val != 0 {
-				return aux.Val, nil
+			if aux.Value != 0 {
+				return aux.Value, nil
 			}
 			return 0, fmt.Errorf("invalid vDSO address in auxv")
 		// _AT_NULL is always the last tag/val pair in the aux vector

internal/vdso_test.go

@@ -18,7 +18,7 @@ func TestAuxvVDSOMemoryAddress(t *testing.T) {
 		t.Fatal(err)
 	}
 
-	expected := uint64(0x7ffd377e5000)
+	expected := uintptr(0x7ffd377e5000)
 	if addr != expected {
 		t.Errorf("Expected vDSO memory address %x, got %x", expected, addr)
 	}