Commit 53b18b0791

Marc Tiehuis <marctiehuis@gmail.com>
2018-08-18 02:15:39
Add secureZero function
This is identical to `mem.set(u8, slice, 0)` except that it will never be optimized out by the compiler. Intended usage is for clearing secret data. The resulting assembly has been manually verified in --release-* modes. It would be valuable to test the 'never be optimized out' claim in tests but this is harder than initially expected due to how much Zig appears to know locally. May be doable with @intToPtr, @ptrToInt to get around known data dependencies but I could not work it out right now.
master
1 parent 1da93ca
Changed files (1)
std
mem.zig
std/mem.zig
@@ -175,6 +175,24 @@ pub fn set(comptime T: type, dest: []T, value: T) void {
         d.* = value;
 }
 
+pub fn secureZero(comptime T: type, s: []T) void {
+    // NOTE: We do not use a volatile slice cast here since LLVM cannot
+    // see that it can be replaced by a memset.
+    const ptr = @ptrCast([*]volatile u8, s.ptr);
+    const len = s.len * @sizeOf(T);
+    @memset(ptr, 0, len);
+}
+
+test "mem.secureZero" {
+    var a = []u8{0xfe} ** 8;
+    var b = []u8{0xfe} ** 8;
+
+    set(u8, a[0..], 0);
+    secureZero(u8, b[0..]);
+
+    assert(eql(u8, a[0..], b[0..]));
+}
+
 pub fn compare(comptime T: type, lhs: []const T, rhs: []const T) Compare {
     const n = math.min(lhs.len, rhs.len);
     var i: usize = 0;