Fix strict weak ordering requirement of less than operation

operator< on hidl_vec<uint8_t> violates strict weak ordering in the case
that one oparand is shorter that the other and the shorter is a prefix
of the longer.

if x and y are incomparable, i.e., neither x < y nor y < x and
   y and z are incomparable, i.e., neither y < z nor z < y, then
   x and z must be incomparable.
As for the current implementation the first two statements are true but
the third is not given the following example input:
x:="aa", y:="a", z:="ab".

This patch fixes the issue by defining a < b if a is a prefix of b.

As this relation is used in a std::sort algorithm which demands strict
weak ordering this bug leads to undefined behavior.

Change-Id: I4961bb35e2fd4f5fcf561ec0c7c536f81830aab8

1 files changed, 3 insertions, 1 deletions

diff --git a/keymaster/4.0/support/keymaster_utils.cpp b/keymaster/4.0/support/keymaster_utils.cpp
index 729e1c1db..e35fdd36d 100644
--- a/keymaster/4.0/support/keymaster_utils.cpp
+++ b/keymaster/4.0/support/keymaster_utils.cpp
@@ -21,7 +21,9 @@ namespace android {
 namespace hardware {
 
 inline static bool operator<(const hidl_vec<uint8_t>& a, const hidl_vec<uint8_t>& b) {
-    return memcmp(a.data(), b.data(), std::min(a.size(), b.size())) == -1;
+    auto result = memcmp(a.data(), b.data(), std::min(a.size(), b.size()));
+    if (!result) return a.size() < b.size();
+    return result < 0;
 }
 
 template <size_t SIZE>