readCString: implemented with readInplace

All read logic must go through and be validated
in the same few places.

Bug: 376674798
Test: binderClearBufTest covers this, and in presubmit
Change-Id: Icc0ade84b671ecd3026069d8f672ff254d58e995

Author: Steven Moreland

libs/binder/Parcel.cpp

@@ -2223,9 +2223,7 @@ const char* Parcel::readCString() const
         const char* eos = reinterpret_cast<const char*>(memchr(str, 0, avail));
         if (eos) {
             const size_t len = eos - str;
-            mDataPos += pad_size(len+1);
-            ALOGV("readCString Setting data pos of %p to %zu", this, mDataPos);
-            return str;
+            return static_cast<const char*>(readInplace(len + 1));
         }
     }
     return nullptr;

libs/binder/tests/binderParcelUnitTest.cpp

@@ -33,6 +33,38 @@ using android::String8;
 using android::binder::Status;
 using android::binder::unique_fd;
 
+static void checkCString(const char* str) {
+    for (size_t i = 0; i < 3; i++) {
+        Parcel p;
+
+        for (size_t j = 0; j < i; j++) p.writeInt32(3);
+
+        p.writeCString(str);
+        int32_t pos = p.dataPosition();
+
+        p.setDataPosition(0);
+
+        for (size_t j = 0; j < i; j++) p.readInt32();
+        const char* str2 = p.readCString();
+
+        ASSERT_EQ(std::string(str), str2);
+        ASSERT_EQ(pos, p.dataPosition());
+    }
+}
+
+TEST(Parcel, TestReadCString) {
+    // we should remove the *CString APIs, but testing them until
+    // they are deleted.
+    checkCString("");
+    checkCString("a");
+    checkCString("\n");
+    checkCString("32");
+    checkCString("321");
+    checkCString("3210");
+    checkCString("3210b");
+    checkCString("123434");
+}
+
 TEST(Parcel, NonNullTerminatedString8) {
     String8 kTestString = String8("test-is-good");