Polish infer_length_after_decode and add test cases

src/core/ext/transport/chttp2/transport/bin_decoder.cc

@@ -75,12 +75,20 @@ static bool input_is_valid(uint8_t* input_ptr, size_t length) {
 #define COMPOSE_OUTPUT_BYTE_2(input_ptr) \
   (uint8_t)((decode_table[input_ptr[2]] << 6) | decode_table[input_ptr[3]])
 
-size_t grpc_base64_infer_length_after_decode(const grpc_slice& slice) {
+// By RFC 4648, if the length of the encoded string without padding is 4n+r,
+// the length of decoded string is: 1) 3n if r = 0, 2) 3n + 1 if r = 2, 3, or
+// 3) invalid if r = 1.
+size_t grpc_chttp2_base64_infer_length_after_decode(const grpc_slice& slice) {
   size_t len = GRPC_SLICE_LENGTH(slice);
   const uint8_t* bytes = GRPC_SLICE_START_PTR(slice);
   while (len > 0 && bytes[len - 1] == '=') {
     len--;
   }
+  if (GRPC_SLICE_LENGTH(slice) - len > 2) {
+    gpr_log(GPR_ERROR,
+            "Base64 decoding failed. Input has more than 2 paddings.");
+    return 0;
+  }
   size_t tuples = len / 4;
   size_t tail_case = len % 4;
   if (tail_case == 1) {
@@ -88,7 +96,7 @@ size_t grpc_base64_infer_length_after_decode(const grpc_slice& slice) {
             "Base64 decoding failed. Input has a length of %zu (without"
             " padding), which is invalid.\n",
             len);
-    tail_case = 0;
+    return 0;
   }
   return tuples * 3 + tail_xtra[tail_case];
 }

src/core/ext/transport/chttp2/transport/bin_decoder.h

@@ -49,6 +49,6 @@ grpc_slice grpc_chttp2_base64_decode_with_length(grpc_slice input,
                                                  size_t output_length);
 
 /* Infer the length of decoded data from encoded data. */
-size_t grpc_base64_infer_length_after_decode(const grpc_slice& slice);
+size_t grpc_chttp2_base64_infer_length_after_decode(const grpc_slice& slice);
 
 #endif /* GRPC_CORE_EXT_TRANSPORT_CHTTP2_TRANSPORT_BIN_DECODER_H */

test/core/transport/chttp2/bin_decoder_test.cc

@@ -67,6 +67,16 @@ static grpc_slice base64_decode_with_length(const char* s,
   return out;
 }
 
+static size_t base64_infer_length(const char* s) {
+  grpc_slice ss = grpc_slice_from_copied_string(s);
+  size_t out = grpc_chttp2_base64_infer_length_after_decode(ss);
+  grpc_slice_unref_internal(ss);
+  return out;
+}
+
+#define EXPECT_DECODED_LENGTH(s, expected) \
+  GPR_ASSERT((expected) == base64_infer_length((s)));
+
 #define EXPECT_SLICE_EQ(expected, slice)                                    \
   expect_slice_eq(                                                          \
       grpc_slice_from_copied_buffer(expected, sizeof(expected) - 1), slice, \
@@ -131,6 +141,26 @@ int main(int argc, char** argv) {
     // Test illegal charactors in grpc_chttp2_base64_decode_with_length
     EXPECT_SLICE_EQ("", base64_decode_with_length("Zm:v", 3));
     EXPECT_SLICE_EQ("", base64_decode_with_length("Zm=v", 3));
+
+    EXPECT_DECODED_LENGTH("", 0);
+    EXPECT_DECODED_LENGTH("ab", 1);
+    EXPECT_DECODED_LENGTH("abc", 2);
+    EXPECT_DECODED_LENGTH("abcd", 3);
+    EXPECT_DECODED_LENGTH("abcdef", 4);
+    EXPECT_DECODED_LENGTH("abcdefg", 5);
+    EXPECT_DECODED_LENGTH("abcdefgh", 6);
+
+    EXPECT_DECODED_LENGTH("ab==", 1);
+    EXPECT_DECODED_LENGTH("abc=", 2);
+    EXPECT_DECODED_LENGTH("abcd", 3);
+    EXPECT_DECODED_LENGTH("abcdef==", 4);
+    EXPECT_DECODED_LENGTH("abcdefg=", 5);
+    EXPECT_DECODED_LENGTH("abcdefgh", 6);
+
+    EXPECT_DECODED_LENGTH("a", 0);
+    EXPECT_DECODED_LENGTH("a===", 0);
+    EXPECT_DECODED_LENGTH("abcde", 0);
+    EXPECT_DECODED_LENGTH("abcde===", 0);
   }
   grpc_shutdown();
   return all_ok ? 0 : 1;