[jira] [Created] (HDFS-14285) libhdfs hdfsRead copies entire array even if its only partially filled

["Sahil Takiar (JIRA)" <ji...@apache.org>]

Sahil Takiar created HDFS-14285:
-----------------------------------

             Summary: libhdfs hdfsRead copies entire array even if its only partially filled
                 Key: HDFS-14285
                 URL: https://issues.apache.org/jira/browse/HDFS-14285
             Project: Hadoop HDFS
          Issue Type: Bug
          Components: hdfs-client, libhdfs, native
            Reporter: Sahil Takiar


There is a bug in libhdfs {{hdfsRead}}
{code:java}
    jthr = invokeMethod(env, &jVal, INSTANCE, jInputStream, HADOOP_ISTRM,
                               "read", "([B)I", jbRarray);
    if (jthr) {
        destroyLocalReference(env, jbRarray);
        errno = printExceptionAndFree(env, jthr, PRINT_EXC_ALL,
            "hdfsRead: FSDataInputStream#read");
        return -1;
    }
    if (jVal.i < 0) {
        // EOF
        destroyLocalReference(env, jbRarray);
        return 0;
    } else if (jVal.i == 0) {
        destroyLocalReference(env, jbRarray);
        errno = EINTR;
        return -1;
    }
    (*env)->GetByteArrayRegion(env, jbRarray, 0, noReadBytes, buffer);
{code}
The method makes a call to {{FSInputStream#read(byte[])}} to fill in the Java byte array, however, {{#read(byte[])}} is not guaranteed to fill up the entire array, instead it returns the number of bytes written to the array (which could be less than the size of the array). Yet `{{GetByteArrayRegion}} decides to copy the entire contents of the {{jbArray}} into the buffer ({{noReadBytes}} is initialized to the length of the buffer and is never updated). So if {{FSInputStream#read(byte[])}} decides to read less data than the size of the byte array, the call to {{GetByteArrayRegion}} will essentially copy more bytes than necessary.



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