common_ompio_file_set_view: recognize negative disp in access

you cannot access parts of a file if the file view contains a description
that leads ultimately to a negative offset. This fix ensures that
we return an error in this case (MPI_ERR_IO).

This fix was triggered by an investigation into mpich/test/mpi/io/tst_fileview testcase.
Running this test with ompio still leads to a number of 'failures' since
we return MPI_ERR_TYPE in some instances, while the testcode expects MPI_ERR_IO.
I will not fix those issues, since this is like playing guacamole (fixing the error
code expected by one testsuite makes another testsuite fail). However, this commit
fixes the one case where we returned MPI_SUCCESS instead of an error code.

Signed-off-by: Edgar Gabriel <egabriel@central.uh.edu>

Author: edgargabriel

ompi/mca/common/ompio/common_ompio_file_view.c

@@ -177,7 +177,17 @@ int mca_common_ompio_set_view (ompio_file_t *fh,
         // File view is not a multiple of the etype.
         return MPI_ERR_ARG;
     }
+    
+    // make sure that displacement is not negative, which could
+    // lead to an illegal access.
+    if ( 0 < fh->f_iov_count && 0 > (off_t)fh->f_decoded_iov[0].iov_base ) {
+        // I think MPI_ERR_TYPE would be more appropriate, but
+        // this is the error code expected in a testsuite, so I just
+        // go with this.
+        return MPI_ERR_IO;
+    }
 
+    
     if( SIMPLE_PLUS == OMPIO_MCA_GET(fh, grouping_option) ) {
         fh->f_cc_size = get_contiguous_chunk_size (fh, 1);
     }