Read file content from Enclave with OCall

Svart_K_ — Tue, 03 Jan 2017 14:24:26 GMT

I'm trying to read the content of a file from an enclave using OCalls.

This is what I got so far:

//enclave.edl
untrusted {
        void ocall_print_string([in, string] const char *str);
        void ocall_read_IMA_file([in, string] const char *filename, [out] char *buf, [out] int *size);
};

//enclave.cpp

void printf(const char *fmt, ...) {
	ocall_print_string(fmt);
}


void read_IMA_file(const char *filename, char *buf, int *size) {
	ocall_read_IMA_file(filename, buf, size);

	printf(buf);
}

//whereas the read_IMA_file function is called with
char *buf;
int size;
read_IMA_file("test.txt", buf, &size);

// implementation of ocall functions
void ocall_print_string(const char *str) {
	printf("%s\n", str);
}


void ocall_read_IMA_file(const char *filename, char *content, int *size) {
	content = (char*) malloc(sizeof(char) * 10);
	memset(content, '\0', sizeof(char) *10);
	char tmp[] = "1234567890";
	copy(&tmp[0], &tmp[9], content);

	cout << content << endl;
}

But the result I receive is the following:

123456789
(null)

I'm not sure what I'm doing wrong!?

Hello Svart,

Prabu_R_Intel — Wed, 04 Jan 2017 17:41:25 GMT

Hello Svart,

In the above program, the "read_IMA_file" trusted function is called with pointer variable(OUT pointer) of type character.Here we are passing the pointer variable without
any memory allocation.
"read_IMA_file" initiate a OCall that allocate memory and do "Copy" operation.Now the allocated memory is valid within the untrusted region. So we are getting expected result
for the "cout<<content" as "123456789".

Since there is no trusted memory allocated for "content"(before calling Ocall), no copy back operation happens in "OUT" pointer during Ocall returns.
So "buf" doesn't contain any valid data while doing "print(buf)" after Ocall returns in trusted region.

Please try with valid OUT pointer to character buffer(with some memory allocation) or IN and OUT pointer to String buffer.

Thanks,
R.Prabu

topic Hello Svart, in Intel® Software Guard Extensions (Intel® SGX)

Read file content from Enclave with OCall

Hello Svart,