File system sample code - reading data

John_D_4 — Mon, 25 Sep 2017 23:32:35 GMT

Hi there,
I am following the sample code from the file system guide (https://software.intel.com/en-us/articles/overview-of-intel-protected-file-system-library-using-software-guard-extensions). The edl sets the readData buffer as "[in, out, string]":

public size_t ecall_file_read([user_check]SGX_FILE* fp,[in,out,string]char* readData);

Unfortunately, the app code below only returns part of the data:

//Read from File
size_t sizeOfRead = 0;
char data[100];
ret = ecall_file_read(eid, &sizeOfRead, fp, data);
printf("Size of Read= %d\n", sizeOfRead);
data[sizeOfRead] = '\0';
printf("Read Data= %s\n", data);

Output:
"Size of Read= 14
Read Data= Hello E"

The data is indeed read in the enclave but only part of it is returned to the app. If I modify the edl to "[out, size=len]" I need to pass the size but at least I get all data over to the app consistenly.

Output: "Read Data= Hello Enclave!"

Any ideas what might be wrong with the sample code or my setup, is it possible to avoid specifying the size upfront and still make this work?

Thanks!

John,

Hoang_N_Intel — Tue, 26 Sep 2017 15:23:41 GMT

John,

I just compiled and ran the provided example without modification and the output is correct as follows:

Enclave Secret Value: Hello Enclave!
Size of Write= 14
Size of Read= 14
Read Data= Hello Enclave!

From what you posted, the size value is not the issue.

Output:
"Size of Read= 14
Read Data= Hello E"

Size of 14 is correct. and "Hello E" string has only the length of 7.

Given that the code shows putting a NULL character at the end of the buffer at 14

ret = ecall_file_read(eid, &sizeOfRead, fp, data);

printf("Size of Read= %d\n", sizeOfRead);

data[sizeOfRead] = '\0';

printf("Read Data= %s\n", data);

it should be printing out at least 14 characters unless your data has a NULL character in the middle of that string.

Would you please download and run the provided example as is one more time and let us know the result?

Thanks,

topic John, in Intel Confidential Computing

File system sample code - reading data

John,