Fix bug in K64F trng_api.c

[sarahmarshy]

Description

K64F trng_get_byte function assumed memory for byte was zero initialized.

That means that this line would OR random data with data existing in memory. All values will eventually reach 255 if called many times.

Steps to test or reproduce

Use this test program to replicate the issue:

#include "mbed.h"

#define RANDOM_BUFF_SIZE 3                                                           /**< Random numbers buffer size. */

#include "hal/trng_api.h"

int main(void)
{
    uint32_t err_code;

    while (true)
    {
        trng_t trng_obj;
        trng_init(&trng_obj);
        uint8_t p_buff[RANDOM_BUFF_SIZE];
        size_t output_length;
        uint8_t length = trng_get_bytes(&trng_obj, p_buff, RANDOM_BUFF_SIZE, &output_length);
        printf("Random Vector:");
        for(uint8_t i = 0; i < RANDOM_BUFF_SIZE; i++)
        {
            printf(" %u",p_buff[i]);
        }
        printf("\r\n expected: %d length: %d",RANDOM_BUFF_SIZE, output_length);
        printf("\n\r");
        trng_free(&trng_obj);
        wait_ms(3000);
    }
}

[hanno-becker]

Thanks for spotting and fixing this!

@0xc0170 Could you also add @mazimkhan as a reviewer, please?

[hanno-becker]

LGTM

[mazimkhan]

LGTM

[0xc0170]

/morph build

[mbed-ci]

Build : SUCCESS

Build number : 590
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/5550/

Triggering tests

/morph test
/morph uvisor-test
/morph export-build

[mbed-ci]

Exporter Build : SUCCESS

Build number : 202
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/5550/

[mbed-ci]

Test : FAILURE

Build number : 403
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/5550/403

[0xc0170]

/morph test

[mbed-ci]

Test : SUCCESS

Build number : 413
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/5550/413