logging: BLE backend unnecessarily sends full buffer regardless of log message size

[dathpo]

Describe the bug

When the CONFIG_LOG_BACKEND_BLE option is enabled, the BLE Peripheral sends log messages to the Central. The Peripheral appears to send the full buffer on every logging call as opposed to only sending the actual content of a log message.

To Reproduce

1. Configure a BLE peripheral
2. Set CONFIG_LOG_BACKEND_BLE=y in the prj.conf
3. Connect to the Peripheral from a Central
4. Subscribe to the NUS
5. Set the MTU to 495
6. Call LOG_INF("Hello, world")
7. Check the size of message sent for the log above. It will report a size of 492 octets

Expected behavior

The BLE frame size should be 14 octets.

Impact

It causes a waste of resources by sending a BLE frame larger than necessary.

Logs and console output

The following log message received below should not have been 492 octets. The end of the log message is "bonded: 1", the rest of the buffer is either zero-filled or full of old log data.

Environment (please complete the following information):

• OS: Linux
• Toolchain: NCS 2.6
• Commit SHA: 23cf38934c

Additional context

Suggestions:

This code here is the cause of the problem:

    if (attr_data_len < LOG_BACKEND_BLE_BUF_SIZE) {
        notify_len = attr_data_len;
    } else {
        notify_len = LOG_BACKEND_BLE_BUF_SIZE;
    }

The following change addresses the problem:

    notify_len = MIN(length, MIN(attr_data_len, LOG_BACKEND_BLE_BUF_SIZE));

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[hermabe]

CC @vChavezB

[vChavezB]

@dathpo Thanks for raising the issue I will take a look.