/proc/sys/kernel/tainted

[hrt]

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If you load this module, /proc/sys/kernel/tainted will have bits masks set and you will get a pretty message in dmesg:

[   62.037165] example: loading out-of-tree module taints kernel.
[   62.037234] example: module verification failed: signature and/or required key missing - tainting kernel

Whilst it's not a complete giveaway.

1. The module (example) is visible above.
2. Since the tainted module is hidden, the tainted flag indicates that a module is potentially hiding.