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SubjectRe: [PATCH v4 2/2] IMA: Call workqueue functions to measure queued keys
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On 12/17/2019 6:01 PM, James Bottomley wrote:

>
> This code is confusing me:
>
> + /*
> + * To avoid holding the mutex when processing queued keys,
> + * transfer the queued keys with the mutex held to a temp list,
> + * release the mutex, and then process the queued keys from
> + * the temp list.
> + *
> + * Since ima_process_keys is set to true, any new key will be
> + * processed immediately and not be queued.
> + */
> + INIT_LIST_HEAD(&temp_ima_keys);
> +
> + mutex_lock(&ima_keys_mutex);
> +
> + if (!ima_process_keys) {
> + ima_process_keys = true;
> +
> + if (!list_empty(&ima_keys)) {
> + list_for_each_entry_safe(entry, tmp, &ima_keys, list)
> + list_move_tail(&entry->list, &temp_ima_keys);
> + process = true;
> + }
> + }
> +
> + mutex_unlock(&ima_keys_mutex);
> +
>
> The direct implication of the comment and the lock dance with the
> temporary list and the processed flag is that stuff can be added to the
> ima_keys list after you drop the mutex. Your explanation in the prior
> couple of emails says that nothing can be added because the
> ima_process_keys flag setting prevents it. If the latter is true, you
> can simply drop the lock after setting the flag and rely on ima_keys
> not changing to run it through process_buffer_measurement without
> needing any of the intermediate list or the processed flag. If the
> latter isn't true then any key added to ima_keys after the mutex is
> dropped is never processed.
>
> James

Once the flag is set no new key will be added to ima_keys list.

You are right - if the flag is set with the lock taken, then there is no
need for the temp list. After dropping the lock, measurement can be done
directly from ima_keys list.

Thanks for reviewing the code. I'll send an update tomorrow.

-lakshmi

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