The sched_get_stackinfo() interface was just added. However, it occurs to me that it is a dangerous feature and could lead to security problems. In FLAT and PROTECTED modes, if you get access to any other threads stack, you could do harm.
This commit adds some level of security. Basically, it implements these rules:
1. Any thread may query its own stack,
2. A kernel thread may query the stack of any other thread
3. Application threads, however, may query only the stacks of threads within the same task group, i.e., the main thread and any of the child pthreads created with the main thread as a parent or grandparent or great-grandpart ...
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