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ktrace: Avoid recursion in namei()
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Authored by markj on May 18 2021, 8:50 PM.

Details

Summary

I tried fuzzing ktrace(2) with syzkaller and it found the following bug:
sys_ktrace() calls namei(), which may call ktrnamei(). But sys_ktrace()
also calls ktrace_enter() first, so if the caller is itself being
traced, the assertion in ktrace_enter() is triggered. And, ktrnamei()
does not check for recursion like most other ktrace ops do.

Fix the bug by simply deferring the ktrace_enter() call. I believe it
is safe to perform name resolution from sys_ktrace() even when traced.
Also make the parameter to ktrnamei() const and convert to ANSI.

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