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SubjectBPF tracing trampoline synchronization between update/freeing and execution?
Hi!

I was chatting with kpsingh about BPF trampolines, and I noticed that
it looks like BPF trampolines (as of current bpf-next/master) seem to
be missing synchronization between trampoline code updates and
trampoline execution. Or maybe I'm missing something?

If I understand correctly, trampolines are executed directly from the
fentry placeholders at the start of arbitrary kernel functions, so
they can run without any locks held. So for example, if task A starts
executing a trampoline on entry to sys_open(), then gets preempted in
the middle of the trampoline, and then task B quickly calls
BPF_RAW_TRACEPOINT_OPEN twice, and then task A continues execution,
task A will end up executing the middle of newly-written machine code,
which can probably end up crashing the kernel somehow?

I think that at least to synchronize trampoline text freeing with
concurrent trampoline execution, it is necessary to do something
similar to what the livepatching code does with klp_check_stack(), and
then either use a callback from the scheduler to periodically re-check
tasks that were in the trampoline or let the trampoline tail-call into
a cleanup helper that is part of normal kernel text. And you'd
probably have to gate BPF trampolines on
CONFIG_HAVE_RELIABLE_STACKTRACE.

[Trampoline *updates* could probably be handled more easily if a
trampoline consisted of an immutable header that increments something
like a percpu refcount followed by a mutable body, but given that that
doesn't solve freeing trampolines, I'm not sure if it'd be worth the
effort. Unless you just never free trampoline memory, but that's
probably not a great idea.]

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