selftests/bpf: Add exception handling selftests for tp_bpf program

Exception handling is triggered in BPF tracing programs when a NULL pointer
is dereferenced; the exception handler zeroes the targe

selftests/bpf: Add exception handling selftests for tp_bpf program

Exception handling is triggered in BPF tracing programs when a NULL pointer
is dereferenced; the exception handler zeroes the target register and
execution of the BPF program progresses.

To test exception handling then, we need to trigger a NULL pointer dereference
for a field which should never be zero; if it is, the only explanation is the
exception handler ran. task->task_works is the NULL pointer chosen (for a new
task from fork() no work is associated), and the task_works->func field should
not be zero if task_works is non-NULL. The test verifies that task_works and
task_works->func are 0.

Signed-off-by: Alan Maguire <[email protected]>
Signed-off-by: Daniel Borkmann <[email protected]>
Acked-by: Andrii Nakryiko <[email protected]>
Link: https://lore.kernel.org/bpf/[email protected]

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