the return address on the stack and only then "dereferences" %pc.
Therefore, in the case of a call to an invalid address, we arrive
to the trap handler with the invalid value in tf_eip. This used
to prevent db_backtrace() from assigning the most recent and interesting
frame on the stack to the right spot in the right function, from
which the invalid call was attempted.
Try to detect and work around that by recovering the return address
from the stack.
The work-around requires the fault address be passed to db_backtrace().
Smuggle it as tf_err.
MFC after: 1 month
Sponsored by: RiNet (Cronyx Plus LLC)
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