consequence, fill_regs() and fill_fpregs() access random data, usually
on the thread kernel stack. Most often the td_frame points to the
previous frame saved by last kernel entry sequence, but this is not
guaranteed.
For /proc/<pid>/{regs,fpregs} read access, require the thread to be in
stopped state. Otherwise, return EBUSY as is done for write case.
Reported and tested by: pho
Approved by: des (procfs maintainer)
MFC after: 1 week
847e02e941