The routine ccwchain_calc_length() is tasked with looking at a
channel program, seeing how many CCWs are chained together by
the presence of the Chain-Command flag, and returning a count
to the caller.
Previously, it also considered a Transfer-in-Channel CCW as being
an appropriate mechanism for chaining. The problem at the time
was that the TIC CCW will almost certainly not go to the next CCW
in memory (because the CC flag would be sufficient), and so
advancing to the next 8 bytes will cause us to read potentially
invalid memory. So that comparison was removed, and the target
of the TIC is processed as a new chain.
This is fine when a TIC goes to a new chain (consider a NOP+TIC to
a channel program that is being redriven), but there is another
scenario where this falls apart. A TIC can be used to "rewind"
a channel program, for example to find a particular record on a
disk with various orientation CCWs. In this case, we DO want to
consider the memory after the TIC since the TIC will be skipped
once the requested criteria is met. This is due to the Status
Modifier presented by the device, though software doesn't need to
operate on it beyond understanding the behavior change of how the
channel program is executed.
So to handle this, we will re-introduce the check for a TIC CCW
but limit it by examining the target of the TIC. If the TIC
doesn't go back into the current chain, then current behavior
applies; we should stop counting CCWs and let the target of the
TIC be handled as a new chain. But, if the TIC DOES go back into
the current chain, then we need to keep looking at the memory after
the TIC for when the channel breaks out of the TIC loop. We can't
use tic_target_chain_exists() because the chain in question hasn't
been built yet, so we will redefine that comparison with some small
functions to make it more readable and to permit refactoring later.
Fixes: 405d566f98 ("vfio-ccw: Don't assume there are more ccws after a TIC")
Signed-off-by: Eric Farman <farman@linux.ibm.com>
Message-Id: <20190222183941.29596-2-farman@linux.ibm.com>
Reviewed-by: Halil Pasic <pasic@linux.ibm.com>
Reviewed-by: Farhan Ali <alifm@linux.ibm.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>
48bd0eee8e