OPENAFS-SA-2019-003: ubik: Avoid unlocked ubik_currentTrans deref

OPENAFS-SA-2019-003: ubik: Avoid unlocked ubik_currentTrans deref

Currently, SVOTE_Debug/SVOTE_DebugOld examine some ubik internal state
without any locks, because the speed of these functions is more
important than accuracy.

However, one of the pieces of data we examine is ubik_currentTrans,
which we dereference to get ubik_currentTrans->type. ubik_currentTrans
could be set to NULL while this code is running, so there is a small
chance of this code causing a segfault, if SVOTE_Debug() is running
when the current transaction ends.

We only ever initialize ubik_currentTrans as a write transation (via
SDISK_Begin), so this check is pointless anyway. Accordingly, skip the
type check, and always assume that any active transaction is a write
transaction.  This means we only ever access ubik_currentTrans once,
avoiding any risk of the value changing between accesses (and we no
longer need to dereference it, anyway).

Note that, since ubik_currentTrans is not marked as 'volatile', some C
compilers, with certain options, can and do assume that its value will
not change between accesses, and thus only fetch the pointer value once.
This avoids the risk of NULL dereference (and thus, crash, if pointer
stores/loads are atomic), but the value pointed to by ubik_currentTrans->type
would be incorrect when the transaction ends during the execution of
SVOTE_Debug().

Change-Id: Ia36c58e5906f5e8df59936f845ae11e886e8ec38
Reviewed-on: https://gerrit.openafs.org/13915
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: Benjamin Kaduk <kaduk@mit.edu>