On 2/12/21 11:07 AM, Erik Skultety wrote:
On Fri, Feb 12, 2021 at 10:43:56AM +0100, Martin Kletzander wrote:
On Fri, Feb 12, 2021 at 12:54:02AM -0500, Laine Stump wrote:
I've looked at a few of these, and one thing I've found is that very
often we have a function called somethingSomethingClear(), and:
1) The only places it is ever called will immediately free the memory
of the object as soon as they clear it.
and very possibly
2) It doesn't actually *clear* everything. Some items are cleared via VIR_FREE(), but then some of the other pointers call
bobLoblawFree(def->bobloblaw)
and then don't actually set def->bobloblaw to NULL - so the functions
aren't actually "Clearing", they're "Freeing and then clearing a few
things, but not everything".
One thing I am wondering is whether this is really only used where it makes
sense. As far as I understand, and please correct me if I am way off, the
purpose of the Clear functions is to:
a) provide a way to remove everything from a structure that the current
function cannot recreate (there is a pointer to it somewhere else which
would not be updated) and
b) provide a way to reset a structure so that it can be filled again without
needless reallocation.
I think (b) is obviously pointless, especially lately, so the only reasonable
usage would be for the scenario (a). However, I think I remember this also
being used in places where it would be perfectly fine to free the variable and
recreate it. Maybe it could ease up the decision, at least by eliminating some
of the code, if my hunch is correct.
In my quick search I only found virDomainVideoDefClear to be used in this manner
and I am not convinced that it is worth having this extra function with extra
You could always memset it explicitly, someone might find the code more
readable then. IMO I'd vote for an explicit memset just for the sake of better
security practice (since we'll have to wait a little while for something like
SGX to be convenient to deploy and develop with...). Anyhow, I'm not sure how
many cycles exactly would be wasted, but IIRC a recent discussion memset can be
optimized away (correct me if I don't remember it well!), so Dan P.B.
suggested to gradually convert to some platform-specific ways on how to
sanitize the memory safely - with that in mind, I'd say we use an explicit
memset in all the functions in question and convert them later?
I think one can argue that if there's a memset() called inside a
function that is supposed to clear out a member of a struct and later
the member is tested against NULL, but compiler decides to "optimize"
memset out - it's a compiler bug.
Michal
