On Sun, 20 Apr 2025 20:09:50 -0700, Kees Cook wrote:
> On Sun, Apr 20, 2025 at 09:27:26PM -0400, Kent Overstreet wrote:
> > On Sun, Apr 20, 2025 at 06:08:41PM -0700, Kees Cook wrote:
> > > On Sun, Apr 20, 2025 at 11:59:13AM -0400, Kent Overstreet wrote:
> > > > Not sure we have a list for library code, but this might be of interest
> > > > to anyone who's had to debug refcount issues on refs with lots of users
> > > > (filesystem people), and I know the hardening folks deal with refcounts
> > > > a lot.
> > >
> > > Why not use refcount_t instead of atomic_t?
> > Out of curiousity, has overflow of an atomic_long_t refcount ever been
> > observed?
>
> Not to my knowledge. :)
Equivalent systems have observed it, but only in the presence of compiler
optimizations that deduce they could increment the refcount multiple times.
NEVER_INLINE void naughty_ref_increment(ref* ref) {
long i;
for (i = 0; i < LONG_MAX/2; i++) {
ref_get(ref);
}
}
Running the above code 3 times will saturate the refcount, if it ever
terminates in our lifetimes (due to being optimized into an
atomic_fetch_add(LONG_MAX/2)).
So: don't write the above code!
