Re: [RFC PATCH v3 1/8] Use refcount_t for ucounts reference counting

Kaiwan N Billimoria
2021-01-19T04:42:57Z

(Sorry for the gmail client)
My 0.2, HTH:
a) AFAIK, refcount_inc() (and similar friends) don't return any value
b) they're designed to just WARN() if they saturate or if you're
attempting to increment the value 0 (as it's possibly a UAF bug)
c) refcount_inc_checked() is documented as "Similar to atomic_inc(),
but will saturate at UINT_MAX and WARN"
d) we should avoid using the __foo() when foo() 's present as far as
is sanely possible...

So is one expected to just fix things when they break? - as signalled
by the WARN firing?

--
Regards, kaiwan.

On Tue, Jan 19, 2021 at 2:26 AM Alexey Gladkov <gladkov.alexey@gmail.com> wrote:
>
> On Mon, Jan 18, 2021 at 12:34:29PM -0800, Linus Torvalds wrote:
> > On Mon, Jan 18, 2021 at 11:46 AM Alexey Gladkov
> > <gladkov.alexey@gmail.com> wrote:
> > >
> > > Sorry about that. I thought that this code is not needed when switching
> > > from int to refcount_t. I was wrong.
> >
> > Well, you _may_ be right. I personally didn't check how the return
> > value is used.
> >
> > I only reacted to "it certainly _may_ be used, and there is absolutely
> > no comment anywhere about why it wouldn't matter".
>
> I have not found examples where checked the overflow after calling
> refcount_inc/refcount_add.
>
> For example in kernel/fork.c:2298 :
>
> current->signal->nr_threads++;
> atomic_inc(&current->signal->live);
> refcount_inc(&current->signal->sigcnt);
>
> $ semind search signal_struct.sigcnt
> def include/linux/sched/signal.h:83 refcount_t sigcnt;
> m-- kernel/fork.c:723 put_signal_struct if (refcount_dec_and_test(&sig->sigcnt))
> m-- kernel/fork.c:1571 copy_signal refcount_set(&sig->sigcnt, 1);
> m-- kernel/fork.c:2298 copy_process refcount_inc(&current->signal->sigcnt);
>
> It seems to me that the only way is to use __refcount_inc and then compare
> the old value with REFCOUNT_MAX
>
> Since I have not seen examples of such checks, I thought that this is
> acceptable. Sorry once again. I have not tried to hide these changes.
>
> --
> Rgrds, legion
>
>