On Fri, Aug 20, 2021 at 08:43:40AM +0200, Peter Zijlstra wrote:

Fine with me; although the immediate complaint from Andrew was about
size, hence my UD1 hackery.

quoted

So if we do this, I think it should be something like

   static inline __must_check bool refcount_dec_and_test(refcount_t *r)
   {
        asm_volatile_goto (LOCK_PREFIX "decl %[var]\n\t"
                "jz %l[cc_zero]\n\t"
                "jl %l[cc_error]"
                : : [var] "m" (r->refs.counter)
                : "memory" : cc_zero, cc_error);

        return false;

   cc_zero:
        return true;
   cc_error:
        refcount_warn_saturate(r, REFCOUNT_SUB_UAF);
        return false;
   }

and we can discuss whether we could improve on the
refcount_warn_saturate() separately.

I can do the refcount_warn_saturate() change separately.

Let me go check how small I can get it...

gcc-10.2.1, x86_64-defconfig

kernel/event/core.o-inline-ud1:     96454
kernel/event/core.o-outofline-ud1:  96604
kernel/event/core.o-outofline-call: 97072

(42 refcount_warn_saturate/ud1 instances in that file,
 10 of which are refcount_dec_and_test)