Re: [PATCH] err_ptr.h: introduce ERR_PTR_SAFE()

On Sat, May 16, 2026 at 2:42 PM David Laight
<[email protected]> wrote:
>
> On Sat, 16 May 2026 13:39:11 +0200
> Amir Goldstein <[email protected]> wrote:
>
> > On Sat, May 16, 2026 at 10:42 AM David Laight
> > <[email protected]> wrote:
> > >
> > > On Fri, 15 May 2026 21:26:04 +0200
> > > Amir Goldstein <[email protected]> wrote:
> > >
> > > > On Fri, May 15, 2026 at 8:30 PM David Laight
> > > > <[email protected]> wrote:
> > > > >
> > > > > On Thu, 14 May 2026 22:01:29 +0200
> > > > > Amir Goldstein <[email protected]> wrote:
> > > > >
> > > ...
> > > > >
> > > > > The object code bloat would be noticeable if this were used everywhere.
> > > > > But you could make it a bit simpler:
> > > > >         if (__builtin_constant_p(__e))
> > > > >                 BUILD_BUG_ON(__e && !IS_ERR_VALUE(__e));
> > > > >         else if WARN_ON(__e && !IS_ERR_VALUE(__e))
> > > > >                 __e = -MAX_ERRNO; // Or maybe -EINVAL to stop and other boundary errors
> > > > >         (void *)__e;
> > > >
> > > > Yeh that's nicer thanks.
> > >
> > > Actually this might be better still (or just more succinct):
> > >         void *__e = (void *)error;
> > >         BUILD_BUG_ON(!statically_true(IS_ERR_OR_NULL(__e));
> >
> > This condition is wrong but also my compiler does not evaluate
> > __builtin_constant_p(IS_ERR_OR_NULL(__e)) as true.
> >
> > This works
> > BUILD_BUG_ON(statically_true(!IS_ERR_VALUE(__e)));
>
> Yes, it is easy to get those wrong - especially when typing quickly.
>
> >
> > I think it is enough to statically assert on ERR_PTR(EINVAL)
> > and no need to bother with ERR_PTR(0)
>
> Then the tests don't match - which looks funny.
> IS_ERR_VALUE(val) should be: val += 4095; jump_carry ...
> and IS_ERR_OR_NULL(val): val--; val += 4096; jump_carry ...
> but I can't remember whether gcc manages to do that.
>
> >
> > >         if (WARN_ON(!IS_ERR_OR_NULL(__e))
> > >                 __e = (void *)-EINVAL;
> >
> > Oh, anything but EINVAL please - the most overloaded error value
> > My choice of meaningful error value would be EFAULT
> > because without the safe helper we would be returning an address
> > which is in most likelihood bad, so better be explicit about it.
>
> I'm not sure about EFAULT; it is only really used for user copy failures.
> IIRC at least one Unix (I've forgotten which) generates SIGSEGV when a
> system call return of EFAULT.
>
> There is also the 'problem' of PANIC_ON_WARN which is set by a lot
> of distributions.
> That (sort of) means than you might as well use BUG_ON() and get the
> associated slightly smaller code size.

Come on. For real?
I might as well use BUG_ON() which is strictly and rightfully
frowned upon instead of using WARN_ON() because some distros
choose to do PANIC_ON_WARN?
That logic is backwards.

> On x86-64 (and maybe a few others) both BUG_ON() and WARN_ON() just
> execute UD2 (an undefined instruction) and the trap handler finds the
> associated info and does the printk().
> That makes the code smaller than pr_warn().
> Someone needs to add a 'I_REALLY_MEAN_WARN_ON()' that never panics.
> (And maybe with an option to not dump all the stack.)
>

Well, the whole point is that I would want to get a stack dump
if my code was doing ERR_PTR(<positive_value>) because without
this stack dump it would have been really hard to understand
the conditions that caused the regression in the Link.

Thanks,
Amir.