Re: (ITS#8988) Undefined Behavior in slapadd

[noloader@gmail.com]

On Sat, Jun 8, 2019 at 3:47 AM Howard Chu <hyc@symas.com> wrote:
>
> Jeffrey Walton wrote:
> > On Fri, Jun 7, 2019 at 2:37 PM Howard Chu <hyc@symas.com> wrote:
> >>
> >> Jeffrey Walton wrote:
> >>> On Fri, Jun 7, 2019 at 10:08 AM Howard Chu <hyc@symas.com> wrote:
> >>>>
> >>>> Jeffrey Walton wrote:
> >>>>> On Fri, Jun 7, 2019 at 9:59 AM Howard Chu <hyc@symas.com> wrote:
> >>>>>>
> >>>>>> noloader@gmail.com wrote:
> >>>>>>> On Fri, Jun 7, 2019 at 9:32 AM Howard Chu <hyc@symas.com> wrote:
> >>>>>>>>
> >>>>>>>> noloader@gmail.com wrote:
> >>>>>>>> ...
> >>>>>>>>> I encourage OpenLDAP to fix the undefined behavior. OpenLDAP is an
> >>>>>>>>> important project, and the undefined behavior is causing too many
> >>>>>>>>> tangential problems.
> >>>>>>>>
> >>>>>>>> Undefined behavior is not a bug, nor is it prohibited by the C spec. It is a necessary
> >>>>>>>> part of the language for its intended use as a system programming language, writing
> >>>>>>>> machine-specific programs. Anyone who says it is prohibited by the spec is wrong.
> >>>>>>>
> >>>>>>> I don't believe this is correct.
> >>>>>>>
> >>>>>>> Maybe you are thinking of implementation defined behavior?
> >>>>>>
> >>>>>> That would apply to how a particular compiler implementation treats some piece of code.
> >>>>>> Whether a CPU supports unaligned access is machine-defined. Since our code is already
> >>>>>> properly ifdef'd for the machines which do or don't support it, the fact that this is
> >>>>>> "non-portable" is not an issue.
> >>>>>
> >>>>> The undefined behavior is causing OpenLDAP to fail testing.
> >>>>>
> >>>>> Worse, it is causing test failures in programs and libraries which use
> >>>>> OpenLDAP. The OpenLDAP bugs have cross-pollinated into other programs
> >>>>> and libraries. It is not simply contained or limited to OpenLDAP.
> >>>>>
> >>>>> That's a big problem for testing a QA folks.
> >>>>
> >>>> Then the tests are broken, because these are not bugs.
> >>>
> >>> They are absolutely OpenLDAP bugs. The unaligned accesses are
> >>> Undefined Behavior.
> >>
> >> Simply because the C standard doesn't specify what the behavior is doesn't make it a bug.
> >
> > The C standard does specify the behavior. It falls clearly in
> > Undefined Behavior.
>
> The fact a behavior is undefined does not make it illegal.

A program with undefined behavior is, by definition, and illegal program.

> > And Appendix J.2, Undefined Behavior, says:
> >
> >     Conversion between two pointer types produces a result
> >     that is incorrectly aligned (6.3.2.3).
> >
> > You are violating the C standard in macros like COPY_PGNO. Effectively
> > you are casting a uint8_t array to an uint16_t*. You cannot do that
> > because uint16_t* increases the alignment requirements.
>
> All internal fields of LMDB are (at least) 2-byte aligned, so these copies always
> meet alignment requirements.

No, they are not. Some of them are 1-byte aligned. UBsan tells you that.

Others require 4-byte alignment. UBsan tells you that, too.

Jeff