Re: gcc10's -Wreturn-local-addr gives FP warning about lib/careadlinkat

[Jim Meyering]

On Fri, Feb 7, 2020 at 5:47 AM Pádraig Brady <address@hidden> wrote:
>
> On 06/02/2020 17:05, Jim Meyering wrote:
> > On Thu, Feb 6, 2020 at 6:03 AM Pádraig Brady <address@hidden> wrote:
> >> On 06/02/2020 00:27, Jim Meyering wrote:
> >>> Building latest latest coreutils using latest-from-git gcc10 evokes
> >>> this false positive:
> >>>
> >>> lib/careadlinkat.c: In function 'careadlinkat':
> >>> cc1: error: function may return address of local variable
> >>> [-Werror=return-local-addr]
> >>> lib/careadlinkat.c:73:8: note: declared here
> >>>      73 |   char stack_buf[1024];
> >>>
> >>> I'm guessing improved flow analysis will eventually suppress this. I
> >>> hesitate to turn off the useful and normally-high-S/N
> >>> -Wreturn-local-addr globally. Maybe just disable it in that one file,
> >>> temporarily?
> >>
> >> The logic of the function looks fine.
> >> Would an `assure (buf != stack_buf)` before the `return buf`
> >> indicate that constraint to gcc with minimal runtime overhead?
> >
> > I would have preferred that, but it has no effect.
>
> surprised.
>
> > I then tried to suppress that warning in the affected file by adding
> > these lines:
> >
> > /* Without this pragma, gcc 10.0.1 20200205 reports that
> >     the "function may return address of local variable".  */
> > # pragma GCC diagnostic ignored "-Wreturn-local-addr"
> >
> > But, surprisingly, that didn't help, either.
>
> surprised++
> That suggests something major is up with that error
> which might be easier for gcc folks to find.
>
> > Also tried Kaz Kylheku's return-early suggestion, to no avail.
> >
> > Hence, my last resort suggestion: disable that warning option for all
> > gnulib compiles. Note that simply omitting the -W... option was
> > insufficient (probably pulled in by some high-level one), so I
> > resorted to adding -Wno-return-local-addr:
>
> +1
>
> This is a significant enough regression that it should be reported.
> I didn't see a gcc bug report, only this meta bug:
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90556

I've pushed that commit.
Thanks for looking. I'll file a gcc report tomorrow.