[systemd-devel] [PATCH 2/5] libsystemd:terminal :fix uninitialized warning

[David Herrmann]

Hi

On Mon, Nov 3, 2014 at 1:44 PM, Zbigniew Jędrzejewski-Szmek
<zbyszek at in.waw.pl> wrote:
> On Mon, Nov 03, 2014 at 01:41:20PM +0100, Lennart Poettering wrote:
>> On Mon, 03.11.14 13:12, David Herrmann (dh.herrmann at gmail.com) wrote:
>>
>> > Hi
>> >
>> > On Thu, Oct 16, 2014 at 11:43 PM,  <philippedeswert at gmail.com> wrote:
>> > > From: Philippe De Swert <philippedeswert at gmail.com>
>> > >
>> > > Remove the following warning during the compilation:
>> > > src/libsystemd-terminal/grdev-drm.c: In function 'grdrm_card_hotplug':
>> > > src/libsystemd-terminal/grdev-drm.c:1087:45: warning: 'fb' may be used uninitialized in this function [-Wmaybe-uninitialized]
>> > > src/libsystemd-terminal/grdev-drm.c:1035:19: note: 'fb' was declared here
>> > > ---
>> > >  src/libsystemd-terminal/grdev-drm.c | 2 +-
>> > >  1 file changed, 1 insertion(+), 1 deletion(-)
>> > >
>> > > diff --git a/src/libsystemd-terminal/grdev-drm.c b/src/libsystemd-terminal/grdev-drm.c
>> > > index dba6db2..415755e 100644
>> > > --- a/src/libsystemd-terminal/grdev-drm.c
>> > > +++ b/src/libsystemd-terminal/grdev-drm.c
>> > > @@ -1032,7 +1032,7 @@ error:
>> > >
>> > >  static void grdrm_crtc_expose(grdrm_crtc *crtc) {
>> > >          grdrm_pipe *pipe;
>> > > -        grdrm_fb *fb;
>> > > +        grdrm_fb *fb  = NULL;
>> >
>> > Ewww, this is not nice. It does fix the warning, indeed, but the
>> > underlying problem is more generic. Lets look at this:
>> >
>> > int some_function(void **out) {
>> > ...
>> >         r = ioctl(...);
>> >         if (r < 0)
>> >                 return -errno;
>> > ...
>> > }
>> >
>> > gcc has no guarantee that "errno" is <0 if r is <0. Therefore,
>> > whenever it inlines those functions (-O2 etc.), it will spew a warning
>> > that "out" might be uninitialized if r<0 but errno==0. With -O0 gcc
>> > doesn't complain as it probably does not optimize across functions.
>> > However, with -O2 I get those warnings for "static" functions all the
>> > time.
>> >
>> > Not sure what to do here. I dislike initializing the pointer to NULL
>> > as it might hide other real warnings. I'd prefer something like:
>> >
>> > r = -SANE_ERRNO;
>> >
>> > with:
>> >
>> > #define SANE_ERRNO (abs(errno) ? : EMAGIC)
>> >
>> > ...not sure what we do in other places, though. Lennart? Tom?
>>
>> So far we went the simple way out and merged patches like the original
>> one you are replying to.
>>
>> Adding a call like this would make sane to me though:
>>
>> static inline negative_errno(void) {
>>        return _likely_(errno > 0) ? -errno : -EINVAL;
>> }
>>
>> Which is then invoked as:
>>
>>       return negative_errno();
>>
>> or so...
> I like the assert more, because having errno <= 0 would be a signficant bug
> in the system, not something that we want to ever hide. So basically this
> is a way to tell the compiler what we already know, and assert is better.

Indeed. So how about:

static inline int negative_errno(void) {
        assert_return(errno > 0, -EINVAL);
        return -errno;
}

Thanks
David

> I also filed https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61846, but
> without much response so far.
>
> Zbyszek