Re: [PATCH v3 1/3] qemu-keymap: Free xkb allocations

[Peter Maydell]

On Wed, 22 May 2024 at 12:47, Daniel P. Berrangé <berrange@redhat.com> wrote:
>
> On Wed, May 22, 2024 at 12:35:23PM +0100, Peter Maydell wrote:
> > On Wed, 22 May 2024 at 11:49, Akihiko Odaki <akihiko.odaki@daynix.com> 
> > wrote:
> > >
> > > This fixes LeakSanitizer complaints with xkbcommon 1.6.0.
> > >
> > > Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com>
> > > ---
> > >  qemu-keymap.c | 3 +++
> > >  1 file changed, 3 insertions(+)
> > >
> > > diff --git a/qemu-keymap.c b/qemu-keymap.c
> > > index 8c80f7a4ed65..7a9f38cf9863 100644
> > > --- a/qemu-keymap.c
> > > +++ b/qemu-keymap.c
> > > @@ -237,6 +237,9 @@ int main(int argc, char *argv[])
> > >      xkb_state_unref(state);
> > >      state = NULL;
> > >
> > > +    xkb_keymap_unref(map);
> > > +    xkb_context_unref(ctx);
> > > +
> > >      /* add quirks */
> > >      fprintf(outfile,
> > >              "\n"
> >
> > This is surely a sanitizer bug. We're unconditionally about
> > to exit() the program here, where everything is freed, so nothing
> > is leaked.
>
> I'm not sure I'd call it a sanitizer bug, rather its expected behaviour
> of sanitizers. Even if you're about to exit, its important to see info
> about all memory that is not freed by that time, since it can reveal
> leaks that were ongoing in the process that are valid things to fix.
> To make the sanitizers usable you need to get rid of the noise. IOW,
> either have to provide a file to supress reports of memory that is
> expected to remain allocated, or have to free it despite being about
> to exit.  Free'ing is the more maintainable strategy, as IME, supression
> files get outdated over time.

I think if there's still a live variable pointing to the unfreed
memory at point of exit the compiler/sanitizer should be able to
deduce that that's not a real leak. And if you believe that these
really are leaks then you also need to be fixing them on the early
exit paths, like the one where we exit(1) if xkb_keymap_new_from_names()
fails.

I don't object to this change, but I think that if the sanitizer
complains about this kind of thing it's a bug, because it obscures
real leaks.

-- PMM