Re: g_autoptr(Error)

[Peter Xu <peterx@redhat.com>]

On Tue, Nov 25, 2025 at 12:46:01PM +0100, Markus Armbruster wrote:
> Daniel P. Berrangé <berrange@redhat.com> writes:
> 
> > On Tue, Nov 25, 2025 at 08:40:07AM +0100, Markus Armbruster wrote:
> >> g_autoptr(T) is quite useful when the object's extent matches the
> >> function's.
> >> 
> >> This isn't the case for an Error object the function propagates to its
> >> caller.  It is the case for an Error object the function reports or
> >> handles itself.  However, the functions to report Error also free it.

I'd confess I didn't pay enough attention on how the error API was designed
deliberately to always free the Error objects before almost whenever
possible.  But I see now, thanks for the write up.

> >> 
> >> Thus, g_autoptr(Error) is rarely applicable.  We have just three
> >> instances out of >1100 local Error variables, all in migration code.
> >> 
> >> Two want to move the error to the MigrationState for later handling /
> >> reporting.  Since migrate_set_error() doesn't move, but stores a copy,
> >> the original needs to be freed, and g_autoptr() is correct there.  We
> >> have 17 more that instead manually free with error_free() or
> >> error_report_err() right after migrate_set_error().
> >> 
> >> We recently discussed storing a copy vs. move the original:
> >> 
> >>     From: Peter Xu <peterx@redhat.com>
> >>     Subject: Re: [PATCH 0/3] migration: Error fixes and improvements
> >>     Date: Mon, 17 Nov 2025 11:03:37 -0500
> >>     Message-ID: <aRtHWbWcTh3OF2wY@x1.local>
> >> 
> >> The two g_autoptr() gave me pause when I investigated this topic, simply
> >> because they deviate from the common pattern migrate_set_error(s, err)
> >> followed by error_free() or error_report_err().
> >> 
> >> The third one became wrong when I cleaned up the reporting (missed in
> >> the cleanup patch, fixed in the patch I'm replying to).  I suspect my
> >> mistake escaped review for the same reason I made it: g_autoptr(Error)
> >> is unusual and not visible in the patch hunk.
> >> 
> >> Would you like me to replace the two correct uses of g_autoptr(Error) by
> >> more common usage?

Works for me.

Now I also think it should be good migrate_set_error() follow QEMU's Error
API design if we decide to stick with it freeing errors in such APIs.

Said that, I wonder if you think we could still consider passing Error**
into migrate_set_error(), though, which will be a merged solution of
current Error API and what Marc-Andre proposed on resetting pointers to
avoid any possible UAF, which I would still slightly prefer personally.

If we rework migrate_set_error() to take ownership first, then we can
naturally drop the two use cases, and remove the cleanup function.

Markus, please also let me know if you want me to do it.

> >
> > I had previously proposed g_autoptr(Error) a year or two back and you
> > rejected it then, so I'm surprised to see that it got into the code,
> > because it requires explicit opt-in via a G_DEFINE_AUTOPTR_CLEANUP_FUNC.
> >
> > Unfortunately it appears exactly that was added earlier this year in
> >
> >   commit 18eb55546a54e443d94a4c49286348176ad4b00a
> >   Author: Maciej S. Szmigiero <maciej.szmigiero@oracle.com>
> >   Date:   Tue Mar 4 23:03:35 2025 +0100
> >
> >     error: define g_autoptr() cleanup function for the Error type
> >     
> >     Automatic memory management helps avoid memory safety issues.
> >     
> >     Reviewed-by: Peter Xu <peterx@redhat.com>
> >     Signed-off-by: Maciej S. Szmigiero <maciej.szmigiero@oracle.com>
> >     Link: https://lore.kernel.org/qemu-devel/a5843c5fa64d7e5239a4316092ec0ef0d10c2320.1741124640.git.maciej.szmigiero@oracle.com
> >     Signed-off-by: Cédric Le Goater <clg@redhat.com>
> 
> I missed it.  Not he submitter's fault; it was cc'ed to me.

If someone to blame, it's the reviewer.

Thanks!

-- 
Peter Xu