Subject: Re: [PATCH] reflog-walk: don't segfault on non-commit sha1's in the reflog

From: Dennis Kaarsemaker <[email-masked]>
Date: Wed, 30 Dec 2015 22:33:28 +0100

On wo, 2015-12-30 at 13:20 -0800, Junio C Hamano wrote:
> Dennis Kaarsemaker <[email-masked]> writes:
> 
> > diff --git a/reflog-walk.c b/reflog-walk.c
> > index 85b8a54..b85c8e8 100644
> > --- a/reflog-walk.c
> > +++ b/reflog-walk.c
> > @@ -236,8 +236,8 @@ void fake_reflog_parent(struct reflog_walk_info
> > *info, struct commit *commit)
> >  	reflog = &commit_reflog->reflogs->items[commit_reflog
> > ->recno];
> >  	info->last_commit_reflog = commit_reflog;
> >  	commit_reflog->recno--;
> > -	commit_info->commit = (struct commit *)parse_object(reflog
> > ->osha1);
> > -	if (!commit_info->commit) {
> > +	commit_info->commit = lookup_commit(reflog->osha1);
> > +	if (!commit_info->commit || parse_commit(commit_info
> > ->commit)) {
> >  		commit->parents = NULL;
> >  		return;
> 
> This looks somewhat roundabout and illogical.  The original was bad
> because it blindly assumed reflgo->osha1 refers to a commit without
> making sure that assumption holds.  Calling lookup_commit() blindly
> is not much better, even though you are helped that the function
> happens not to barf if the given object is not a commit.
> 
> Also this changes semantics, no?  Trace the original flow and think
> what happens, when we see a commit object that cannot be parsed in
> parse_commit_buffer().  parse_object() calls parse_object_buffer()
> which in turn calls parse_commit_buffer() and the entire callchain
> returns NULL.  commit_info->commit will become NULL in such a case.
> 
> With your code, lookup_commit() will store a non NULL in
> commit_info->commit, and parse_commit() calls parse_commit_buffer()
> and that would fail, so you clear commit->parents to NULL but fail
> to set commit_info->commit to NULL.
>
> Why not keep the parse_object() as-is and make sure we error out
> unless the result is a commit with a more explicit check, perhaps
> like this, instead?

lookup_commit actually returns NULL (via object_as_type) for objects
that are not commits, so I don't think the above is true. The code
below also loses the diagnostic message about the object not being a
commit.

>  reflog-walk.c | 7 +++++--
>  1 file changed, 5 insertions(+), 2 deletions(-)
> 
> diff --git a/reflog-walk.c b/reflog-walk.c
> index 85b8a54..861d7c4 100644
> --- a/reflog-walk.c
> +++ b/reflog-walk.c
> @@ -221,6 +221,7 @@ void fake_reflog_parent(struct reflog_walk_info
> *info, struct commit *commit)
>  	struct commit_info *commit_info =
>  		get_commit_info(commit, &info->reflogs, 0);
>  	struct commit_reflog *commit_reflog;
> +	struct object *logobj;
>  	struct reflog_info *reflog;
>  
>  	info->last_commit_reflog = NULL;
> @@ -236,11 +237,13 @@ void fake_reflog_parent(struct reflog_walk_info
> *info, struct commit *commit)
>  	reflog = &commit_reflog->reflogs->items[commit_reflog
> ->recno];
>  	info->last_commit_reflog = commit_reflog;
>  	commit_reflog->recno--;
> -	commit_info->commit = (struct commit *)parse_object(reflog
> ->osha1);
> -	if (!commit_info->commit) {
> +	logobj = parse_object(reflog->osha1);
> +	if (!logobj || logobj->type != OBJ_COMMIT) {
> +		commit_info->commit = NULL;
>  		commit->parents = NULL;
>  		return;
>  	}
> +	commit_info->commit = (struct commit *)logobj;
>  
>  	commit->parents = xcalloc(1, sizeof(struct commit_list));
>  	commit->parents->item = commit_info->commit;
> 
> 
> > +test_expect_success 'reflog containing non-commit sha1s' '
> > +	git checkout -b broken-reflog &&
> > +	echo "$(git rev-parse HEAD^{tree}) $(git rev-parse HEAD)
> > abc <xyz> 0000000001 +0000" >> .git/logs/refs/heads/broken-reflog
> > &&
> > +	git reflog broken-reflog
> > +'
> > +
> 
> This will negatively affect the ongoing effort to abstract out the
> on-disk implementation of the reflog.  In some future installation
> of Git, the reflog may not even be in .git/logs/refs/whatever file.

I was following the style of the test above it, will fix.

> Use a non-branch ref, so that you can store any valid object not
> just commits, and use a Git command (e.g. "git update-ref" or "git
> tag") instead of the raw filesystem access to update it, perhaps
> like this?
> 
> 	git tag --create-reflog test-logs HEAD^ &&
> 	git tag -f test-logs HEAD^{tree} &&
> 	git tag -f test-logs HEAD &&
> 	git reflog test-logs

-- 
Dennis Kaarsemaker
www.kaarsemaker.net