Re: [PATCH] commit-reach: avoid NULL dereference

Eric Wong <e@80x24.org> writes:

Good comment.

The flag clearing rule of this function smells somewhat iffy. There are three primary callers of the function:

 * commit-reach.c::can_all_from_reach() calls the function, but it
   has its own loop to clear the flag it asked the function to add.
   If the function uses the flag as a temporary mark and is designed
   to clear it from all the objects, as 4067a646 (commit-reach: fix
   memory and flag leaks, 2018-09-21) states, why should the caller
   have a separate loop to clear them?

 * fetch-pack.c::negotiate_using_fetch() calls this function in a
   loop, so it does depend on it to clear the flag upon returning.

 * upload-pack.c::ok_to_give_up() is a thin wrapper around this
   function and none callers of it have any logic to clear flag, so
   it clearly depends on the function to clear the flag.

The above seems to indicate that the expectation by callers is a bit uneven. Shouldn't the first onetrust the callee to clear the flag?

Even before 4067a646 (commit-reach: fix memory and flag leaks, 2018-09-21), the function had a call to clear_commit_marks() to clear two bits it used temporarily. The reason why 4067a646 needed to add this additional flag clearing, whose NULL-dereference bug is being fixed with the patch in this thread, is because it marks any incoming object that peels to a non-commit (e.g. a blob, a tree, or a tag that points at a non-commit) with the flag bit, but such a non-commit object is not added to the list[] of commits to be processed, before the main processing of this function.

		from_one = deref_tag(the_repository, from_one,
				     "a from object", 0);
		if (!from_one || from_one->type != OBJ_COMMIT) {
			/*
			 * no way to tell if this is reachable by
			 * looking at the ancestry chain alone, so
			 * leave a note to ourselves not to worry about
			 * this object anymore.
			 */
			from->objects[i].item->flags |= assign_flag;
			continue;
		}

		list[nr_commits] = (struct commit *)from_one;

But I am not sure if it is even necessary to smudge the flag for the object that was a non-commit (or the tag that peeled down to a non-commit). The main process of this function is a history traversal that stops when the "assign_flag" bit is already set on the found object, but the object that was part of the incoming objects (i.e. in from->objects[] array) that turned out not to be a non-commit would not be discovered during this history walk, would it? In other words, if we walk from list[] that is an array or commits to the parents (but not its trees and blobs), we won't encounter anything but commit. What does it help to smudge an object that peeled down to a non-commit in the incoming set of objects, if it would not appear in the walk from list[]? It would not stop the traversal by having the flag.

So I wonder if we can just stop smudging the assign_flag bit for these objects in from->objects[] that do not make it into list[] as a simpler fix? Wouldn't that make the follow-up cleaning loop added by 4067a646 (commit-reach: fix memory and flag leaks, 2018-09-21) unneeded?