[PATCH 4/3] list-objects-filter: use empty string instead of NULL for sparse "base"

[Jeff King <peff@peff.net>]

On Sat, Sep 14, 2019 at 09:09:42PM -0400, Jeff King wrote:

> On Mon, Sep 09, 2019 at 01:08:24PM -0400, Jeff King wrote:
> 
> > I'll work up what I sent earlier into a real patch, and include some of
> > this discussion.
> 
> Here it is. I pulled Jon's tests out into their own patch (mostly
> because it makes it easier to give credit). Then patch 2 is my fix, and
> patch 3 is the message fixups he had done.
> 
> This replaces what's queued in js/partial-clone-sparse-blob.
> 
>   [1/3]: t5616: test cloning/fetching with sparse:oid=<oid> filter
>   [2/3]: list-objects-filter: delay parsing of sparse oid
>   [3/3]: list-objects-filter: give a more specific error sparse parsing error

And here's a bonus patch that I found while running under ASan/UBSan
(since I wanted to double-check the memory handling of patch 2 when
merged with 'next').

-- >8 --
Subject: list-objects-filter: use empty string instead of NULL for sparse "base"

We use add_excludes_from_blob_to_list() to parse a sparse blob. Since
we don't have a base path, we pass NULL and 0 for the base and baselen,
respectively. But the rest of the exclude code passes a literal empty
string instead of NULL for this case. And indeed, we eventually end up
with match_pathname() calling fspathncmp(), which then calls the system
strncmp(path, base, baselen).

This works on many platforms, which notice that baselen is 0 and do not
look at the bytes of "base" at all. But it does violate the C standard,
and building with SANITIZE=undefined will complain. You can also see it
by instrumenting fspathncmp like this:

	diff --git a/dir.c b/dir.c
	index d021c908e5..4bb3d3ec96 100644
	--- a/dir.c
	+++ b/dir.c
	@@ -71,6 +71,8 @@ int fspathcmp(const char *a, const char *b)

	 int fspathncmp(const char *a, const char *b, size_t count)
	 {
	+	if (!a || !b)
	+		BUG("null fspathncmp arguments");
	 	return ignore_case ? strncasecmp(a, b, count) : strncmp(a, b, count);
	 }

We could perhaps be more defensive in match_pathname(), but even if we
did so, it makes sense for this code to match the rest of the exclude
callers.

Signed-off-by: Jeff King <peff@peff.net>
---
 list-objects-filter.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/list-objects-filter.c b/list-objects-filter.c
index 50f0c6d07b..83c788e8b5 100644
--- a/list-objects-filter.c
+++ b/list-objects-filter.c
@@ -472,7 +472,7 @@ static void *filter_sparse_oid__init(
 		die(_("unable to access sparse blob in '%s'"),
 		    filter_options->sparse_oid_name);
 	d->omits = omitted;
-	if (add_excludes_from_blob_to_list(&sparse_oid, NULL, 0, &d->el) < 0)
+	if (add_excludes_from_blob_to_list(&sparse_oid, "", 0, &d->el) < 0)
 		die(_("unable to parse sparse filter data in %s"),
 		    oid_to_hex(&sparse_oid));
 
-- 
2.23.0.667.gcccf1fbb03