[PATCH 2/4] avoid computing zero offsets from NULL pointer

[Jeff King <peff@xxxxxxxx>]

The Undefined Behavior Sanitizer in clang-11 seems to have learned a new
trick: it complains about computing offsets from a NULL pointer, even if
that offset is 0. This causes numerous test failures. For example, from
t1090:

  unpack-trees.c:1355:41: runtime error: applying zero offset to null pointer
  ...
  not ok 6 - in partial clone, sparse checkout only fetches needed blobs

The code in question looks like this:

  struct cache_entry **cache_end = cache + nr;
  ...
  while (cache != cache_end)

and we sometimes pass in a NULL and 0 for "cache" and "nr". This is
conceptually fine, as "cache_end" would be equal to "cache" in this
case, and we wouldn't enter the loop at all. But computing even a zero
offset violates the C standard. And given the fact that UBSan is
noticing this behavior, this might be a potential problem spot if the
compiler starts making unexpected assumptions based on undefined
behavior.

So let's just avoid it, which is pretty easy. In some cases we can just
switch to iterating with a numeric index (as we do in sequencer.c here).
In other cases (like the cache_end one) the use of an end pointer is
more natural; we can keep that by just explicitly checking for NULL when
assigning the end pointer.

Signed-off-by: Jeff King <peff@xxxxxxxx>
---
 sequencer.c       | 6 +++---
 unpack-trees.c    | 2 +-
 xdiff-interface.c | 4 ++--
 3 files changed, 6 insertions(+), 6 deletions(-)

diff --git a/sequencer.c b/sequencer.c
index b9dbf1adb0..4d31ec3296 100644
--- a/sequencer.c
+++ b/sequencer.c
@@ -588,7 +588,7 @@ static int do_recursive_merge(struct repository *r,
 	struct merge_options o;
 	struct tree *next_tree, *base_tree, *head_tree;
 	int clean;
-	char **xopt;
+	int i;
 	struct lock_file index_lock = LOCK_INIT;
 
 	if (repo_hold_locked_index(r, &index_lock, LOCK_REPORT_ON_ERROR) < 0)
@@ -608,8 +608,8 @@ static int do_recursive_merge(struct repository *r,
 	next_tree = next ? get_commit_tree(next) : empty_tree(r);
 	base_tree = base ? get_commit_tree(base) : empty_tree(r);
 
-	for (xopt = opts->xopts; xopt != opts->xopts + opts->xopts_nr; xopt++)
-		parse_merge_opt(&o, *xopt);
+	for (i = 0; i < opts->xopts_nr; i++)
+		parse_merge_opt(&o, opts->xopts[i]);
 
 	clean = merge_trees(&o,
 			    head_tree,
diff --git a/unpack-trees.c b/unpack-trees.c
index d5f4d997da..b4292d2be8 100644
--- a/unpack-trees.c
+++ b/unpack-trees.c
@@ -1352,7 +1352,7 @@ static int clear_ce_flags_1(struct index_state *istate,
 			    enum pattern_match_result default_match,
 			    int progress_nr)
 {
-	struct cache_entry **cache_end = cache + nr;
+	struct cache_entry **cache_end = cache ? cache + nr : cache;
 
 	/*
 	 * Process all entries that have the given prefix and meet
diff --git a/xdiff-interface.c b/xdiff-interface.c
index 8509f9ea22..2f1fe48512 100644
--- a/xdiff-interface.c
+++ b/xdiff-interface.c
@@ -84,8 +84,8 @@ static void trim_common_tail(mmfile_t *a, mmfile_t *b)
 {
 	const int blk = 1024;
 	long trimmed = 0, recovered = 0;
-	char *ap = a->ptr + a->size;
-	char *bp = b->ptr + b->size;
+	char *ap = a->ptr ? a->ptr + a->size : a->ptr;
+	char *bp = b->ptr ? b->ptr + b->size : b->ptr;
 	long smaller = (a->size < b->size) ? a->size : b->size;
 
 	while (blk + trimmed <= smaller && !memcmp(ap - blk, bp - blk, blk)) {
-- 
2.25.0.421.gb74d19af79