Subject: [PATCH] hashmap: ensure hashmaps are reusable after hashmap_clear()

From: "Elijah Newren via GitGitGadget" <[email-masked]>
Date: Tue, 29 Apr 2025 15:47:43 +0000

From: Elijah Newren <[email-masked]>

In the series merged at bf0a430f70b5 (Merge branch 'en/strmap',
2020-11-21), strmap was built on top of hashmap and hashmap was extended
in a few ways to support strmap and be more generally useful.  One of
the extensions was that hashmap_partial_clear() was introduced to allow
reuse of the hashmap without freeing the table.  Peff believed that it
also made sense to introduce a hashmap_clear() which freed everything
while allowing reuse.

I added hashmap_clear(), but in doing so, overlooked the fact that for
a hashmap to be reusable, it needs a defined cmpfn and data (the
HASHMAP_INIT macro requires these fields as parameters, for example).
So, if we want the hashmap to be reusable, we shouldn't zero out those
fields.  We probably also shouldn't zero out do_count_items.  (We could
zero out grow_at and shrink_at, but whether we zero those or not is
irrelevant as they'll be automatically updated whenever a new entry is
inserted.)

Since clearing is associated with freeing map->table, and the only thing
required for consistency after freeing map->table is zeroing tablesize
and private_size, let's only zero those fields out.

Signed-off-by: Elijah Newren <[email-masked]>
---
    hashmap: ensure hashmaps are reusable after hashmap_clear()
    
    Ran into a NULL pointer dereference of cmpfn a few months ago when
    trying to reuse one of {strmap, strset, strintmap} (don't remember which
    one) after calling the relevant ${TYPE}_clear() variant, and tracked the
    NULL pointer back to hashmap_clear(). Turned out to not be relevant to
    those patches because I ended up not needing to reuse the map after all,
    but I kept a note to myself to send in a fix.
    
    I was surprised this wasn't a bug we were already hitting somewhere, but
    I looked through the codebase and it appears that the only time we
    attempt to reuse a hashmap after clearing is when we specifically use
    hashmap_partial_clear(). So, this is just a latent bug waiting as a trap
    for someone.

Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-1911%2Fnewren%2Ffix-hashmap-clear-v1
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-1911/newren/fix-hashmap-clear-v1
Pull-Request: https://github.com/gitgitgadget/git/pull/1911

 hashmap.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/hashmap.c b/hashmap.c
index ee45ef00852..a711377853f 100644
--- a/hashmap.c
+++ b/hashmap.c
@@ -205,8 +205,9 @@ void hashmap_clear_(struct hashmap *map, ssize_t entry_offset)
 		return;
 	if (entry_offset >= 0)  /* called by hashmap_clear_and_free */
 		free_individual_entries(map, entry_offset);
-	free(map->table);
-	memset(map, 0, sizeof(*map));
+	FREE_AND_NULL(map->table);
+	map->tablesize = 0;
+	map->private_size = 0;
 }
 
 struct hashmap_entry *hashmap_get(const struct hashmap *map,

base-commit: f65182a99e545d2f2bc22e6c1c2da192133b16a3
-- 
gitgitgadget