Re: [PATCH v2 03/10] hashmap: allow re-use after hashmap_free()

From: Elijah Newren <newren@gmail.com>
To: Jeff King <peff@peff.net>
Cc: Elijah Newren via GitGitGadget <gitgitgadget@gmail.com>,
	Git Mailing List <git@vger.kernel.org>
Subject: Re: [PATCH v2 03/10] hashmap: allow re-use after hashmap_free()
Date: Fri, 30 Oct 2020 08:37:42 -0700	[thread overview]
Message-ID: <CABPp-BHRSottxaYO+Ot5tPpNCf4scxmtwBLZicMJ4ChNEhmGHA@mail.gmail.com> (raw)
In-Reply-To: <20201030133518.GC3277724@coredump.intra.peff.net>

On Fri, Oct 30, 2020 at 6:35 AM Jeff King <peff@peff.net> wrote:
>
> On Tue, Oct 13, 2020 at 12:40:43AM +0000, Elijah Newren via GitGitGadget wrote:
>
> > Previously, once map->table had been freed, any calls to hashmap_put(),
> > hashmap_get(), or hashmap_remove() would cause a NULL pointer
> > dereference (since hashmap_free_() also zeros the memory; without that
> > zeroing, calling these functions would cause a use-after-free problem).
> >
> > Modify these functions to check for a NULL table and automatically
> > allocate as needed.
>
> Unsurprisingly, I like this direction. The code looks correct to me,
> though I think you could reduce duplication slightly by checking
> map->table in find_entry_ptr(). That covers both hashmap_get() and
> hashmap_remove(). But I'm happy either way.
>
> > I also thought about creating a HASHMAP_INIT macro to allow initializing
> > hashmaps on the stack without calling hashmap_init(), but virtually all
> > uses of hashmap specify a usecase-specific equals_function which defeats
> > the utility of such a macro.
>
> This part I disagree with. If we did:
>
>   #define HASHMAP_INIT(fn, data) = { .cmpfn = cmpfn, cmpfn_data = data }
>
> then many callers could avoid handling the lazy-init themselves. E.g.:

Ah, gotcha.  That makes sense to me.  Given that 43 out of 47 callers
of hashmap_init use cmpfn_data = NULL, should I shorten it to just one
parameter for the macro, and let the four special cases keep calling
hashmap_init() to specify a non-NULL cmpfn_data?

>
>  attr.c | 16 +++-------------
>  1 file changed, 3 insertions(+), 13 deletions(-)
>
> diff --git a/attr.c b/attr.c
> index a826b2ef1f..55a2783f1b 100644
> --- a/attr.c
> +++ b/attr.c
> @@ -57,7 +57,9 @@ static inline void hashmap_unlock(struct attr_hashmap *map)
>   * is a singleton object which is shared between threads.
>   * Access to this dictionary must be surrounded with a mutex.
>   */
> -static struct attr_hashmap g_attr_hashmap;
> +static struct attr_hashmap g_attr_hashmap = {
> +       HASHMAP_INIT(attr_hash_entry_cmp, NULL)
> +};
>
>  /* The container for objects stored in "struct attr_hashmap" */
>  struct attr_hash_entry {
> @@ -80,12 +82,6 @@ static int attr_hash_entry_cmp(const void *unused_cmp_data,
>         return (a->keylen != b->keylen) || strncmp(a->key, b->key, a->keylen);
>  }
>
> -/* Initialize an 'attr_hashmap' object */
> -static void attr_hashmap_init(struct attr_hashmap *map)
> -{
> -       hashmap_init(&map->map, attr_hash_entry_cmp, NULL, 0);
> -}
> -
>  /*
>   * Retrieve the 'value' stored in a hashmap given the provided 'key'.
>   * If there is no matching entry, return NULL.
> @@ -96,9 +92,6 @@ static void *attr_hashmap_get(struct attr_hashmap *map,
>         struct attr_hash_entry k;
>         struct attr_hash_entry *e;
>
> -       if (!map->map.tablesize)
> -               attr_hashmap_init(map);
> -
>         hashmap_entry_init(&k.ent, memhash(key, keylen));
>         k.key = key;
>         k.keylen = keylen;
> @@ -114,9 +107,6 @@ static void attr_hashmap_add(struct attr_hashmap *map,
>  {
>         struct attr_hash_entry *e;
>
> -       if (!map->map.tablesize)
> -               attr_hashmap_init(map);
> -
>         e = xmalloc(sizeof(struct attr_hash_entry));
>         hashmap_entry_init(&e->ent, memhash(key, keylen));
>         e->key = key;