Subject: Re: [PATCH 02/14] combine-diff: add combine_diff_path_new()

From: Patrick Steinhardt <[email-masked]>
Date: Mon, 13 Jan 2025 16:40:19 +0100

On Thu, Jan 09, 2025 at 03:32:36AM -0500, Jeff King wrote:
> The combine_diff_path struct has variable size, since it embeds both the
> memory allocation for the path field as well as a variable-sized parent
> array. This makes allocating one a bit tricky.
> 
> We have a helper to compute the required size, but it's up to individual
> sites to actually initialize all of the fields. Let's provide a
> constructor function to make that a little nicer. Besides being shorter,
> it also hides away tricky bits like the computation of the "path"
> pointer (which is right after the "parent" flex array).
> 
> As a bonus, using the same constructor everywhere means that we'll
> consistently initialize all parts of the struct. A few code paths left
> the parent array unitialized. This didn't cause any bugs, but we'll be
> able to simplify some code in the next few patches knowing that the
> parent fields have all been zero'd.
> 
> This also gets rid of some questionable uses of "int" to store buffer
> lengths. Though we do use them to allocate, I don't think there are any
> integer overflow vulnerabilities here (the allocation helper promotes
> them to size_t and checks arithmetic for overflow, and the actual memcpy
> of the bytes is done using the possibly-truncated "int" value).
> 
> Sadly we can't use the FLEX_* macros to simplify the allocation here,
> because there are two variable-sized parts to the struct (and those
> macros only handle one).
> 
> Nor can we get stop publicly declaring combine_diff_path_size(). This

s/we get stop/we stop/

> diff --git a/combine-diff.c b/combine-diff.c
> index 641bc92dbd..45548fd438 100644
> --- a/combine-diff.c
> +++ b/combine-diff.c
> @@ -47,22 +47,13 @@ static struct combine_diff_path *intersect_paths(
>  
>  	if (!n) {
>  		for (i = 0; i < q->nr; i++) {
> -			int len;
> -			const char *path;
>  			if (diff_unmodified_pair(q->queue[i]))
>  				continue;
> -			path = q->queue[i]->two->path;
> -			len = strlen(path);
> -			p = xmalloc(combine_diff_path_size(num_parent, len));
> -			p->path = (char *) &(p->parent[num_parent]);
> -			memcpy(p->path, path, len);
> -			p->path[len] = 0;
> -			p->next = NULL;
> -			memset(p->parent, 0,
> -			       sizeof(p->parent[0]) * num_parent);
> -
> -			oidcpy(&p->oid, &q->queue[i]->two->oid);
> -			p->mode = q->queue[i]->two->mode;
> +			p = combine_diff_path_new(q->queue[i]->two->path,
> +						  strlen(q->queue[i]->two->path),
> +						  q->queue[i]->two->mode,
> +						  &q->queue[i]->two->oid,
> +						  num_parent);
>  			oidcpy(&p->parent[n].oid, &q->queue[i]->one->oid);
>  			p->parent[n].mode = q->queue[i]->one->mode;
>  			p->parent[n].status = q->queue[i]->status;
> @@ -1667,3 +1658,24 @@ void diff_tree_combined_merge(const struct commit *commit,
>  	diff_tree_combined(&commit->object.oid, &parents, rev);
>  	oid_array_clear(&parents);
>  }
> +
> +struct combine_diff_path *combine_diff_path_new(const char *path,
> +						size_t path_len,
> +						unsigned int mode,
> +						const struct object_id *oid,
> +						size_t num_parents)
> +{
> +	struct combine_diff_path *p;
> +
> +	p = xmalloc(combine_diff_path_size(num_parents, path_len));
> +	p->path = (char *)&(p->parent[num_parents]);
> +	memcpy(p->path, path, path_len);
> +	p->path[path_len] = 0;
> +	p->next = NULL;
> +	p->mode = mode;
> +	oidcpy(&p->oid, oid);
> +
> +	memset(p->parent, 0, sizeof(p->parent[0]) * num_parents);
> +
> +	return p;
> +}

If I were to write this anew I'd probably use `xcalloc()` instead of
manually `memset()`ing parts of it to zero. But it's a faithful
transplant of the code from `intersect_paths()`, so that's probably
okay.

Patrick