Re: [PATCH v5 3/7] submodule: always validate gitdirs inside submodule_name_to_gitdir

[Adrian Ratiu <adrian.ratiu@collabora.com>]

On Fri, 05 Dec 2025, Patrick Steinhardt <ps@pks.im> wrote:
> On Wed, Nov 19, 2025 at 11:10:26PM +0200, Adrian Ratiu wrote:
>> diff --git a/builtin/submodule--helper.c b/builtin/submodule--helper.c
>> index 2873b2780e..9914ca0786 100644
>> --- a/builtin/submodule--helper.c
>> +++ b/builtin/submodule--helper.c
>> @@ -1780,23 +1776,6 @@ static int clone_submodule(const struct module_clone_data *clone_data,
>>  		free(path);
>>  	}
>>  
>> -	/*
>> -	 * We already performed this check at the beginning of this function,
>> -	 * before cloning the objects. This tries to detect racy behavior e.g.
>> -	 * in parallel clones, where another process could easily have made the
>> -	 * gitdir nested _after_ it was created.
>> -	 *
>> -	 * To prevent further harm coming from this unintentionally-nested
>> -	 * gitdir, let's disable it by deleting the `HEAD` file.
>> -	 */
>> -	if (validate_submodule_git_dir(sm_gitdir, clone_data->name) < 0) {
>> -		char *head = xstrfmt("%s/HEAD", sm_gitdir);
>> -		unlink(head);
>> -		free(head);
>> -		die(_("refusing to create/use '%s' in another submodule's "
>> -		      "git dir"), sm_gitdir);
>> -	}
>> -
>>  	connect_work_tree_and_git_dir(clone_data_path, sm_gitdir, 0);
>>  
>>  	p = repo_submodule_path(the_repository, clone_data_path, "config");
>
> Hm. This one is a bit puzzling to me. This seems to explicitly be a
> check about a TOCTOU-style race, where a concurrent process might have
> created the parent repository after our initial validation of the path.
> We don't call `submodule_name_to_gitdir()` inbetween those two calls
> though, so why is this not a concern anymore with the unified API?

Excellent catch.

It still is a concern and this specific check needs to be added back.

My aim with this patch is to:
1. Avoid redundant validation calls for submodule_name_to_gitdir().
2. Avoid the risk of callers forgetting to validate.
3. Ensure gitdir paths provided by users via configs are always valid,
so I added the validation directly into submodule_name_to_gitdir().

Nothing prevents us from running the validation as many times as we
need and, in this case, it's perfectly justified to run it twice.

I'll add it back in v6.

>> diff --git a/submodule.c b/submodule.c
>> index 35c55155f7..8ef028f26b 100644
>> --- a/submodule.c
>> +++ b/submodule.c
>> @@ -2153,30 +2153,11 @@ int submodule_move_head(const char *path, const char *super_prefix,
>>  
>>  	if (!(flags & SUBMODULE_MOVE_HEAD_DRY_RUN)) {
>>  		if (old_head) {
>> -			if (!submodule_uses_gitfile(path))
>> -				absorb_git_dir_into_superproject(path,
>> -								 super_prefix);
>> -			else {
>> -				char *dotgit = xstrfmt("%s/.git", path);
>> -				char *git_dir = xstrdup(read_gitfile(dotgit));
>> -
>> -				free(dotgit);
>> -				if (validate_submodule_git_dir(git_dir,
>> -							       sub->name) < 0)
>> -					die(_("refusing to create/use '%s' in "
>> -					      "another submodule's git dir"),
>> -					    git_dir);
>> -				free(git_dir);
>> -			}
>> +			absorb_git_dir_into_superproject(path, super_prefix);
>>  		} else {
>>  			struct strbuf gitdir = STRBUF_INIT;
>>  			submodule_name_to_gitdir(&gitdir, the_repository,
>>  						 sub->name);
>> -			if (validate_submodule_git_dir(gitdir.buf,
>> -						       sub->name) < 0)
>> -				die(_("refusing to create/use '%s' in another "
>> -				      "submodule's git dir"),
>> -				    gitdir.buf);
>>  			connect_work_tree_and_git_dir(path, gitdir.buf, 0);
>>  			strbuf_release(&gitdir);
>
> The second case here makes sense to me, as we do call
> `submodule_name_to_gitdir()`. But in the first branch of the condition
> we retrieve the path directly, so we're not guarded by the validation
> anymore, are we?

The !submodule_uses_gitfile(path) case never validated and relied on the
validation done by absorb_git_dir_into_superproject() which calls both
validate_submodule_path() and submodule_name_to_gitdir() and has
additional checks.

So in essence the validation moved to absorb_git_dir_into_superproject().

If you prefer I can try to refactor absorb_git_dir_into_superproject() a
bit to make it clearer, otherwise I can just add back the validation
here as well... we can validate as many times as we like. :)