[v4,3/5] config: read protected config with `git_protected_config()`

Commit Message

Glen Choo June 7, 2022, 8:57 p.m. UTC

From: Glen Choo <[e-mail]>

`uploadpack.packObjectsHook` is the only 'protected config only'
variable today, but we've noted that `safe.directory` and the upcoming
`discovery.bare` should also be 'protected config only'. So, for
consistency, we'd like to have a single implementation for protected
config.

The primary constraints are:

1. Reading from protected config should be as fast as possible. Nearly
   all "git" commands inside a bare repository will read both
   `safe.directory` and `discovery.bare`, so we cannot afford to be
   slow.

2. Protected config must be readable when the gitdir is not known.
   `safe.directory` and `discovery.bare` both affect repository
   discovery and the gitdir is not known at that point [1].

The chosen implementation in this commit is to read protected config and
cache the values in a global configset. This is similar to the caching
behavior we get with the_repository->config.

Introduce git_protected_config(), which reads protected config and
caches them in the global configset protected_config. Then, refactor
`uploadpack.packObjectsHook` to use git_protected_config().

The protected config functions are named similarly to their
non-protected counterparts, e.g. git_protected_config_check_init() vs
git_config_check_init().

In light of constraint 1, this implementation can still be improved
since git_protected_config() iterates through every variable in
protected_config, which may still be too expensive. There exist constant
time lookup functions for non-protected config (repo_config_get_*()),
but for simplicity, this commit does not implement similar functions for
protected config.

An alternative that avoids introducing another configset is to continue
to read all config using git_config(), but only accept values that have
the correct config scope [2]. This technically fulfills constraint 2,
because git_config() simply ignores the local and worktree config when
the gitdir is not known. However, this would read incomplete config into
the_repository->config, which would need to be reset when the gitdir is
known and git_config() needs to read the local and worktree config.
Resetting the_repository->config might be reasonable while we only have
these 'protected config only' variables, but it's not clear whether this
extends well to future variables.

[1] In this case, we do have a candidate gitdir though, so with a little
refactoring, it might be possible to provide a gitdir.
[2] This is how `uploadpack.packObjectsHook` was implemented prior to
this commit.

Signed-off-by: Glen Choo <[e-mail]>
---
 config.c                     | 51 ++++++++++++++++++++++++++++++++++++
 config.h                     | 17 ++++++++++++
 t/t5544-pack-objects-hook.sh |  7 ++++-
 upload-pack.c                | 27 ++++++++++++-------
 4 files changed, 91 insertions(+), 11 deletions(-)

Comments

Junio C Hamano June 7, 2022, 10:49 p.m. UTC | #1

"Glen Choo via GitGitGadget" <[e-mail]> writes:

> diff --git a/upload-pack.c b/upload-pack.c
> index 3a851b36066..09f48317b02 100644
> --- a/upload-pack.c
> +++ b/upload-pack.c
> @@ -1321,18 +1321,27 @@ static int upload_pack_config(const char *var, const char *value, void *cb_data)
>  		data->advertise_sid = git_config_bool(var, value);
>  	}
>  
> -	if (current_config_scope() != CONFIG_SCOPE_LOCAL &&
> -	    current_config_scope() != CONFIG_SCOPE_WORKTREE) {
> -		if (!strcmp("uploadpack.packobjectshook", var))
> -			return git_config_string(&data->pack_objects_hook, var, value);
> -	}
> -

The lossage of this block is because this general git_config()
callback routine that is used to read from any scope is no longer
used to pick up the sensitive variable.  Instead, we need to get it
with a different API, namely, git_protected_config().

It is probably is good that in the new code we are not encouraging
folks to write random comparisons on current_config_scope(), and
instead uniformly use a git_protected_config().  That may promote
consistency.

An obvious alternative to achieve the same consistency would be to
introduce a helper, and rewrite (instead of removing) the above part
like so:

	if (in_protected_scope()) {
		... parse sensitive variable ...
	}

We would not need any other change to this file in this patch if we
go that route, I suspect.

>  	if (parse_object_filter_config(var, value, data) < 0)
>  		return -1;
>  
>  	return parse_hide_refs_config(var, value, "uploadpack");
>  }
>  
> +static int upload_pack_protected_config(const char *var, const char *value, void *cb_data)
> +{
> +	struct upload_pack_data *data = cb_data;
> +
> +	if (!strcmp("uploadpack.packobjectshook", var))
> +		return git_config_string(&data->pack_objects_hook, var, value);
> +	return 0;
> +}
> +
> +static void get_upload_pack_config(struct upload_pack_data *data)
> +{
> +	git_config(upload_pack_config, data);
> +	git_protected_config(upload_pack_protected_config, data);
> +}

Where we used to just do git_config(upload_pack_config), we now need
to do a separate git_protected_config().  It feels a bit wasteful to
iterate over the same configset twice, but it is not like we are
doing the IO and text file parsing multiple times.  This looks quite
straight-forward.

Glen Choo June 8, 2022, 12:22 a.m. UTC | #2

Junio C Hamano <[e-mail]> writes:

> "Glen Choo via GitGitGadget" <[e-mail]> writes:
>
>> diff --git a/upload-pack.c b/upload-pack.c
>> index 3a851b36066..09f48317b02 100644
>> --- a/upload-pack.c
>> +++ b/upload-pack.c
>> @@ -1321,18 +1321,27 @@ static int upload_pack_config(const char *var, const char *value, void *cb_data)
>>  		data->advertise_sid = git_config_bool(var, value);
>>  	}
>>  
>> -	if (current_config_scope() != CONFIG_SCOPE_LOCAL &&
>> -	    current_config_scope() != CONFIG_SCOPE_WORKTREE) {
>> -		if (!strcmp("uploadpack.packobjectshook", var))
>> -			return git_config_string(&data->pack_objects_hook, var, value);
>> -	}
>> -
>
> The lossage of this block is because this general git_config()
> callback routine that is used to read from any scope is no longer
> used to pick up the sensitive variable.  Instead, we need to get it
> with a different API, namely, git_protected_config().
>
> It is probably is good that in the new code we are not encouraging
> folks to write random comparisons on current_config_scope(), and
> instead uniformly use a git_protected_config().  That may promote
> consistency.
>
> An obvious alternative to achieve the same consistency would be to
> introduce a helper, and rewrite (instead of removing) the above part
> like so:
>
> 	if (in_protected_scope()) {
> 		... parse sensitive variable ...
> 	}
>
> We would not need any other change to this file in this patch if we
> go that route, I suspect.

Yes, and as noted in the commit message, this approach seems to work for
`safe.directory` and `discovery.bare` too.

>>  	if (parse_object_filter_config(var, value, data) < 0)
>>  		return -1;
>>  
>>  	return parse_hide_refs_config(var, value, "uploadpack");
>>  }
>>  
>> +static int upload_pack_protected_config(const char *var, const char *value, void *cb_data)
>> +{
>> +	struct upload_pack_data *data = cb_data;
>> +
>> +	if (!strcmp("uploadpack.packobjectshook", var))
>> +		return git_config_string(&data->pack_objects_hook, var, value);
>> +	return 0;
>> +}
>> +
>> +static void get_upload_pack_config(struct upload_pack_data *data)
>> +{
>> +	git_config(upload_pack_config, data);
>> +	git_protected_config(upload_pack_protected_config, data);
>> +}
>
> Where we used to just do git_config(upload_pack_config), we now need
> to do a separate git_protected_config().  It feels a bit wasteful to
> iterate over the same configset twice, but it is not like we are
> doing the IO and text file parsing multiple times.  This looks quite
> straight-forward.

Yeah it's not optimal, but at the very least, I think it's easy enough
to understand that we could replace it with something more economical in
the future.

Patch

diff --git a/config.c b/config.c
index fa471dbdb89..56b7ed5ffe8 100644
--- a/config.c
+++ b/config.c
@@ -81,6 +81,18 @@  static enum config_scope current_parsing_scope;
 static int pack_compression_seen;
 static int zlib_compression_seen;
 
+/*
+ * Config that comes from trusted sources, namely:
+ * - system config files (e.g. /etc/gitconfig)
+ * - global config files (e.g. $HOME/.gitconfig,
+ *   $XDG_CONFIG_HOME/git)
+ * - the command line.
+ *
+ * This is declared here for code cleanliness, but unlike the other
+ * static variables, this does not hold config parser state.
+ */
+static struct config_set protected_config;
+
 static int config_file_fgetc(struct config_source *conf)
 {
 	return getc_unlocked(conf->u.file);
@@ -2373,6 +2385,11 @@  int git_configset_add_file(struct config_set *cs, const char *filename)
 	return git_config_from_file(config_set_callback, filename, cs);
 }
 
+int git_configset_add_parameters(struct config_set *cs)
+{
+	return git_config_from_parameters(config_set_callback, cs);
+}
+
 int git_configset_get_value(struct config_set *cs, const char *key, const char **value)
 {
 	const struct string_list *values = NULL;
@@ -2614,6 +2631,40 @@  int repo_config_get_pathname(struct repository *repo,
 	return ret;
 }
 
+/* Read values into protected_config. */
+static void read_protected_config(void)
+{
+	char *xdg_config = NULL, *user_config = NULL, *system_config = NULL;
+
+	git_configset_init(&protected_config);
+
+	system_config = git_system_config();
+	git_global_config(&user_config, &xdg_config);
+
+	git_configset_add_file(&protected_config, system_config);
+	git_configset_add_file(&protected_config, xdg_config);
+	git_configset_add_file(&protected_config, user_config);
+	git_configset_add_parameters(&protected_config);
+
+	free(system_config);
+	free(xdg_config);
+	free(user_config);
+}
+
+/* Ensure that protected_config has been initialized. */
+static void git_protected_config_check_init(void)
+{
+	if (protected_config.hash_initialized)
+		return;
+	read_protected_config();
+}
+
+void git_protected_config(config_fn_t fn, void *data)
+{
+	git_protected_config_check_init();
+	configset_iter(&protected_config, fn, data);
+}
+
 /* Functions used historically to read configuration from 'the_repository' */
 void git_config(config_fn_t fn, void *data)
 {
diff --git a/config.h b/config.h
index 7654f61c634..e3ff1fcf683 100644
--- a/config.h
+++ b/config.h
@@ -446,6 +446,15 @@  void git_configset_init(struct config_set *cs);
  */
 int git_configset_add_file(struct config_set *cs, const char *filename);
 
+/**
+ * Parses command line options and environment variables, and adds the
+ * variable-value pairs to the `config_set`. Returns 0 on success, or -1
+ * if there is an error in parsing. The caller decides whether to free
+ * the incomplete configset or continue using it when the function
+ * returns -1.
+ */
+int git_configset_add_parameters(struct config_set *cs);
+
 /**
  * Finds and returns the value list, sorted in order of increasing priority
  * for the configuration variable `key` and config set `cs`. When the
@@ -505,6 +514,14 @@  int repo_config_get_maybe_bool(struct repository *repo,
 int repo_config_get_pathname(struct repository *repo,
 			     const char *key, const char **dest);
 
+/*
+ * Functions for reading protected config. By definition, protected
+ * config ignores repository config, so it is unnecessary to read
+ * protected config from any `struct repository` other than
+ * the_repository.
+ */
+void git_protected_config(config_fn_t fn, void *data);
+
 /**
  * Querying For Specific Variables
  * -------------------------------
diff --git a/t/t5544-pack-objects-hook.sh b/t/t5544-pack-objects-hook.sh
index dd5f44d986f..54f54f8d2eb 100755
--- a/t/t5544-pack-objects-hook.sh
+++ b/t/t5544-pack-objects-hook.sh
@@ -56,7 +56,12 @@  test_expect_success 'hook does not run from repo config' '
 	! grep "hook running" stderr &&
 	test_path_is_missing .git/hook.args &&
 	test_path_is_missing .git/hook.stdin &&
-	test_path_is_missing .git/hook.stdout
+	test_path_is_missing .git/hook.stdout &&
+
+	# check that global config is used instead
+	test_config_global uploadpack.packObjectsHook ./hook &&
+	git clone --no-local . dst2.git 2>stderr &&
+	grep "hook running" stderr
 '
 
 test_expect_success 'hook works with partial clone' '
diff --git a/upload-pack.c b/upload-pack.c
index 3a851b36066..09f48317b02 100644
--- a/upload-pack.c
+++ b/upload-pack.c
@@ -1321,18 +1321,27 @@  static int upload_pack_config(const char *var, const char *value, void *cb_data)
 		data->advertise_sid = git_config_bool(var, value);
 	}
 
-	if (current_config_scope() != CONFIG_SCOPE_LOCAL &&
-	    current_config_scope() != CONFIG_SCOPE_WORKTREE) {
-		if (!strcmp("uploadpack.packobjectshook", var))
-			return git_config_string(&data->pack_objects_hook, var, value);
-	}
-
 	if (parse_object_filter_config(var, value, data) < 0)
 		return -1;
 
 	return parse_hide_refs_config(var, value, "uploadpack");
 }
 
+static int upload_pack_protected_config(const char *var, const char *value, void *cb_data)
+{
+	struct upload_pack_data *data = cb_data;
+
+	if (!strcmp("uploadpack.packobjectshook", var))
+		return git_config_string(&data->pack_objects_hook, var, value);
+	return 0;
+}
+
+static void get_upload_pack_config(struct upload_pack_data *data)
+{
+	git_config(upload_pack_config, data);
+	git_protected_config(upload_pack_protected_config, data);
+}
+
 void upload_pack(const int advertise_refs, const int stateless_rpc,
 		 const int timeout)
 {
@@ -1340,8 +1349,7 @@  void upload_pack(const int advertise_refs, const int stateless_rpc,
 	struct upload_pack_data data;
 
 	upload_pack_data_init(&data);
-
-	git_config(upload_pack_config, &data);
+	get_upload_pack_config(&data);
 
 	data.stateless_rpc = stateless_rpc;
 	data.timeout = timeout;
@@ -1695,8 +1703,7 @@  int upload_pack_v2(struct repository *r, struct packet_reader *request)
 
 	upload_pack_data_init(&data);
 	data.use_sideband = LARGE_PACKET_MAX;
-
-	git_config(upload_pack_config, &data);
+	get_upload_pack_config(&data);
 
 	while (state != FETCH_DONE) {
 		switch (state) {