Re: [PATCH 03/11] config: prefer git_config_string_dup() for temp variables

[Eric Sunshine <sunshine@sunshineco.com>]

On Sat, Apr 6, 2024 at 9:00 PM Jeff King <peff@peff.net> wrote:
> In some cases we may use git_config_string() or git_config_pathname() to
> read a value into a temporary variable, and then either hand off memory
> ownership of the new variable or free it. These are not potential leaks,
> since we know that there is no previous value we are overwriting.
>
> However, it's worth converting these to use git_config_string_dup() and
> git_config_pathname_dup(). It makes it easier to audit for leaky cases,
> and possibly we can get rid of the leak-prone functions in the future.
> Plus it lets the const-ness of our variables match their expected memory
> ownership, which avoids some casts when calling free().
>
> Signed-off-by: Jeff King <peff@peff.net>
> ---
> diff --git a/builtin/config.c b/builtin/config.c
> @@ -282,11 +282,11 @@ static int format_config(struct strbuf *buf, const char *key_,
> -                       const char *v;
> -                       if (git_config_pathname(&v, key_, value_) < 0)
> +                       char *v = NULL;
> +                       if (git_config_pathname_dup(&v, key_, value_) < 0)
>                                 return -1;
>                         strbuf_addstr(buf, v);
> -                       free((char *)v);
> +                       free(v);

This revised code implies that git_config_pathname_dup() doesn't
assign allocated memory to `v` in the "failure" case, thus it is safe
to `return` immediately without calling free(v). However, the
documentation for git_config_pathname_dup() and cousins doesn't state
this explicitly, which means the caller needs to peek into the
implementation of git_config_pathname_dup() to verify that it is safe
to write code such as the above. Hence, should the documentation be
updated to explain that `v` won't be modified in the "failure" case?