[PATCH 5/7] util/repair: add _notmuch_repair_crypto_payload_skip_legacy_display

[William Casarin]

Daniel Kahn Gillmor <dkg at fifthhorseman.net> writes:

> This is a utility function designed to make it easier to
> "fast-forward" past a legacy-display part associated with a
> cryptographic envelope, and show the user the intended message body.
>
> The bulk of the ugliness in here is in the test function
> _notmuch_crypto_payload_has_legacy_display, which tests all of the
> things we'd expect to be true in a a cryptographic payload that
> contains a legacy display part.
>
> Signed-off-by: Daniel Kahn Gillmor <dkg at fifthhorseman.net>
> ---
>  util/repair.c | 98 +++++++++++++++++++++++++++++++++++++++++++++++++++
>  util/repair.h | 17 +++++++++
>  2 files changed, 115 insertions(+)
>
> diff --git a/util/repair.c b/util/repair.c
> index f91c1244..0809f7b4 100644
> --- a/util/repair.c
> +++ b/util/repair.c
> @@ -18,4 +18,102 @@
>   * Authors: Daniel Kahn Gillmor <dkg at fifthhorseman.net>
>   */
>  
> +#include <stdbool.h>
>  #include "repair.h"
> +
> +
> +static bool
> +_notmuch_crypto_payload_has_legacy_display (GMimeObject *payload)
> +{
> +    GMimeMultipart *mpayload;
> +    const char *protected_header_parameter;
> +    GMimeTextPart *legacy_display;
> +    char *legacy_display_header_text = NULL;
> +    GMimeStream *stream = NULL;
> +    GMimeParser *parser = NULL;
> +    GMimeObject *legacy_header_object = NULL, *first;
> +    GMimeHeaderList *legacy_display_headers = NULL, *protected_headers = NULL;
> +    bool ret = false;
> +
> +    if (! g_mime_content_type_is_type (g_mime_object_get_content_type (payload),
> +				       "multipart", "mixed"))
> +	return false;
> +    protected_header_parameter = g_mime_object_get_content_type_parameter (payload, "protected-headers");
> +    if ((! protected_header_parameter) || strcmp (protected_header_parameter, "v1"))
> +	return false;
> +    if (! GMIME_IS_MULTIPART (payload))
> +	return false;
> +    mpayload = GMIME_MULTIPART (payload);
> +    if (mpayload == NULL)
> +	return false;
> +    if (g_mime_multipart_get_count (mpayload) != 2)
> +	return false;
> +    first = g_mime_multipart_get_part (mpayload, 0);
> +    if (! g_mime_content_type_is_type (g_mime_object_get_content_type (first),
> +				       "text", "rfc822-headers"))
> +	return false;
> +    protected_header_parameter = g_mime_object_get_content_type_parameter (first, "protected-headers");
> +    if ((! protected_header_parameter) || strcmp (protected_header_parameter, "v1"))
> +	return false;
> +    if (! GMIME_IS_TEXT_PART (first))
> +	return false;
> +
> +    /* ensure that the headers in the first part all match the values
> +     * found in the payload's own protected headers!  if they don't,
> +     * we should not treat this as a valid "legacy-display" part.
> +     *
> +     * Crafting a GMimeHeaderList object from the content of the
> +     * text/rfc822-headers part is pretty clumsy; we should probably
> +     * push something into GMime that makes this a one-shot
> +     * operation. */
> +    if ((protected_headers = g_mime_object_get_header_list (payload), protected_headers) &&
> +	(legacy_display = GMIME_TEXT_PART (first), legacy_display) &&
> +	(legacy_display_header_text = g_mime_text_part_get_text (legacy_display), legacy_display_header_text) &&
> +	(stream = g_mime_stream_mem_new_with_buffer (legacy_display_header_text, strlen (legacy_display_header_text)), stream) &&
> +	(g_mime_stream_write (stream, "\r\n\r\n", 4) == 4) &&
> +	(g_mime_stream_seek (stream, 0, GMIME_STREAM_SEEK_SET) == 0) &&
> +	(parser = g_mime_parser_new_with_stream (stream), parser) &&
> +	(legacy_header_object = g_mime_parser_construct_part (parser, NULL), legacy_header_object) &&
> +	(legacy_display_headers = g_mime_object_get_header_list (legacy_header_object), legacy_display_headers)) {
> +	/* walk through legacy_display_headers, comparing them against

This may be a noob question, but why the comma operators after the
assignment expressions? Wouldn't they evaluate to the same thing?

> +	 * their values in the protected_headers: */
> +	ret = true;
> +	for (int i = 0; i < g_mime_header_list_get_count (legacy_display_headers); i++) {
> +	    GMimeHeader *dh = g_mime_header_list_get_header_at (legacy_display_headers, i);
> +	    if (dh == NULL) {
> +		ret = false;
> +		break;
> +	    }
> +	    GMimeHeader *ph = g_mime_header_list_get_header (protected_headers, g_mime_header_get_name (dh));
> +	    if (ph == NULL) {
> +		ret = false;
> +		break;
> +	    }
> +	    if (strcmp (g_mime_header_get_value (dh), g_mime_header_get_value (ph))) {
> +		ret = false;
> +		break;
> +	    }
> +	}
> +    }
> +
> +    if (legacy_display_header_text)
> +	g_free (legacy_display_header_text);
> +    if (stream)
> +	g_object_unref (stream);
> +    if (parser)
> +	g_object_unref (parser);
> +    if (legacy_header_object)
> +	g_object_unref (legacy_header_object);
> +
> +    return ret;
> +}
> +
> +GMimeObject *
> +_notmuch_repair_crypto_payload_skip_legacy_display (GMimeObject *payload)
> +{
> +    if (_notmuch_crypto_payload_has_legacy_display (payload)) {
> +	return g_mime_multipart_get_part (GMIME_MULTIPART (payload), 1);
> +    } else {
> +	return payload;
> +    }
> +}
> diff --git a/util/repair.h b/util/repair.h
> index 70e2b7bc..9974d693 100644
> --- a/util/repair.h
> +++ b/util/repair.h
> @@ -11,6 +11,23 @@ extern "C" {
>   * techniques that are designed to improve the user experience of
>   * notmuch */
>  
> +/* If payload is a cryptographic payload within an encrypted message, and
> + * it has a "legacy display" part, then we can skip over it and jump
> + * to the actual content, because notmuch already handles protected
> + * headers appropriately.
> + *
> + * This function either returns payload directly (if it does not have
> + * a "legacy display" part), or it returns a pointer to its
> + * content-bearing subpart, with the "legacy display" part and the
> + * surrounding multipart/mixed object bypassed.
> + *
> + * No new objects are created by calling this function, and the
> + * returned object will only be released when the original part is
> + * disposed of.
> + */
> +GMimeObject *
> +_notmuch_repair_crypto_payload_skip_legacy_display (GMimeObject *payload);
> +
>  #ifdef __cplusplus
>  }
>  #endif
> -- 
> 2.20.1
>
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