ima: extend the measurement list to include the file signature

This patch defines a new template called 'ima-sig', which includes
the file signature in the template data, in addition to the file's
digest and pathname.

A template is composed of a set of fields.  Associated with each
field is an initialization and display function.  This patch defines
a new template field called 'sig', the initialization function
ima_eventsig_init(), and the display function ima_show_template_sig().

This patch modifies the .field_init() function definition to include
the 'security.ima' extended attribute and length.

Changelog:
- remove unused code (Dmitry Kasatkin)
- avoid calling ima_write_template_field_data() unnecesarily (Roberto Sassu)
- rename DATA_FMT_SIG to DATA_FMT_HEX
- cleanup ima_eventsig_init() based on Roberto's comments

Signed-off-by: Mimi Zohar <zohar@linux.vnet.ibm.com>
Signed-off-by: Dmitry Kasatkin <d.kasatkin@samsung.com>
Signed-off-by: Roberto Sassu <roberto.sassu@polito.it>

security/integrity/ima/ima_template.c

@@ -20,6 +20,7 @@
 static struct ima_template_desc defined_templates[] = {
 	{.name = IMA_TEMPLATE_IMA_NAME, .fmt = IMA_TEMPLATE_IMA_FMT},
 	{.name = "ima-ng",.fmt = "d-ng|n-ng"},
+	{.name = "ima-sig",.fmt = "d-ng|n-ng|sig"},
 };
 
 static struct ima_template_field supported_fields[] = {
@@ -31,6 +32,8 @@ static struct ima_template_field supported_fields[] = {
 	 .field_show = ima_show_template_digest_ng},
 	{.field_id = "n-ng",.field_init = ima_eventname_ng_init,
 	 .field_show = ima_show_template_string},
+	{.field_id = "sig",.field_init = ima_eventsig_init,
+	 .field_show = ima_show_template_sig},
 };
 
 static struct ima_template_desc *ima_template;