[ptxdist] [PATCH v5] ptxd_lib_code_signing: cs_get_ca(): improve error handling

[Roland Hieber <rhi@pengutronix.de>]

From: Marc Kleine-Budde <mkl@pengutronix.de>

This patch changes cs_get_ca() to only output the CA if it actually
exists, so that this function can be used even if a signing provider
does not provide a CA for a role.

Additionally improve robustness against premature evaluation by printing
an error code if the signing provider was not set up yet. If the error
message is used as part of a URI, the user can at least get a hint about
the fact that an error happened.

Co-authored-by: Roland Hieber <rhi@pengutronix.de>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>
Signed-off-by: Roland Hieber <rhi@pengutronix.de>
---
PATCH v5:
 - print error if keydir doesn't exist; and only print CA if it was set
   (feedback from Michael Olbrich)
 - update docs, and add example

PATCH v4: https://lore.ptxdist.org/ptxdist/20210713115125.15630-1-rhi@pengutronix.de
 - revert to [ -e "${ca}" ] test (feeback from Michael Olbrich and Marc
   Kleine-Budde)
 - add documentation too

PATCH v3: https://lore.ptxdist.org/ptxdist/20210708203941.30212-1-rhi@pengutronix.de
 - correctly check for existence of ${keydir} instead of ${ca} (feedback
   from Michael Olbrich)
 - drop controversial re-indentation patches 6/7 and 7/7 from the series

PATCH v2 (rhi): https://lore.ptxdist.org/ptxdist/20210627231121.28313-1-rhi@pengutronix.de
 - reorder from PATCH 3/n to PATCH 1/n
 - echo "ERROR_CA_NOT_YET_SET" in case of error (feedback from Michael
   Olbrich) and also return 1

PATCH v1 (mkl): https://lore.ptxdist.org/ptxdist/20210412161900.2376802-3-mkl@pengutronix.de
---
 doc/ref_code_signing_helpers.rst     | 22 +++++++++++++++++++---
 scripts/lib/ptxd_lib_code_signing.sh | 11 ++++++++++-
 2 files changed, 29 insertions(+), 4 deletions(-)

diff --git a/doc/ref_code_signing_helpers.rst b/doc/ref_code_signing_helpers.rst
index 99a395b287c9..fd16ca763557 100644
--- a/doc/ref_code_signing_helpers.rst
+++ b/doc/ref_code_signing_helpers.rst
@@ -330,8 +330,24 @@ Usage:
 
 Get path to the CA keyring in PEM format for role.
 
+If the provider does not set a CA for this role (see :ref:`cs_append_ca_from_pem`,
+:ref:`cs_append_ca_from_der`, :ref:`cs_append_ca_from_uri`), this function will print an empty
+string.
+
 Preconditions:
 
-- a certificate must have been appended to the CA keyring
-  (see :ref:`cs_append_ca_from_pem`, :ref:`cs_append_ca_from_der`,
-  :ref:`cs_append_ca_from_uri`)
+- The role must have been defined by the provider (see :ref:`cs_define_role`).
+  Otherwise, this function will print ``ERROR_CA_NOT_YET_SET`` and return 1.
+  This can happen if the function is evaluated by a variable expansion in make
+  with ``:=`` instead of ``=`` before the code signing provider is set up.
+
+Example:
+
+.. code-block:: make
+
+   # set up kernel module signing, and add a trusted CA if the provider set one
+   KERNEL_SIGN_OPT =
+   	CONFIG_MODULE_SIG_KEY='"$(shell cs_get_uri kernel-modules)"' \
+   	CONFIG_MODULE_SIG_ALL=y \
+   	$(if $(shell cs_get_ca kernel-trusted), \
+   		CONFIG_SYSTEM_TRUSTED_KEYS=$(shell cs_get_ca kernel-trusted))
diff --git a/scripts/lib/ptxd_lib_code_signing.sh b/scripts/lib/ptxd_lib_code_signing.sh
index 5fa62d8372f9..5ba1a4666af4 100644
--- a/scripts/lib/ptxd_lib_code_signing.sh
+++ b/scripts/lib/ptxd_lib_code_signing.sh
@@ -288,7 +288,16 @@ cs_get_ca() {
     local role="${1}"
     cs_init_variables
 
-    echo "${keydir}/${role}/ca.pem"
+    local ca="${keydir}/${role}/ca.pem"
+
+    if [ ! -d "${keydir}" ]; then
+	echo "ERROR_CA_NOT_YET_SET"
+	return 1
+    fi
+
+    if [ -e "${ca}" ]; then
+	echo "${ca}"
+    fi
 }
 export -f cs_get_ca
 
-- 
2.30.2


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