Allow alternative names for overridden PSA headers

Integrators of Mbed TLS may override the header files
"psa/crypto_platform.h" and "psa/crypto_struct.h" by overwriting the files
or by placing alternative versions earlier in the include file search path.
These two methods are sometimes inconvenient, so allow a third method which
doesn't require overwriting files or having a precise order for the include
path: integrators can now specify alternative names for the headers.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>

ChangeLog.d/psa-alt-headers.txt

@@ -0,0 +1,4 @@
+Features
+   * The configuration macros MBEDTLS_PSA_CRYPTO_PLATFORM_FILE and
+     MBEDTLS_PSA_CRYPTO_STRUCT_FILE specify alternative locations for
+     the headers "psa/crypto_platform.h" and "psa/crypto_struct.h".

include/mbedtls/mbedtls_config.h

@@ -3537,6 +3537,53 @@
  */
 //#define MBEDTLS_PSA_CRYPTO_USER_CONFIG_FILE "/dev/null"
 
+/**
+ * \def MBEDTLS_PSA_CRYPTO_PLATFORM_FILE
+ *
+ * If defined, this is a header which will be included instead of
+ * `"psa/crypto_platform.h"`. This file should declare the same identifiers
+ * as the one in Mbed TLS, but with definitions adapted to the platform on
+ * which the library code will run.
+ *
+ * \note The required content of this header can vary from one version of
+ *       Mbed TLS to the next. Integrators who provide an alternative file
+ *       should review the changes in the original file whenever they
+ *       upgrade Mbed TLS.
+ *
+ * This macro is expanded after an <tt>\#include</tt> directive. This is a popular but
+ * non-standard feature of the C language, so this feature is only available
+ * with compilers that perform macro expansion on an <tt>\#include</tt> line.
+ *
+ * The value of this symbol is typically a path in double quotes, either
+ * absolute or relative to a directory on the include search path.
+ */
+//#define MBEDTLS_PSA_CRYPTO_PLATFORM_FILE "psa/crypto_platform_alt.h"
+
+/**
+ * \def MBEDTLS_PSA_CRYPTO_STRUCT_FILE
+ *
+ * If defined, this is a header which will be included instead of
+ * `"psa/crypto_struct.h"`. This file should declare the same identifiers
+ * as the one in Mbed TLS, but with definitions adapted to the environment
+ * in which the library code will run. The typical use for this feature
+ * is to provide alternative type definitions on the client side in
+ * client-server integrations of PSA crypto, where operation structures
+ * contain handles instead of cryptographic data.
+ *
+ * \note The required content of this header can vary from one version of
+ *       Mbed TLS to the next. Integrators who provide an alternative file
+ *       should review the changes in the original file whenever they
+ *       upgrade Mbed TLS.
+ *
+ * This macro is expanded after an <tt>\#include</tt> directive. This is a popular but
+ * non-standard feature of the C language, so this feature is only available
+ * with compilers that perform macro expansion on an <tt>\#include</tt> line.
+ *
+ * The value of this symbol is typically a path in double quotes, either
+ * absolute or relative to a directory on the include search path.
+ */
+//#define MBEDTLS_PSA_CRYPTO_STRUCT_FILE "psa/crypto_struct_alt.h"
+
 /** \} name SECTION: General configuration options */
 
 /**

include/psa/crypto.h

@@ -22,7 +22,11 @@
 #ifndef PSA_CRYPTO_H
 #define PSA_CRYPTO_H
 
+#if defined(MBEDTLS_PSA_CRYPTO_PLATFORM_FILE)
+#include MBEDTLS_PSA_CRYPTO_PLATFORM_FILE
+#else
 #include "crypto_platform.h"
+#endif
 
 #include <stddef.h>
 
@@ -4677,7 +4681,11 @@ psa_status_t psa_verify_hash_abort(
 
 /* The file "crypto_struct.h" contains definitions for
  * implementation-specific structs that are declared above. */
+#if defined(MBEDTLS_PSA_CRYPTO_STRUCT_FILE)
+#include MBEDTLS_PSA_CRYPTO_STRUCT_FILE
+#else
 #include "crypto_struct.h"
+#endif
 
 /* The file "crypto_extra.h" contains vendor-specific definitions. This
  * can include vendor-defined algorithms, extra functions, etc. */

include/psa/crypto_types.h

@@ -34,7 +34,11 @@
 #define PSA_CRYPTO_TYPES_H
 #include "mbedtls/private_access.h"
 
+#if defined(MBEDTLS_PSA_CRYPTO_PLATFORM_FILE)
+#include MBEDTLS_PSA_CRYPTO_PLATFORM_FILE
+#else
 #include "crypto_platform.h"
+#endif
 
 /* If MBEDTLS_PSA_CRYPTO_C is defined, make sure MBEDTLS_PSA_CRYPTO_CLIENT
  * is defined as well to include all PSA code.