2.2 What is expected of an application

The conversation function

An application must provide a ``conversation function''. It is used for direct communication between a loaded module and the application and will typically provide a means for the module to prompt the user for a password etc. . The structure, pam_conv, is defined by including <security/pam_appl.h>; to be,

struct pam_conv {
    int (*conv)(int num_msg,
        const struct pam_message **msg,
        struct pam_response **resp,
        void *appdata_ptr);
    void *appdata_ptr;
};

It is initialized by the application before it is passed to the library. The contents of this structure are attached to the *pamh handle. The point of this argument is to provide a mechanism for any loaded module to interact directly with the application program. This is why it is called a conversation structure.

When a module calls the referenced conv() function, the argument *appdata_ptr is set to the second element of this structure.

The other arguments of a call to conv() concern the information exchanged by module and application. That is to say, num_msg holds the length of the array of pointers, msg. After a successful return, the pointer *resp points to an array of pam_response structures, holding the application supplied text. Note, *resp is an struct pam_response array and not an array of pointers.

The message (from the module to the application) passing structure is defined by <security/pam_appl.h> as:

struct pam_message {
    int msg_style;
    const char *msg;
};

Valid choices for msg_style are:

PAM_PROMPT_ECHO_OFF

Obtain a string without echoing any text

PAM_PROMPT_ECHO_ON

Obtain a string whilst echoing text

PAM_ERROR_MSG

Display an error

PAM_TEXT_INFO

Display some text.

The point of having an array of messages is that it becomes possible to pass a number of things to the application in a single call from the module. It can also be convenient for the application that related things come at once: a windows based application can then present a single form with many messages/prompts on at once.

The response (from the application to the module) passing structure is defined by including <security/pam_appl.h> as:

struct pam_response {
    char *resp;
    int resp_retcode;
};

Currently, there are no definitions for resp_retcode values; the normal value is 0. The length of the *resp array is known in advance by the module, since it is dependent on the number of prompts sent in the msg array. The memory for this array and its string elements should be dynamically allocated with one of the malloc() library of functions. It will be free()'d by the module.

The maximum length of the pam_msg.msg and pam_response.resp character strings is PAM_MAX_MSG_SIZE.

PAM_SUCCESS is the expected return value of this function. However, should an error occur the application should not set *resp but simply return PAM_CONV_ERR.

Note, if an application wishes to use two conversation functions, it should activate the second with a call to pam_set_item().