The Linux-PAM Application Developers' Guide: A library of miscellaneous helper functions

Next Previous Contents

## 5. A library of miscellaneous helper functions

To aid the work of the application developer a library of miscellaneous functions is provided. It is called `libpam_misc`, and contains functions for allocating memory (securely), a text based conversation function, and routines for enhancing the standard PAM-environment variable support.

## 5.1 Requirements

The functions, structures and macros, made available by this library can be defined by including `<security/pam_misc.h>`. It should be noted that this library is specific to Linux-PAM and is not referred to in the defining DCE-RFC (see the bibliography) below.

## 5.2 Macros supplied

### Safe duplication of strings

> `

>     x_strdup(const char *s)

> `

This macro is a replacement for the `xstrdup()` function that was present in earlier versions of the library and which clashed horribly with a number of applications. It returns a duplicate copy of the `NUL` terminated string, `s`. `NULL` is returned if there is insufficient memory available for the duplicate or if `s` is `NULL` to begin with.

## 5.3 Functions supplied

### A text based conversation function

> `

>     extern int misc_conv(int num_msg, const struct pam_message **msgm,
>                          struct pam_response **response, void *appdata_ptr);

> `

This is a function that will prompt the user with the appropriate comments and obtain the appropriate inputs as directed by authentication modules.

In addition to simply slotting into the appropriate `struct pam_conv`, this function provides some time-out facilities. The function exports five variables that can be used by an application programmer to limit the amount of time this conversation function will spend waiting for the user to type something.

The five variables are as follows:

`extern time_t pam_misc_conv_warn_time;`

  This variable contains the time (as returned by `time()`) that the user should be first warned that the clock is ticking. By default it has the value `0`, which indicates that no such warning will be given. The application may set its value to sometime in the future, but this should be done prior to passing control to the Linux-PAM library.

`extern const char *pam_misc_conv_warn_line;`

  Used in conjuction with `pam_misc_conv_warn_time`, this variable is a pointer to the string that will be displayed when it becomes time to warn the user that the timeout is approaching. Its default value is ``..\a.Time is running out...\n'', but this can be changed by the application prior to passing control to Linux-PAM.

`extern time_t pam_misc_conv_die_time;`

  This variable contains the time (as returned by `time()`) that the conversation will time out. By default it has the value `0`, which indicates that the conversation function will not timeout. The application may set its value to sometime in the future, this should be done prior to passing control to the Linux-PAM library.

`extern const char *pam_misc_conv_die_line;`

  Used in conjuction with `pam_misc_conv_die_time`, this variable is a pointer to the string that will be displayed when the conversation times out. Its default value is ``..\a.Sorry, your time is up!\n'', but this can be changed by the application prior to passing control to Linux-PAM.

`extern int pam_misc_conv_died;`

  Following a return from the Linux-PAM libraray, the value of this variable indicates whether the conversation has timed out. A value of `1` indicates the time-out occurred.

The following two function pointers are available for supporting binary prompts in the conversation function. They are optimized for the current incarnation of the `libpamc` library and are subject to change.

`extern int (*pam_binary_handler_fn)(void *appdata, pamc_bp_t *prompt_p);`

  This function pointer is initialized to `NULL` but can be filled with a function that provides machine-machine (hidden) message exchange. It is intended for use with hidden authentication protocols such as RSA or Diffie-Hellman key exchanges. (This is still under development.)

`extern int (*pam_binary_handler_free)(void *appdata, pamc_bp_t *delete_me);`

  This function pointer is initialized to `PAM_BP_RENEW(delete_me, 0, 0)`, but can be redefined as desired by the application.

### Transcribing an environment to that of Linux-PAM

> `

>     extern int pam_misc_paste_env(pam_handle_t *pamh,
>                                   const char * const * user_env);

> `

This function takes the supplied list of environment pointers and uploads its contents to the Linux-PAM environment. Success is indicated by `PAM_SUCCESS`.

### Liberating a locally saved environment

> `

>     extern char **pam_misc_drop_env(char **env);

> `

This function is defined to complement the `pam_getenvlist()` function. It liberates the memory associated with `env`, overwriting with `0` all memory before `free()`ing it.

### BSD like Linux-PAM environment variable setting

> `

>     extern int pam_misc_setenv(pam_handle_t *pamh, const char *name,
>                                const char *value, int readonly);

> `

This function performs a task equivalent to `pam_putenv()`, its syntax is, however, more like the BSD style function; `setenv()`. The `name` and `value` are concatenated with an ```=`'' to form a `name_value` and passed to `pam_putenv()`. If, however, the Linux-PAM variable is already set, the replacement will only be applied if the last argument, `readonly`, is zero.

Next Previous Contents