PAM_CONV(3) Linux-PAM Manual PAM_CONV(3)
NAME
pam_conv - PAM conversation function
SYNOPSIS
#include <security/pam_appl.h>
struct pam_message {
int msg_style;
const char *msg;
};
struct pam_response {
char *resp;
int resp_retcode;
};
struct pam_conv {
int (*conv)(int num_msg, const struct pam_message **msg,
struct pam_response **resp, void *appdata_ptr);
void *appdata_ptr;
};
DESCRIPTION
The PAM library uses an application-defined callback to allow a direct
communication between a loaded module and the application. This
callback is specified by the struct pam_conv passed to pam_start(3) at
the start of the transaction.
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. The resp_retcode member of this struct is
unused and should be set to zero. It is the caller's responsibility to
release both, this array and the responses themselves, using free(3).
Note, *resp is a struct pam_response array and not an array of
pointers.
The number of responses is always equal to the num_msg conversation
function argument. This does require that the response array is
free(3)'d after every call to the conversation function. The index of
the responses corresponds directly to the prompt index in the
pam_message array.
On failure, the conversation function should release any resources it
has allocated, and return one of the predefined PAM error codes.
Each message can have one of four types, specified by the msg_style
member of struct pam_message:
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 message.
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.
In passing, it is worth noting that there is a discrepancy between the
way Linux-PAM handles the const struct pam_message **msg conversation
function argument and the way that Solaris' PAM (and derivatives, known
to include HP/UX, are there others?) does. Linux-PAM interprets the msg
argument as entirely equivalent to the following prototype const struct
pam_message *msg[] (which, in spirit, is consistent with the commonly
used prototypes for argv argument to the familiar main() function: char
**argv; and char *argv[]). Said another way Linux-PAM interprets the
msg argument as a pointer to an array of num_msg read only 'struct
pam_message' pointers. Solaris' PAM implementation interprets this
argument as a pointer to a pointer to an array of num_msg pam_message
structures. Fortunately, perhaps, for most module/application
developers when num_msg has a value of one these two definitions are
entirely equivalent. Unfortunately, casually raising this number to two
has led to unanticipated compatibility problems.
For what its worth the two known module writer work-arounds for trying
to maintain source level compatibility with both PAM implementations
are:
• never call the conversation function with num_msg greater than one.
• set up msg as doubly referenced so both types of conversation
function can find the messages. That is, make
msg[n] = & (( *msg )[n])
RETURN VALUES
PAM_BUF_ERR
Memory buffer error.
PAM_CONV_ERR
Conversation failure. The application should not set *resp.
PAM_SUCCESS
Success.
SEE ALSO
pam_start(3), pam_set_item(3), pam_get_item(3), pam_strerror(3), pam(7)
Linux-PAM Manual 09/03/2021 PAM_CONV(3)
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