PAM module error (passing argument 1 of ‘snprintf’ discards ‘const’) - c

Question

I have my PAM module:
I try to compile it, using string:
gcc -fPIC -DPIC -shared -rdynamic -o pam_test.so pam_test.c
But I have errors:
pam_test.c: In function ‘pam_sm_authenticate’:
pam_test.c:74:9: warning: passing argument 1 of ‘snprintf’ discards ‘const’ qualifier from pointer target type [enabled by default]
snprintf(msg[0].msg,60,"Second Password:%d:%d:%d:%d:",x1,x2,x3,x4);
^
In file included from pam_test.c:10:0:
/usr/include/stdio.h:386:12: note: expected ‘char * __restrict__’ but argument is of type ‘const char *’
extern int snprintf (char *__restrict __s, size_t __maxlen,
module code:
#include <security/pam_modules.h>
#include <stdarg.h>
#include <time.h>
#include <stdlib.h>
#include <stdio.h>
#define PAM_SM_AUTH
#define MAX_V 30
PAM_EXTERN int pam_sm_authenticate(pam_handle_t * pamh, int flags, int argc, const char **argv)
{
unsigned int ctrl;
int retval;
const char *name, *p;
char *right;
int x1,x2,x3,x4,y;
time_t mytime;
struct tm *mytm;
mytime=time(0);
mytm=localtime(&mytime);
srandom(mytime);
x1=random()%MAX_V;
x2=random()%MAX_V;
x3=random()%MAX_V;
x4=random()%MAX_V;
retval = pam_get_user(pamh, &name, "login: ");
{
struct pam_conv *conv;
struct pam_message *pmsg[3],msg[3];
struct pam_response *response;
retval = pam_get_item( pamh, PAM_CONV, (const void **) &conv ) ;
pmsg[0] = &msg[0];
msg[0].msg_style = PAM_PROMPT_ECHO_OFF;
msg[0].msg=malloc(100);
snprintf(msg[0].msg,60,"Second Password:%d:%d:%d:%d:",x1,x2,x3,x4);
retval = conv->conv(1, ( const struct pam_message ** ) pmsg
, &response, conv->appdata_ptr);
y=2*x1*mytm->tm_mday+x3*mytm->tm_hour;
right=malloc(100);
snprintf(right,20,"%d",y);
if (!(strcmp(right,response->resp))){
return PAM_SUCCESS;
}else{
return PAM_AUTH_ERR;
}
}
return PAM_SUCCESS;
}
PAM_EXTERN int pam_sm_setcred(pam_handle_t * pamh, int flags
,int argc, const char **argv)
{
unsigned int ctrl;
int retval;
retval = PAM_SUCCESS;
}
#ifdef PAM_STATIC
struct pam_module _pam_unix_auth_modstruct = {
"pam_test",
pam_sm_authenticate,
pam_sm_setcred,
NULL,
NULL,
NULL,
NULL,
};
#endif

Answer 1

The problem here is that in struct pam_message, the msg field is declared as const char*; that is, a pointer to an immutable string. So you're not supposed to overwrite it with snprintf.
A simple workaround is to write the string before inserting a pointer to it into msg:
char *s = malloc(100);
snprintf(s, 100, "Second Password:%d:%d:%d:%d:",x1,x2,x3,x4);
msg[0].msg = s;
Note: You should check the return value of snprintf. If the value returned is more than 100, then the string was truncated.