Peer credentials through unix domain socket ipc mechanism - unix

Question

I'm writing a code for extraction of peer credential by the server process connected through ipc using domain sockets to the client process. There is no error in the code but while running it I don't get the euid and gid of the peer process.
Code for server process is:
#include <stdio.h>
#include <sys/socket.h>
#include <sys/un.h>
#include <sys/types.h>
#include <unistd.h>
#include <string.h>
//#include<ucred.h>
#define SCM_CREDENTIALS
# define UNIX_PATH_MAX 100
int getpeereid(int connection_fd,uid_t euid,gid_t gid)
{
struct ucred cred;
socklen_t len = sizeof(cred);
if (getsockopt(connection_fd, SOL_SOCKET, SO_PEERCRED, &cred, &len) < 0)
return (-1);
euid =cred.uid;
gid = cred.gid;
//int passcred=1;
//setsockopt(connection_fd,SOL_SOCKET,SO_PASSCRED,(void *)&passcred,sizeof(passcred));
printf("effective user id", euid);
printf("effective group id",gid);
return 0;
}
int connection_handler(int connection_fd)
{
int nbytes;
char buffer[1024];
char msg[256];
//while(cont=recv(connection_fd,buffer,sizeof(buffer),0)>0)
//{
//write(1,buffer,cont)
nbytes = read(connection_fd, buffer, 256);
buffer[nbytes] = 0;
printf("MESSAGE FROM CLIENT: %s\n", buffer);
printf("enter the message");
scanf("%s",msg);
nbytes = snprintf(buffer, 256,msg);
write(connection_fd, buffer, nbytes);
//}
close(connection_fd);
return 0;
}
int main(void)
{
struct sockaddr_un address;
int socket_fd, connection_fd,res;
socklen_t address_length;
pid_t child;
uid_t eid;
gid_t gid;
socket_fd = socket(PF_UNIX, SOCK_STREAM, 0);
if(socket_fd < 0)
{
printf("socket() failed\n");
return 1;
}
printf("socket created\n");
unlink("./demo_socket");
/* start with a clean address structure */
memset(&address, 0, sizeof(struct sockaddr_un));
address.sun_family = AF_UNIX;
snprintf(address.sun_path, UNIX_PATH_MAX, "./demo_socket");
if(bind(socket_fd,
(struct sockaddr *) &address,
sizeof(struct sockaddr_un)) != 0)
{
printf("bind() failed\n");
return 1;
}
if(listen(socket_fd, 5) != 0)
{
printf("listen() failed\n");
return 1;
}
while((connection_fd = accept(socket_fd,
(struct sockaddr *) &address,
&address_length)) > -1)
{
// get the credentials
res=getpeereid(connection_fd,geteuid(),getgid());
if (res==0)
{
//if(res==0)
//{
child = fork();
if(child == 0)
{
/* now inside newly created connection handling process */
return connection_handler(connection_fd);
}
}
/* still inside server process */
close(connection_fd);
//}
}
close(socket_fd);
unlink("./demo_socket");
return 0;
}
the code for client
#include <stdio.h>
#include <sys/socket.h>
#include <sys/un.h>
#include <unistd.h>
#include <string.h>
#define UNIX_PATH_MAX 100
int connection_handler(int socket_fd)
{
int nbytes;
char buffer[1024];
char mesg[256];
printf("enter the message");
scanf("%s",mesg);
//printf("message is %s",mesg);
nbytes = snprintf(buffer, 256,mesg);
//fgets(buffer,256,mesg);
//i=atoi(mesg);
write(socket_fd,buffer,nbytes);
//send(socket_fd,mesg,sizeof(mesg),0);
}
//nbytes = read(socket_fd, buffer, 256);
//buffer[nbytes] = 0;
//printf("MESSAGE FROM SERVER: %s\n", buffer);
//}
int main(void)
{
struct sockaddr_un address;
int socket_fd, nbytes,i;
pid_t child;
char buffer[256];
//char mesg[100];
socket_fd = socket(PF_UNIX, SOCK_STREAM, 0);
if(socket_fd < 0)
{
printf("socket() failed\n");
return 1;
}
printf("socket created\n");
/* start with a clean address structure */
memset(&address, 0, sizeof(struct sockaddr_un));
address.sun_family = AF_UNIX;
snprintf(address.sun_path, UNIX_PATH_MAX, "./demo_socket");
if(connect(socket_fd,
(struct sockaddr *) &address,
sizeof(struct sockaddr_un)) != 0)
{
printf("connect() failed\n");
return 1;
}
child=fork();
while(child==0)
{
return connection_handler(socket_fd);
//printf("connection established\n");
//printf("enter the message");
//scanf("%s",mesg);
//printf("message is %s",mesg);
//bytes = snprintf(buffer, 256,mesg);
//fgets(buffer,256,mesg);
//i=atoi(mesg);
//write(socket_fd,buffer,nbytes);
//send(socket_fd,mesg,sizeof(mesg),0);
}
nbytes = read(socket_fd, buffer, 256);
buffer[nbytes] = 0;
printf("MESSAGE FROM SERVER: %s\n", buffer);
close(socket_fd);
return 0;
}

Answer 1

See http://www.thomasstover.com/uds.html for good overview.
If the server is Linux, the main problem is missing _GNU_SOURCE. Full patch below, it has a few minor changes just to get rid of gcc -Wall -Werror -pedantic -std=c99 errors and warnings.
--- server.c.orig 2014-12-06 13:23:09.138472871 +0200
+++ server.c 2014-12-06 13:21:31.962475754 +0200
## -1,3 +1,4 ##
+#define _GNU_SOURCE
#include <stdio.h>
#include <sys/socket.h>
#include <sys/un.h>
## -5,7 +6,6 ##
#include <unistd.h>
#include <string.h>
//#include<ucred.h>
-#define SCM_CREDENTIALS
# define UNIX_PATH_MAX 100
int getpeereid(int connection_fd,uid_t euid,gid_t gid)
## -20,8 +20,8 ##
//int passcred=1;
//setsockopt(connection_fd,SOL_SOCKET,SO_PASSCRED,(void *)&passcred,sizeof(passcred));
-printf("effective user id", euid);
-printf("effective group id",gid);
+printf("effective user id %d", euid);
+printf("effective group id %d",gid);
return 0;
}
## -40,7 +40,7 ##
printf("MESSAGE FROM CLIENT: %s\n", buffer);
printf("enter the message");
scanf("%s",msg);
-nbytes = snprintf(buffer, 256,msg);
+nbytes = snprintf(buffer, 256, "%s", msg);
write(connection_fd, buffer, nbytes);
//}
close(connection_fd);
## -54,8 +54,6 ##
int socket_fd, connection_fd,res;
socklen_t address_length;
pid_t child;
-uid_t eid;
-gid_t gid;
socket_fd = socket(PF_UNIX, SOCK_STREAM, 0);
if(socket_fd < 0)