Function Interposition in Linux

Hello,

Please I try to apply the mechanism Interposition Function in Linux on the function write (write a socket) as follows:

Code:

---

# include <sys/types.h>
# include <sys/socket.h>
# include <netinet/in.h>
# include <arpa/inet.h>
# include <netdb.h>
# include <stdio.h>
# include <string.h>
int write (int n, char buffer [32], int sb)
{


int nb = write (so, "Ceci est un exemple de socket!!!", 32);
}

---

​I also have the server code and client code, when the client connects to the server, the server sends a message to the client but when running I got the following error:

Erreur de segmentation
Have you an idea please ?

Thank you so much

Post your whole code.

Two lines, which don't compile, are insufficient to track down a segmentation fault.

Quote:

---

Originally Posted by Salem

Post your whole code.

Two lines, which don't compile, are insufficient to track down a segmentation fault.

---

test.c

Code:

---

#include <sys/types.h>  #include <sys/socket.h> 
#include <netinet/in.h> 
#include <arpa/inet.h> 
#include <netdb.h> 
#include <stdio.h> 
#include <stdlib.h>
#include <string.h>


int write(int so, char buffer[32], int sb)
{
int nb=write(so, "Ceci est un exemple de Socket!!!", sb);


return nb;
}

---

server.c

Code:

---

//Socket Serveur#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define BUFFER_TAILLE 32






int creer_socket_serveur(int port)


//Fonction qui attache une socket a un port, l'ecoute et renvoie le descripteur de la socket


{
  int                  listen_fd;
  struct sockaddr_in    sin;


  //Creation de la socket
  if ((listen_fd = socket(AF_INET, SOCK_STREAM, 0)) == -1) exit(1);
 
  //Definition de la socket
  sin.sin_family = AF_INET;
  sin.sin_port = htons(port);
  sin.sin_addr.s_addr = INADDR_ANY;
 
  //Attachement de la socket au port
  if (bind(listen_fd, (struct sockaddr *)&sin, sizeof (sin)) == -1) exit(1);
 
  //Socket en mode ecoute   
  listen(listen_fd, 5);
 
  return (listen_fd);
}






main (int argc, char *argv[])
{
  int port, so, client_sin_len, nb, socketserveur;
  struct sockaddr_in    client_sin;
  char buffer[BUFFER_TAILLE]="Ceci est un exemple de socket!!!";


  printf("Entrez le port du serveur : "); //Demande port
  scanf("%d", &port);


  socketserveur = creer_socket_serveur(port); //Creation du serveur et socket
  printf("Serveur pret ... En attente de requetes ...\n");


  client_sin_len = sizeof (client_sin);
  so = accept(socketserveur, (struct sockaddr *)&client_sin, &client_sin_len);
  if (so == -1)
  {
      printf ("Probleme de creation d'une socket de traitement\n");
      exit (1);
  }
       
  nb = write(so, buffer, sizeof (buffer));
  printf("Transmission effectuee. Nombre de caracteres transmis = %d\n", nb);
  close (so);
  close(socketserveur);                   
  exit (0);
}

---

client.c

Code:

---

//Socket Client 
#include <sys/types.h> 
#include <sys/socket.h> 
#include <netinet/in.h> 
#include <arpa/inet.h> 
#include <netdb.h> 
#include <stdio.h> 
#include <unistd.h>
#include <stdlib.h>
#include <string.h>
 
#define BUFFER_TAILLE 32 
 
int main (int argc, char *argv[]) { 
   
  int socketclient,rc; 
  int port, retour;
  struct sockaddr_in localAddr, servAddr; 
  struct hostent *h; 
  char adresseserveurip[32]; 
  char buffer[BUFFER_TAILLE]; 
   
  //Demande le serveur et no de port 
  printf("Entrez l'adresse IP ou le nom du serveur :"); 
  scanf("%s", adresseserveurip); 
  printf("Entrez le port d'ecoute du serveur :"); 
  scanf("%d", &port); 
 
  //Chercher serveur IP 
  h = gethostbyname(adresseserveurip); 
  if(h==NULL) { 
    printf("%s: serveur inconnu '%s'\n",argv[0],adresseserveurip); 
    exit(1); 
  } 
                                                                                                                           
  //Creation socket 
  servAddr.sin_family = h->h_addrtype; 
  memcpy((char *) &servAddr.sin_addr.s_addr, h->h_addr_list[0], h->h_length); 
  servAddr.sin_port = htons(port);


  //Creation de la socket
  socketclient = socket(AF_INET, SOCK_STREAM, 0); 
  if(socketclient<0) { 
    perror("Probleme de creation de socket\n "); 
    exit(1); 
  }
  //Connexion au serveur
  rc = connect(socketclient, (struct sockaddr *) &servAddr, sizeof(servAddr)); 
  if(rc < 0)
  { 
    printf("Probleme de connexion\n"); 
    exit(1); 
  }
  retour = read (socketclient, buffer, 32);
  buffer[32] = '\0';
  printf("\nMessage :%s. Nombre de caracteres lus = %d\n", buffer, retour);
  close (socketclient); // Fermeture de la socket
  return 0; 
}

---

I proceeded as follows:
gcc -shared -ldl -fPIC test.c -o test.so
i run the server like that:
LD_PRELOAD="/home/test.so" /home/server
i run the client like that: ./client
when the client connects to server the server sends a message ,
i have this error:
Erreur de segmentation



Thank you for help

Your write() function is infinitely recursive. That would explain the segmentation fault, when the server attempts to send a message.

O_o

grumpy is right.

If you are trying to wrap an exported function, as you say, you can't just call the same named function with the same function signature.

If you need to call the wrapped function you must jump through some hoops.

Ultimately, just stop trying to "wing it" after hearing the phrase "Interposition Function".

Search for tutorials on doing this and you'll discover how to do what you want without the infinite recursion.

Soma

Quote:

---

Originally Posted by phantomotap

O_o

grumpy is right.

If you are trying to wrap an exported function, as you say, you can't just call the same named function with the same function signature.

---

i have modified the function like that

Code:

---

size_t write(int fd, const void *buf, size_t count)
{
static size_t (*write_func)(int, const void *, size_t) = NULL;


/* get reference to original (libc provided) write */
if (!write_func)
{
write_func = (size_t(*)(int, const void *, size_t)) dlsym(RTLD_NEXT, "write");
}
return write_func(fd, buffer, sizeof (buffer));
}

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I have a question please :how canget the buffer "const void *buf" because I want to change it before sending ?


Thank you so much for help