Return a struct with string from a multi threaded RPC server

Question

I'm trying to write a multithreaded RPC server that returns a struct. Here's my XDR file. I'm running rpcgen -MN foo.x to generate multithreaded compatible code.

// foo.x
struct foo_out {
  string name<128>;
};

program FOO_PROG {
  version FOO_VERS {
  foo_out foo(void) = 2;
  } = 2 ;
} = 0x31230000;

Here's my server, you can see it takes a pointer to my struct, allocates some memory for the name string and then copies over the string. I also added a debug output

// foo_server.c
#include "foo.h"
#include <stdio.h>
#include <stdlib.h>
#include <rpc/pmap_clnt.h>
#include <string.h>
#include <memory.h>
#include <sys/socket.h>
#include <netinet/in.h>

bool_t foo_2_svc(foo_out *out, struct svc_req *req) {
  out->name = malloc(sizeof("foo"));
  strcpy(out->name, "foo");
  printf("Value: '%s'\n", out->name);
  return TRUE;
}

int
foo_prog_2_freeresult(SVCXPRT *transp, xdrproc_t xdr_result, caddr_t result) {
  xdr_free(xdr_result, result);
  return(1);
}

Here's the client. It creates a variable, passes in a pointer to that variable and then outputs the value:

// foo_client.c
#include "memory.h" /* for memset */
#include "foo.h"
#include "stdio.h"
#include "stdlib.h"
#include "rpc/pmap_clnt.h"
#include "string.h"
#include "memory.h"
#include "sys/socket.h"
#include "netinet/in.h"
#include <unistd.h>

int
main (int argc,char **argv)
{
  CLIENT *cl;

  cl = clnt_create("127.0.0.1", FOO_PROG, FOO_VERS, "tcp");
  if (cl == NULL) {
    clnt_perror(cl, "call failed");
    exit (1);
  }

  foo_out out;
  if (foo_2(&out, cl) != RPC_SUCCESS) {
    printf("failed \n");
    // exit(1);
  }
  printf("foo out: %s\n",   out.name);

  sleep(1);
  exit(0);
}

When I run this, the server is fine I see Value: 'foo' every time it is called via the client. However the client crashes with a segv. Using address sanitizer I'm getting this:

=================================================================
==8269==ERROR: AddressSanitizer: SEGV on unknown address 0x0000004b0503 (pc 0x7fc9e2b5160f bp 0x000000000003 sp 0x7ffe264190c0 T0)
    #0 0x7fc9e2b5160e in xdr_string (/lib/x86_64-linux-gnu/libc.so.6+0x13b60e)
    #1 0x460751 in __interceptor_xdr_string.part.268 (/vagrant/foo/foo_client+0x460751)
    #2 0x4b04a7 in xdr_foo_out /vagrant/foo/foo_xdr.c:13
    #3 0x7fc9e2b4b3ea  (/lib/x86_64-linux-gnu/libc.so.6+0x1353ea)
    #4 0x4b03b8 in foo_2 /vagrant/foo/foo_clnt.c:15
    #5 0x4b042e in main /vagrant/foo/foo_client.c:28
    #6 0x7fc9e2a3682f in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x2082f)
    #7 0x405298 in _start (/vagrant/foo/foo_client+0x405298)

AddressSanitizer can not provide additional info.
SUMMARY: AddressSanitizer: SEGV ??:0 xdr_string
==8269==ABORTING

When I use GDB to inspect the contents of the struct before and after thefoo_2 call, it hasn't changed. It looks like the server didn't modify the value at all.

Question: How do you modify a struct with a string using multithreaded RPC calls? If I change my example to use an integer instead of a string, it works fine. So it seems like i'm missing something basic. Any ideas on what i'm doing wrong or steps to take to debug this?

Someone on Reddit found the issue reddit.com/r/C_Programming/comments/6p8smu/… — Schneems

Jordan Owens Jordan Owens · Accepted Answer · 2017-07-24T15:30:25

In your client try allocating memory for the foo_out struct:

foo_out *out = malloc(sizeof(foo_out));
if (foo_2(out, cl) != RPC_SUCCESS) {
  printf("failed \n");
  // exit(1);
}
printf("foo out: %s\n",   out->name);

Return a struct with string from a multi threaded RPC server

1 Answers