LD_PRELOAD implementation of SSLKEYLOGFILE in OpenSSL

ldkeylog.c

/*
 * ldkeylog.c - tool for dumping TLS keys from applications which
 *              dynamically link against OpenSSL on *nix systems
 *
 * Compile:
 *   cc -shared -fPIC -O3 -o ldkeylog.so ldkeylog.c
 * 
 * Use:
 *   LD_PRELOAD=.../ldkeylog.so SSLKEYLOGFILE=.../keylog.txt \
 *     openssl s_client -connect www.google.com:443
 *
 * 2023 Hugo Landau <hlandau@devever.net>
 * Released into the public domain.
 */
#define _GNU_SOURCE
#include <dlfcn.h>
#include <fcntl.h>
#include <unistd.h>
#include <sys/uio.h>
#include <pthread.h>
#include <stdlib.h>
#include <string.h>

typedef void (*keylog_callback_t)(const void *ssl, const char *line);
typedef void *(*SSL_CTX_new_t)(const void *method);
typedef void *(*SSL_CTX_new_ex_t)(void *libctx, const char *propq, const void *method);
typedef void (*SSL_CTX_set_keylog_callback_t)(void *ctx, keylog_callback_t fp);

static pthread_once_t g_initOnce = PTHREAD_ONCE_INIT;
static SSL_CTX_new_t                  fp_SSL_CTX_new;
static SSL_CTX_new_ex_t               fp_SSL_CTX_new_ex;
static SSL_CTX_set_keylog_callback_t  fp_SSL_CTX_set_keylog_callback;
static int g_logFile = -1;

static void _Teardown(void)
{
  close(g_logFile);
  g_logFile = -1;
}

static void _InitActual(void)
{
  fp_SSL_CTX_new                  = dlsym(RTLD_NEXT, "SSL_CTX_new");
  fp_SSL_CTX_new_ex               = dlsym(RTLD_NEXT, "SSL_CTX_new_ex");
  fp_SSL_CTX_set_keylog_callback  = dlsym(RTLD_NEXT, "SSL_CTX_set_keylog_callback");
  if (!fp_SSL_CTX_set_keylog_callback)
    return;

  const char *klFilename = getenv("SSLKEYLOGFILE");
  if (!klFilename || !*klFilename)
    return;

  g_logFile = open(klFilename, O_WRONLY | O_CREAT | O_APPEND | O_CLOEXEC, 0644);
  if (g_logFile >= 0)
    atexit(_Teardown);
}

static void _Init(void)
{
  pthread_once(&g_initOnce, _InitActual);
}

static void _KeylogCallback(const void *ssl, const char *line)
{
  struct iovec iov[2];

  if (g_logFile < 0)
    return;

  iov[0].iov_base = (char *)line;
  iov[0].iov_len  = strlen(line);
  iov[1].iov_base = "\n";
  iov[1].iov_len  = 1;
  writev(g_logFile, iov, 2);
}

static void _ConfigureKeylog(void *ctx)
{
  if (g_logFile < 0)
    return;

  fp_SSL_CTX_set_keylog_callback(ctx, _KeylogCallback);
}

void *SSL_CTX_new(const void *method)
{
  _Init();
  if (!fp_SSL_CTX_new)
    abort();

  void *p = fp_SSL_CTX_new(method);

  if (p)
    _ConfigureKeylog(p);

  return p;
}

void *SSL_CTX_new_ex(void *libctx, const char *propq,
                     const void *method)
{
  _Init();
  if (!fp_SSL_CTX_new_ex)
    abort();

  void *p = fp_SSL_CTX_new_ex(libctx, propq, method);

  if (p)
    _ConfigureKeylog(p);

  return p;
}

void SSL_CTX_set_keylog_callback(void *ctx, keylog_callback_t fp)
{
  /* ignore */
}

Makefile

ldkeylog.so: ldkeylog.c
	gcc -Wall -shared -fPIC -O3 "$<" -o "$@"