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clean_write.c
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clean_write.c
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/**
* Copyright (c) 2012 Jean-Christophe DUBOIS.
* All rights reserved.
*
* This program is free software; you can redistribute it and/or modify
* it under the terms of the GNU Lesser General Public License as published
* by the Free Software Foundation; either version 2.1, or (at your option)
* any later version.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU General Public License for more details.
*
* You should have received a copy of the GNU Lesser General Public License
* along with this program; if not, write to the Free Software
* Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
*
* @file clear_write.c
* @author Jean-Christophe DUBOIS (jcd@tribudubois.net)
* @brief write/send interposing library.
*
* This library redefines the following functions:
* - write
* - send
* - sendto
* - sendmsg
*
* In turn these functions will use the following functions from glibc.
* - write
* - send
* - sendto
* - sendmsg
*
* Usage: LD_PRELOAD=./clean_write.so command args ...
*
* Changes:
*
*/
#include <stdio.h>
#include <unistd.h>
#include <errno.h>
#include <string.h>
#include <sys/types.h>
#include <sys/socket.h>
#define __USE_GNU
#include <dlfcn.h>
static ssize_t default_write(int fd, const void *buf, size_t count);
static ssize_t default_sendto(int sockfd, const void *buf, size_t len,
int flags, const struct sockaddr *dest_addr,
socklen_t addrlen);
static ssize_t default_sendmsg(int sockfd, const struct msghdr *msg, int flags);
static ssize_t(*real_write) (int fd, const void *buf, size_t count) =
default_write;
static ssize_t(*real_sendto) (int sockfd, const void *buf, size_t len,
int flags, const struct sockaddr * dest_addr,
socklen_t addrlen) = default_sendto;
static ssize_t(*real_sendmsg) (int sockfd, const struct msghdr * msg,
int flags) = default_sendmsg;
#define MIN(a,b) ((a>b) ? b : a)
#ifdef DEBUG
#define debug(fmt, ...) fprintf(stderr, "%s " fmt, __func__, ## __VA_ARGS__)
#else
#define debug(fmt, ...)
#endif
/**
* We use a constructor to lookup the write addresses
* of the glibc functions.
* However it might happen that our functions are called before the
* constructor is called. In such instance we setup some default versions
* of these functions that will force the call to the constructor.
*/
__attribute__ ((constructor))
void init_write(void)
{
static int init_done;
void *ptr;
/*
* TBD: We should protect init_done access/modification in case of
* multithreaded application. But as it is mostly used by the
* dynamic linker it might not be necessary as we should not be
* in multithreaded mode a this time.
*/
if (init_done) {
return;
}
init_done = 1;
/* We resolve the various symbols we are going to overload and use */
ptr = dlsym(RTLD_NEXT, "write");
if (ptr) {
real_write = ptr;
} else {
debug("write %s\n", dlerror());
}
ptr = dlsym(RTLD_NEXT, "sendto");
if (ptr) {
real_sendto = ptr;
} else {
debug("sendto %s\n", dlerror());
}
ptr = dlsym(RTLD_NEXT, "sendmsg");
if (ptr) {
real_sendmsg = ptr;
} else {
debug("sendmsg %s\n", dlerror());
}
}
/*
*/
static ssize_t default_write(int fd, const void *buf, size_t count)
{
init_write();
if (real_write == default_write) {
debug("Failed to resolve 'write', returning EINVAL\n");
errno = EINVAL;
return -1;
}
return real_write(fd, buf, count);
}
static ssize_t default_sendto(int sockfd, const void *buf, size_t len,
int flags, const struct sockaddr *dest_addr,
socklen_t addrlen)
{
init_write();
if (real_sendto == default_sendto) {
debug("Failed to resolve 'sendto', returning EINVAL\n");
errno = EINVAL;
return -1;
}
return real_sendto(sockfd, buf, len, flags, dest_addr, addrlen);
}
static ssize_t default_sendmsg(int sockfd, const struct msghdr *msg, int flags)
{
init_write();
if (real_sendmsg == default_sendmsg) {
debug("Failed to resolve 'sendmsg', returning EINVAL\n");
errno = EINVAL;
return -1;
}
return real_sendmsg(sockfd, msg, flags);
}
/**
*/
ssize_t write(int fd, const void *buf, size_t count)
{
ssize_t rc = real_write(fd, buf, count);
if (buf && count) {
/**
* We violate the prototype here as buf is a const void
* We should not change the content of buf but ...
* most of the time it is harmless and it allows to
* guaranty that the data is indeed zeroized as it is
* not needed anymore
* This might prevent some application to work if they do expect
* their data to be still available after the write.
*/
memset((void *)buf, 0, count);
}
return rc;
}
ssize_t sendto(int sockfd, const void *buf, size_t len, int flags,
const struct sockaddr * dest_addr, socklen_t addrlen)
{
ssize_t rc = real_sendto(sockfd, buf, len, flags, dest_addr, addrlen);
if (buf && len) {
/**
* We violate the prototype here as buf is a const void
* We should not change the content of buf but ...
* most of the time it is harmless and it allows to
* guaranty that the data is indeed zeroized as it is
* not needed anymore
* This might prevent some application to work if they do expect
* their data to be still available after the write.
*/
memset((void *)buf, 0, len);
}
return rc;
}
ssize_t sendmsg(int sockfd, const struct msghdr * msg, int flags)
{
ssize_t rc = real_sendmsg(sockfd, msg, flags);
int count = msg->msg_iovlen;
if (msg) {
while (count) {
count--;
memset(msg->msg_iov[count].iov_base, 0,
msg->msg_iov[count].iov_len);
}
}
return rc;
}
ssize_t send(int sockfd, const void *buf, size_t len, int flags)
{
return sendto(sockfd, buf, len, flags, NULL, 0);
}