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SubjectRe: [RFC] lttngtrace (signal forwarding)
This new version forwards SIGINT and SIGTERM to the child process. Signals are
disabled around execution of the parent process until it reaches the point where
it can forward them to the child.

Mathieu

/*
* lttngtrace.c
*
* lttngtrace starts/stop system wide tracing around program execution.
*
* Copyright (c) 2010 Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
*
* This program is free software; you can redistribute it and/or modify
* it under the terms of the GNU General Public License as published by
* the Free Software Foundation; either version 2 of the License, 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 General Public License along
* with this program; if not, write to the Free Software Foundation, Inc.,
* 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
*
* This file should be setuid root, and belong to a "tracing" group. Only users
* part of the tracing group can trace and view the traces gathered.
*
* TODO: LTTng should support per-session tracepoint activation.
* TODO: use mkstemp() and save last trace name in user's home directory.
*/

#include <errno.h>
#include <stdio.h>
#include <fcntl.h>
#include <signal.h>
#include <unistd.h>
#include <stdlib.h>
#include <string.h>
#include <limits.h>
#include <dirent.h>
#include <sys/wait.h>
#include <sys/stat.h>
#include <sys/types.h>

#if DEBUG
#define printf_dbg(fmt, args...) printf(fmt, args)
#else
#define printf_dbg(fmt, ...)
#endif

static char *trace_path;
static char trace_path_pid[PATH_MAX];
static int autotrace; /*
* Is the trace_path automatically chosen in /tmp ? Can
* we unlink if needed ?
*/
static int sigfwd_pid;

static int recunlink(const char *dirname)
{
DIR *dir;
struct dirent *entry;
char path[PATH_MAX];

dir = opendir(dirname);
if (dir == NULL) {
if (errno == ENOENT)
return 0;
perror("Error opendir()");
return -errno;
}

while ((entry = readdir(dir)) != NULL) {
if (strcmp(entry->d_name, ".") && strcmp(entry->d_name, "..")) {
snprintf(path, (size_t) PATH_MAX, "%s/%s", dirname,
entry->d_name);
if (entry->d_type == DT_DIR)
recunlink(path);
else
unlink(path);
}
}
closedir(dir);
rmdir(dirname);

return 0;
}

static int start_tracing(void)
{
int ret;
char command[PATH_MAX];

if (autotrace) {
ret = recunlink(trace_path);
if (ret)
return ret;
}

/*
* Create the directory in /tmp to deal with races (refuse if fail).
* Only allow user and group to read the trace data (to limit
* information disclosure).
*/
ret = mkdir(trace_path, S_IRWXU|S_IRWXG);
if (ret) {
perror("Trace directory creation error");
return ret;
}
ret = system("ltt-armall > /dev/null");
if (ret)
return ret;

ret = snprintf(command, PATH_MAX - 1,
"lttctl -C -w %s autotrace1 > /dev/null",
trace_path);
ret = ret < 0 ? ret : 0;
if (ret)
return ret;
ret = system(command);
if (ret)
return ret;
}

static int stop_tracing(uid_t uid, gid_t egid)
{
int ret;

ret = system("lttctl -D autotrace1 > /dev/null");
if (ret)
return ret;
ret = system("ltt-disarmall > /dev/null");
if (ret)
return ret;
/* Hand the trace back to the user after tracing is over */
ret = chown(trace_path, uid, egid);
if (ret) {
perror("chown error");
return ret;
}
}

static int write_child_pid(pid_t pid, uid_t uid, gid_t gid)
{
int fd;
FILE *fp;
int ret;

/* Create the file as exclusive to deal with /tmp file creation races */
fd = open(trace_path_pid, O_WRONLY | O_CREAT | O_EXCL | O_TRUNC,
S_IRUSR | S_IWUSR | S_IRGRP | S_IWGRP);
fp = fdopen(fd, "w");
if (!fp) {
perror("Error writing child pid");
return -errno;
}

fprintf(fp, "%u", (unsigned int) pid);
ret = fclose(fp);
if (ret)
perror("Error in fclose");
/* Hand pid information file back to user */
ret = chown(trace_path_pid, uid, gid);
if (ret)
perror("chown error");
return ret;
}

static int parse_options(int argc, char *argv[], int *arg)
{
int ret = 0;

for (;;) {
if (*arg >= argc
|| argv[*arg][0] != '-'
|| argv[*arg][0] == '\0'
|| argv[*arg][1] == '\0'
|| !strcmp(argv[*arg], "--"))
break;
switch (argv[*arg][1]) {
case 'o': if (*arg + 1 >= argc) {
printf("Missing -o trace name\n");
ret = -EINVAL;
break;
}
trace_path = argv[*arg + 1];
(*arg) += 2;
break;
default: printf("Unknown option -%c\n", argv[*arg][1]);
ret = -EINVAL;
return ret;
}
}
return ret;
}

static int init_trace_path(void)
{
int ret;

if (!trace_path) {
trace_path = "/tmp/autotrace1";
autotrace = 1;
}
ret = snprintf(trace_path_pid, PATH_MAX - 1, "%s/%s",
trace_path, "pid");
ret = ret < 0 ? ret : 0;
return ret;
}

static void sighandler(int signo, siginfo_t *siginfo, void *context)
{
kill(sigfwd_pid, signo);
}

static int init_sighand(sigset_t *saved_mask)
{
sigset_t sig_all_mask;
int gret = 0, ret;

/* Block signals */
ret = sigfillset(&sig_all_mask);
if (ret)
perror("Error in sigfillset");
gret = (gret == 0) ? ret : gret;
ret = sigprocmask(SIG_SETMASK, &sig_all_mask, saved_mask);
if (ret)
perror("Error in sigprocmask");
gret = (gret == 0) ? ret : gret;
return gret;
}

static int forward_signals(pid_t pid, sigset_t *saved_mask)
{
struct sigaction act;
int gret = 0, ret;

/* Forward SIGINT and SIGTERM */
sigfwd_pid = pid;
act.sa_sigaction = sighandler;
act.sa_flags = SA_SIGINFO | SA_RESTART;
sigemptyset(&act.sa_mask);
ret = sigaction(SIGINT, &act, NULL);
if (ret)
perror("Error in sigaction");
gret = (gret == 0) ? ret : gret;
ret = sigaction(SIGTERM, &act, NULL);
if (ret)
perror("Error in sigaction");
gret = (gret == 0) ? ret : gret;

/* Reenable signals */
ret = sigprocmask(SIG_SETMASK, saved_mask, NULL);
if (ret)
perror("Error in sigprocmask");
gret = (gret == 0) ? ret : gret;
return gret;
}

int main(int argc, char *argv[])
{
uid_t euid, uid;
gid_t egid, gid;
pid_t pid;
int gret = 0, ret = 0;
int arg = 1;
sigset_t saved_mask;

if (argc < 2)
return -ENOENT;

euid = geteuid();
uid = getuid();
egid = getegid();
gid = geteuid();

if (euid != 0 && uid != 0) {
printf("%s must be setuid root\n", argv[0]);
return -EPERM;
}

printf_dbg("euid: %d\n", euid);
printf_dbg("uid: %d\n", uid);
printf_dbg("egid: %d\n", egid);
printf_dbg("gid: %d\n", gid);

if (arg < argc) {
ret = parse_options(argc, argv, &arg);
if (ret)
return ret;
}

ret = init_trace_path();
gret = (gret == 0) ? ret : gret;

ret = init_sighand(&saved_mask);
gret = (gret == 0) ? ret : gret;

ret = start_tracing();
if (ret)
return ret;

pid = fork();
if (pid > 0) { /* parent */
int status;

ret = forward_signals(pid, &saved_mask);
gret = (gret == 0) ? ret : gret;
pid = wait(&status);
if (pid == -1)
gret = (gret == 0) ? -errno : gret;

ret = stop_tracing(uid, egid);
gret = (gret == 0) ? ret : gret;
ret = write_child_pid(pid, uid, egid);
gret = (gret == 0) ? ret : gret;
} else if (pid == 0) { /* child */
/* Drop root euid before executing child program */
seteuid(uid);
/* Reenable signals */
ret = sigprocmask(SIG_SETMASK, &saved_mask, NULL);
if (ret) {
perror("Error in sigprocmask");
return ret;
}
ret = execvp(argv[arg], &argv[arg]);
if (ret)
perror("Execution error");
return ret;
} else { /* error */
perror("Error in fork");
return -errno;
}
return ret;
}


--
Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.
http://www.efficios.com


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