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SubjectRe: [RFC] lttngtrace available to non-root
Here is an updated version taking care of /tmp file creation races. Thanks to
Frank Ch. Eigler for pointing this out.

A small note: pending a large change in LTTng that will allow arming tracepoints
on a per-session basis, the current version of the script cannot be run in
multiple instances concurrently (interaction between the multiple
ltt-armall/ltt-disarmall). I'm also thinking to use mktemp for the next version
to create a temporary file rather than the hardcoded file I'm using now. I could
save the information about the last file name used in the user's home directory.

Thanks,

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.
*/
#include <errno.h>
#include <stdio.h>
#include <fcntl.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

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;
}
int start_tracing(void)
{
int ret;
ret = recunlink("/tmp/autotrace1");
if (ret)
return ret;
ret = unlink("/tmp/autotrace1-pid");
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("/tmp/autotrace1", S_IRWXU|S_IRWXG);
if (ret) {
perror("Trace directory creation race");
return ret;
}
ret = system("ltt-armall > /dev/null");
if (ret)
return ret;
ret = system("lttctl -C -w /tmp/autotrace1 autotrace1 > /dev/null");
if (ret)
return ret;
}
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 */
printf("uid: %d egid %d\n", uid, egid);
ret = chown("/tmp/autotrace1", uid, egid);
if (ret) {
perror("chown error");
return ret;
}
}
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("/tmp/autotrace1-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("/tmp/autotrace1-pid", uid, gid);
if (ret)
perror("chown error");
return ret;
}
int main(int argc, char *argv[])
{
uid_t euid, uid;
gid_t egid, gid;
pid_t pid;
int gret = 0, ret = 0;
if (argc < 2)
return -ENOENT;
euid = geteuid();
uid = getuid();
egid = getegid();
gid = geteuid();

if (euid != 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);

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

pid = fork();
if (pid > 0) { /* parent */
int status;
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);
ret = execvp(argv[1], &argv[1]);
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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