Introduce helper functions for safe fork+exec of processes
In some situations, we want to execute an external shell command
in a non-blocking way. Similar to 'system', but without waiting for
the child to complete. We also want to close all file descriptors
ahead of the exec() and filter + modify the environment.
Change-Id: Ib24ac8a083db32e55402ce496a5eabd8749cc888
Related: OS#4332
diff --git a/src/exec.c b/src/exec.c
new file mode 100644
index 0000000..a9d8ce0
--- /dev/null
+++ b/src/exec.c
@@ -0,0 +1,238 @@
+/* (C) 2019 by Harald Welte <laforge@gnumonks.org>
+ *
+ * All Rights Reserved
+ *
+ * SPDX-License-Identifier: GPL-2.0+
+ *
+ * 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.
+ *
+ */
+
+#include "config.h"
+#ifndef EMBEDDED
+
+#include <unistd.h>
+
+#include <errno.h>
+#include <string.h>
+
+#include <stdio.h>
+#include <dirent.h>
+#include <sys/types.h>
+
+#include <osmocom/core/logging.h>
+#include <osmocom/core/utils.h>
+#include <osmocom/core/exec.h>
+
+/*! suggested list of environment variables to pass (if they exist) to a sub-process/script */
+const char *osmo_environment_whitelist[] = {
+ "USER", "LOGNAME", "HOME",
+ "LANG", "LC_ALL", "LC_COLLATE", "LC_CTYPE", "LC_MESSAGES", "LC_MONETARY", "LC_NUMERIC", "LC_TIME",
+ "PATH",
+ "PWD",
+ "SHELL",
+ "TERM",
+ "TMPDIR",
+ "LD_LIBRARY_PATH",
+ "LD_PRELOAD",
+ "POSIXLY_CORRECT",
+ "HOSTALIASES",
+ "TZ", "TZDIR",
+ "TERMCAP",
+ "COLUMNS", "LINES",
+ NULL
+};
+
+static bool str_in_list(const char **list, const char *key)
+{
+ const char **ent;
+
+ for (ent = list; *ent; ent++) {
+ if (!strcmp(*ent, key))
+ return true;
+ }
+ return false;
+}
+
+/*! filtered a process environment by whitelist; only copying pointers, no actual strings.
+ *
+ * This function is useful if you'd like to generate an environment to pass exec*e()
+ * functions. It will create a new environment containing only those entries whose
+ * keys (as per environment convention KEY=VALUE) are contained in the whitelist. The
+ * function will not copy the actual strings, but just create a new pointer array, pointing
+ * to the same memory as the input strings.
+ *
+ * Constraints: Keys up to a maximum length of 255 characters are supported.
+ *
+ * \oaram[out] out caller-allocated array of pointers for the generated output
+ * \param[in] out_len size of out (number of pointers)
+ * \param[in] in input environment (NULL-terminated list of pointers like **environ)
+ * \param[in] whitelist whitelist of permitted keys in environment (like **environ)
+ * \returns number of entries filled in 'out'; negtive on error */
+int osmo_environment_filter(char **out, size_t out_len, char **in, const char **whitelist)
+{
+ char tmp[256];
+ char **ent;
+ size_t out_used = 0;
+
+ /* invalid calls */
+ if (!out || out_len == 0 || !whitelist)
+ return -EINVAL;
+
+ /* legal, but unusual: no input to filter should generate empty, terminated out */
+ if (!in) {
+ out[0] = NULL;
+ return 1;
+ }
+
+ /* iterate over input entries */
+ for (ent = in; *ent; ent++) {
+ char *eq = strchr(*ent, '=');
+ unsigned long eq_pos;
+ if (!eq) {
+ /* no '=' in string, skip it */
+ continue;
+ }
+ eq_pos = eq - *ent;
+ if (eq_pos >= ARRAY_SIZE(tmp))
+ continue;
+ strncpy(tmp, *ent, eq_pos);
+ tmp[eq_pos] = '\0';
+ if (str_in_list(whitelist, tmp)) {
+ if (out_used == out_len-1)
+ break;
+ /* append to output */
+ out[out_used++] = *ent;
+ }
+ }
+ OSMO_ASSERT(out_used < out_len);
+ out[out_used++] = NULL;
+ return out_used;
+}
+
+/*! append one environment to another; only copying pointers, not actual strings.
+ *
+ * This function is useful if you'd like to append soem entries to an environment
+ * befoer passing it to exec*e() functions.
+ *
+ * It will append all entries from 'in' to the environment in 'out', as long as
+ * 'out' has space (determined by 'out_len').
+ *
+ * Constraints: If the same key exists in 'out' and 'in', duplicate keys are
+ * generated. It is a simple append, without any duplicate checks.
+ *
+ * \oaram[out] out caller-allocated array of pointers for the generated output
+ * \param[in] out_len size of out (number of pointers)
+ * \param[in] in input environment (NULL-terminated list of pointers like **environ)
+ * \returns number of entries filled in 'out'; negative on error */
+int osmo_environment_append(char **out, size_t out_len, char **in)
+{
+ size_t out_used = 0;
+
+ if (!out || out_len == 0)
+ return -EINVAL;
+
+ /* seek to end of existing output */
+ for (out_used = 0; out[out_used]; out_used++) {}
+
+ if (!in) {
+ if (out_used == 0)
+ out[out_used++] = NULL;
+ return out_used;
+ }
+
+ for (; *in && out_used < out_len-1; in++)
+ out[out_used++] = *in;
+
+ OSMO_ASSERT(out_used < out_len);
+ out[out_used++] = NULL;
+
+ return out_used;
+}
+
+/* Iterate over files in /proc/self/fd and close all above lst_fd_to_keep */
+int osmo_close_all_fds_above(int last_fd_to_keep)
+{
+ struct dirent *ent;
+ DIR *dir;
+ int rc;
+
+ dir = opendir("/proc/self/fd");
+ if (!dir) {
+ LOGP(DLGLOBAL, LOGL_ERROR, "Cannot open /proc/self/fd: %s\n", strerror(errno));
+ return -ENODEV;
+ }
+
+ while ((ent = readdir(dir))) {
+ int fd = atoi(ent->d_name);
+ if (fd <= last_fd_to_keep)
+ continue;
+ if (fd == dirfd(dir))
+ continue;
+ rc = close(fd);
+ if (rc)
+ LOGP(DLGLOBAL, LOGL_ERROR, "Error closing fd=%d: %s\n", fd, strerror(errno));
+ }
+ closedir(dir);
+ return 0;
+}
+
+/* Seems like POSIX has no header file for this, and even glibc + __USE_GNU doesn't help */
+extern char **environ;
+
+/*! call an external shell command without waiting for it.
+ *
+ * This mimics the behavior of system(3), with the following differences:
+ * - it doesn't wait for completion of the child process
+ * - it closes all non-stdio file descriptors by iterating /proc/self/fd
+ * - it constructs a reduced environment where only whitelisted keys survive
+ * - it (optionally) appends additional variables to the environment
+ *
+ * \param[in] command the shell command to be executed, see system(3)
+ * \param[in] env_whitelist A white-list of keys for environment variables
+ * \param[in] addl_env any additional environment variables to be appended
+ * \returns PID of generated child process; negative on error
+ */
+int osmo_system_nowait(const char *command, const char **env_whitelist, char **addl_env)
+{
+ int rc;
+
+ rc = fork();
+ if (rc == 0) {
+ /* we are in the child */
+ char *new_env[1024];
+
+ /* close all file descriptors above stdio */
+ osmo_close_all_fds_above(2);
+
+ /* build the new environment */
+ if (env_whitelist)
+ osmo_environment_filter(new_env, ARRAY_SIZE(new_env), environ, env_whitelist);
+ if (addl_env)
+ osmo_environment_append(new_env, ARRAY_SIZE(new_env), addl_env);
+
+ /* if we want to behave like system(3), we must go via the shell */
+ execle("/bin/sh", "sh", "-c", command, (char *) NULL, new_env);
+ /* only reached in case of error */
+ LOGP(DLGLOBAL, LOGL_ERROR, "Error executing command '%s' after fork: %s\n",
+ command, strerror(errno));
+ return -EIO;
+ } else {
+ /* we are in the parent */
+ return rc;
+ }
+}
+
+#endif /* EMBEDDED */