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

2 files changed, 239 insertions, 0 deletions

diff --git a/src/Makefile.am b/src/Makefile.am
index 99432811..eeb3f7df 100644
--- a/src/Makefile.am
+++ b/src/Makefile.am
@@ -27,6 +27,7 @@ libosmocore_la_SOURCES = context.c timer.c timer_gettimeofday.c timer_clockgetti
 			 tdef.c \
 			 sockaddr_str.c \
 			 use_count.c \
+			 exec.c \
 			 $(NULL)
 
 if HAVE_SSSE3
diff --git a/src/exec.c b/src/exec.c
new file mode 100644
index 00000000..a9d8ce0f
--- /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 */