set the monitor process title to something useful

Instead of having a parent process that's called whatever the caller of the
library is called, we instead set it to "[lxc monitor] <lxcpath> <container>"

Closes #180

v2: check for null in tok for loop, only truncate environment when necessary

Signed-off-by: Tycho Andersen <tycho.andersen@canonical.com>
Acked-by: Serge E. Hallyn <serge.hallyn@ubuntu.com>

src/lxc/lxccontainer.c

@@ -598,6 +598,7 @@ static bool lxcapi_start(struct lxc_container *c, int useinit, char * const argv
 	* while container is running...
 	*/
 	if (daemonize) {
+		char title[2048];
 		lxc_monitord_spawn(c->config_path);
 
 		pid_t pid = fork();
@@ -612,6 +613,14 @@ static bool lxcapi_start(struct lxc_container *c, int useinit, char * const argv
 			return wait_on_daemonized_start(c, pid);
 		}
 
+		/* We don't really care if this doesn't print all the
+		 * characters; all that it means is that the proctitle will be
+		 * ugly. Similarly, we also don't care if setproctitle()
+		 * fails. */
+		snprintf(title, sizeof(title), "[lxc monitor] %s %s", c->config_path, c->name);
+		INFO("Attempting to set proc title to %s", title);
+		setproctitle(title);
+
 		/* second fork to be reparented by init */
 		pid = fork();
 		if (pid < 0) {

src/lxc/utils.c

@@ -39,6 +39,7 @@
 #include <sys/types.h>
 #include <sys/wait.h>
 #include <assert.h>
+#include <sys/prctl.h>
 
 #include "utils.h"
 #include "log.h"
@@ -1540,3 +1541,69 @@ char *get_template_path(const char *t)
 
 	return tpath;
 }
+
+/*
+ * Sets the process title to the specified title. Note:
+ *   1. this function requires root to succeed
+ *   2. it clears /proc/self/environ
+ *   3. it may not succed (e.g. if title is longer than /proc/self/environ +
+ *      the original title)
+ */
+int setproctitle(char *title)
+{
+	char buf[2048], *tmp;
+	FILE *f;
+	int i, len, ret = 0;
+	unsigned long arg_start, arg_end, env_start, env_end;
+
+	f = fopen_cloexec("/proc/self/stat", "r");
+	if (!f) {
+		return -1;
+	}
+
+	tmp = fgets(buf, sizeof(buf), f);
+	fclose(f);
+	if (!tmp) {
+		return -1;
+	}
+
+	/* Skip the first 47 fields, column 48-51 are ARG_START and
+	 * ARG_END. */
+	tmp = strchr(buf, ' ');
+	for (i = 0; i < 46; i++) {
+		if (!tmp)
+			return -1;
+		tmp = strchr(tmp+1, ' ');
+	}
+
+	i = sscanf(tmp, "%lu %lu %lu %lu", &arg_start, &arg_end, &env_start, &env_end);
+	if (i != 4) {
+		return -1;
+	}
+
+	/* We're truncating the environment, so we should use at most the
+	 * length of the argument + environment for the title. */
+	len = strlen(title);
+	if (len > env_end - arg_start) {
+		arg_end = env_end;
+		len = env_end - arg_start;
+	} else {
+		/* Only truncate the environment if we're actually going to
+		 * overwrite part of it. */
+		if (len >= arg_end - arg_start) {
+			env_start = env_end;
+		}
+		arg_end = arg_start + len;
+	}
+
+
+	/* memcpy instead of strcpy since this isn't null terminated */
+	memcpy((void*)arg_start, title, len);
+
+	ret |= prctl(PR_SET_MM, PR_SET_MM_ARG_START,   (long)arg_start, 0, 0);
+	ret |= prctl(PR_SET_MM, PR_SET_MM_ARG_END,     (long)arg_end, 0, 0);
+	ret |= prctl(PR_SET_MM, PR_SET_MM_ENV_START,   (long)env_start, 0, 0);
+	ret |= prctl(PR_SET_MM, PR_SET_MM_ENV_END,     (long)env_end, 0, 0);
+
+	return ret;
+}

src/lxc/utils.h

@@ -286,3 +286,4 @@ int print_to_file(const char *file, const char *content);
 bool switch_to_ns(pid_t pid, const char *ns);
 int is_dir(const char *path);
 char *get_template_path(const char *t);
+int setproctitle(char *title);