cgroup: never try to create files in cgroupfs, only open them for writing

This should have the benefit that cg_set_attribute() returns ENOENT
instead of EACCESS when we use it for non-existing attributes.

src/shared/cgroup-util.c

@@ -643,7 +643,7 @@ int cg_attach(const char *controller, const char *path, pid_t pid) {
 
         snprintf(c, sizeof(c), PID_FMT"\n", pid);
 
-        return write_string_file(fs, c);
+        return write_string_file_no_create(fs, c);
 }
 
 int cg_attach_fallback(const char *controller, const char *path, pid_t pid) {
@@ -817,7 +817,7 @@ int cg_install_release_agent(const char *controller, const char *agent) {
 
         sc = strstrip(contents);
         if (sc[0] == 0) {
-                r = write_string_file(fs, agent);
+                r = write_string_file_no_create(fs, agent);
                 if (r < 0)
                         return r;
         } else if (!streq(sc, agent))
@@ -837,7 +837,7 @@ int cg_install_release_agent(const char *controller, const char *agent) {
 
         sc = strstrip(contents);
         if (streq(sc, "0")) {
-                r = write_string_file(fs, "1");
+                r = write_string_file_no_create(fs, "1");
                 if (r < 0)
                         return r;
 
@@ -858,7 +858,7 @@ int cg_uninstall_release_agent(const char *controller) {
         if (r < 0)
                 return r;
 
-        r = write_string_file(fs, "0");
+        r = write_string_file_no_create(fs, "0");
         if (r < 0)
                 return r;
 
@@ -869,7 +869,7 @@ int cg_uninstall_release_agent(const char *controller) {
         if (r < 0)
                 return r;
 
-        r = write_string_file(fs, "");
+        r = write_string_file_no_create(fs, "");
         if (r < 0)
                 return r;
 
@@ -1591,7 +1591,7 @@ int cg_set_attribute(const char *controller, const char *path, const char *attri
         if (r < 0)
                 return r;
 
-        return write_string_file(p, value);
+        return write_string_file_no_create(p, value);
 }
 
 static const char mask_names[] =

src/shared/fileio.c

@@ -58,6 +58,28 @@ int write_string_file(const char *fn, const char *line) {
         return write_string_stream(f, line);
 }
 
+int write_string_file_no_create(const char *fn, const char *line) {
+        _cleanup_fclose_ FILE *f = NULL;
+        int fd;
+
+        assert(fn);
+        assert(line);
+
+        /* We manually build our own version of fopen(..., "we") that
+         * without O_CREAT */
+        fd = open(fn, O_WRONLY|O_CLOEXEC|O_NOCTTY);
+        if (fd < 0)
+                return -errno;
+
+        f = fdopen(fd, "we");
+        if (!f) {
+                safe_close(fd);
+                return -errno;
+        }
+
+        return write_string_stream(f, line);
+}
+
 int write_string_file_atomic(const char *fn, const char *line) {
         _cleanup_fclose_ FILE *f = NULL;
         _cleanup_free_ char *p = NULL;

src/shared/fileio.h

@@ -27,6 +27,7 @@
 
 int write_string_stream(FILE *f, const char *line);
 int write_string_file(const char *fn, const char *line);
+int write_string_file_no_create(const char *fn, const char *line);
 int write_string_file_atomic(const char *fn, const char *line);
 
 int read_one_line_file(const char *fn, char **line);