path-util: add flavour of path_startswith() that leaves a leading slash in place (#37572)

src/basic/cgroup-util.c

@@ -959,13 +959,12 @@ int cg_get_root_path(char **ret_path) {
 }
 
 int cg_shift_path(const char *cgroup, const char *root, const char **ret_shifted) {
-        _cleanup_free_ char *rt = NULL;
-        char *p;
         int r;
 
         assert(cgroup);
         assert(ret_shifted);
 
+        _cleanup_free_ char *rt = NULL;
         if (!root) {
                 /* If the root was specified let's use that, otherwise
                  * let's determine it from PID 1 */
@@ -977,12 +976,7 @@ int cg_shift_path(const char *cgroup, const char *root, const char **ret_shifted
                 root = rt;
         }
 
-        p = path_startswith(cgroup, root);
-        if (p && p > cgroup)
-                *ret_shifted = p - 1;
-        else
-                *ret_shifted = cgroup;
-
+        *ret_shifted = path_startswith_full(cgroup, root, PATH_STARTSWITH_RETURN_LEADING_SLASH|PATH_STARTSWITH_REFUSE_DOT_DOT) ?: cgroup;
         return 0;
 }
 

src/basic/fs-util.c

@@ -67,7 +67,7 @@ int rmdir_parents(const char *path, const char *stop) {
                 assert(*slash == '/');
                 *slash = '\0';
 
-                if (path_startswith_full(stop, p, /* accept_dot_dot= */ false))
+                if (path_startswith_full(stop, p, PATH_STARTSWITH_REFUSE_DOT_DOT))
                         return 0;
 
                 if (rmdir(p) < 0 && errno != ENOENT)

src/basic/mkdir.c

@@ -151,7 +151,7 @@ int mkdir_parents_internal(const char *prefix, const char *path, mode_t mode, ui
         assert(_mkdirat != mkdirat);
 
         if (prefix) {
-                p = path_startswith_full(path, prefix, /* accept_dot_dot= */ false);
+                p = path_startswith_full(path, prefix, PATH_STARTSWITH_REFUSE_DOT_DOT);
                 if (!p)
                         return -EINVAL;
 

src/basic/path-util.c

@@ -433,8 +433,8 @@ int path_simplify_alloc(const char *path, char **ret) {
         return 0;
 }
 
-char* path_startswith_full(const char *path, const char *prefix, bool accept_dot_dot) {
-        assert(path);
+char* path_startswith_full(const char *original_path, const char *prefix, PathStartWithFlags flags) {
+        assert(original_path);
         assert(prefix);
 
         /* Returns a pointer to the start of the first component after the parts matched by
@@ -447,28 +447,45 @@ char* path_startswith_full(const char *path, const char *prefix, bool accept_dot
          * Returns NULL otherwise.
          */
 
+        const char *path = original_path;
+
         if ((path[0] == '/') != (prefix[0] == '/'))
                 return NULL;
 
         for (;;) {
                 const char *p, *q;
-                int r, k;
+                int m, n;
 
-                r = path_find_first_component(&path, accept_dot_dot, &p);
-                if (r < 0)
+                m = path_find_first_component(&path, !FLAGS_SET(flags, PATH_STARTSWITH_REFUSE_DOT_DOT), &p);
+                if (m < 0)
                         return NULL;
 
-                k = path_find_first_component(&prefix, accept_dot_dot, &q);
-                if (k < 0)
+                n = path_find_first_component(&prefix, !FLAGS_SET(flags, PATH_STARTSWITH_REFUSE_DOT_DOT), &q);
+                if (n < 0)
                         return NULL;
 
-                if (k == 0)
-                        return (char*) (p ?: path);
+                if (n == 0) {
+                        if (!p)
+                                p = path;
 
-                if (r != k)
+                        if (FLAGS_SET(flags, PATH_STARTSWITH_RETURN_LEADING_SLASH)) {
+
+                                if (p <= original_path)
+                                        return NULL;
+
+                                p--;
+
+                                if (*p != '/')
+                                        return NULL;
+                        }
+
+                        return (char*) p;
+                }
+
+                if (m != n)
                         return NULL;
 
-                if (!strneq(p, q, r))
+                if (!strneq(p, q, m))
                         return NULL;
         }
 }

src/basic/path-util.h

@@ -43,9 +43,15 @@ int safe_getcwd(char **ret);
 int path_make_absolute_cwd(const char *p, char **ret);
 int path_make_relative(const char *from, const char *to, char **ret);
 int path_make_relative_parent(const char *from_child, const char *to, char **ret);
-char* path_startswith_full(const char *path, const char *prefix, bool accept_dot_dot) _pure_;
+
+typedef enum PathStartWithFlags {
+        PATH_STARTSWITH_REFUSE_DOT_DOT       = 1U << 0,
+        PATH_STARTSWITH_RETURN_LEADING_SLASH = 1U << 1,
+} PathStartWithFlags;
+
+char* path_startswith_full(const char *path, const char *prefix, PathStartWithFlags flags) _pure_;
 static inline char* path_startswith(const char *path, const char *prefix) {
-        return path_startswith_full(path, prefix, true);
+        return path_startswith_full(path, prefix, 0);
 }
 
 int path_compare(const char *a, const char *b) _pure_;

src/core/cgroup.c

@@ -2808,19 +2808,13 @@ static int unit_prune_cgroup_via_bus(Unit *u) {
                 return -EOWNERDEAD;
 
         /* Determine this unit's cgroup path relative to our cgroup root */
-        const char *pp = path_startswith(crt->cgroup_path, u->manager->cgroup_root);
+        const char *pp = path_startswith_full(
+                        crt->cgroup_path,
+                        u->manager->cgroup_root,
+                        PATH_STARTSWITH_RETURN_LEADING_SLASH|PATH_STARTSWITH_REFUSE_DOT_DOT);
         if (!pp)
                 return -EINVAL;
 
-        _cleanup_free_ char *absolute = NULL;
-        if (!path_is_absolute(pp)) { /* RemoveSubgroupFromUnit() wants an absolute path */
-                absolute = strjoin("/", pp);
-                if (!absolute)
-                        return -ENOMEM;
-
-                pp = absolute;
-        }
-
         r = bus_call_method(u->manager->system_bus,
                             bus_systemd_mgr,
                             "RemoveSubgroupFromUnit",

src/test/test-path-util.c

@@ -756,6 +756,22 @@ TEST(path_startswith) {
         test_path_startswith_one("/foo/bar/barfoo/", "/fo", NULL, NULL);
 }
 
+static void test_path_startswith_return_leading_slash_one(const char *path, const char *prefix, const char *expected) {
+        const char *p;
+
+        log_debug("/* %s(%s, %s) */", __func__, path, prefix);
+
+        p = path_startswith_full(path, prefix, PATH_STARTSWITH_RETURN_LEADING_SLASH);
+        ASSERT_STREQ(p, expected);
+}
+
+TEST(path_startswith_return_leading_slash) {
+        test_path_startswith_return_leading_slash_one("/foo/bar", "/", "/foo/bar");
+        test_path_startswith_return_leading_slash_one("/foo/bar", "/foo", "/bar");
+        test_path_startswith_return_leading_slash_one("/foo/bar", "/foo/bar", NULL);
+        test_path_startswith_return_leading_slash_one("/foo/bar/", "/foo/bar", "/");
+}
+
 static void test_prefix_root_one(const char *r, const char *p, const char *expected) {
         _cleanup_free_ char *s = NULL;