fs-util: readlinkat() supports an empty string

From readlinkat(2):
Since Linux 2.6.39, pathname can be an empty string, in which case the
call operates on the symbolic link referred to by dirfd (which should
have been obtained using open(2) with the O_PATH and O_NOFOLLOW flags).

diff --git a/src/basic/fs-util.c b/src/basic/fs-util.c
index ee38e0266a6f9d4ea8d6191a9c28e83e6e3ea1ba..9ba9268d773b860759fa736e23f49b8fe96f1680 100644 (file)
--- a/src/basic/fs-util.c
+++ b/src/basic/fs-util.c
@@ -116,7 +116,11 @@ int rename_noreplace(int olddirfd, const char *oldpath, int newdirfd, const char
 int readlinkat_malloc(int fd, const char *p, char **ret) {
         size_t l = PATH_MAX;
 
-        assert(p);
+        assert(fd >= 0 || fd == AT_FDCWD);
+
+        if (fd < 0 && isempty(p))
+                return -EISDIR; /* In this case, the fd points to the current working directory, and is
+                                 * definitely not a symlink. Let's return earlier. */
 
         for (;;) {
                 _cleanup_free_ char *c = NULL;
@@ -126,7 +130,7 @@ int readlinkat_malloc(int fd, const char *p, char **ret) {
                 if (!c)
                         return -ENOMEM;
 
-                n = readlinkat(fd, p, c, l);
+                n = readlinkat(fd, strempty(p), c, l);
                 if (n < 0)
                         return -errno;
 

diff --git a/src/test/test-fs-util.c b/src/test/test-fs-util.c
index 5de1eea0d4d4850412e29a5204aed4173f7e30cf..ef335b43ae14fb32b550f01849546181f870bdc9 100644 (file)
--- a/src/test/test-fs-util.c
+++ b/src/test/test-fs-util.c
@@ -758,4 +758,39 @@ static int intro(void) {
         return EXIT_SUCCESS;
 }
 
+TEST(readlinkat_malloc) {
+        _cleanup_(rm_rf_physical_and_freep) char *t = NULL;
+        _cleanup_close_ int tfd = -EBADF, fd = -EBADF;
+        _cleanup_free_ char *p = NULL, *q = NULL;
+        const char *expect = "hgoehogefoobar";
+
+        tfd = mkdtemp_open(NULL, O_PATH, &t);
+        assert_se(tfd >= 0);
+
+        assert_se(symlinkat(expect, tfd, "linkname") >= 0);
+
+        assert_se(readlinkat_malloc(tfd, "linkname", &p) >= 0);
+        assert_se(streq(p, expect));
+        p = mfree(p);
+
+        fd = openat(tfd, "linkname", O_PATH | O_NOFOLLOW | O_CLOEXEC);
+        assert_se(fd >= 0);
+        assert_se(readlinkat_malloc(fd, NULL, &p) >= 0);
+        assert_se(streq(p, expect));
+        p = mfree(p);
+        assert_se(readlinkat_malloc(fd, "", &p) >= 0);
+        assert_se(streq(p, expect));
+        p = mfree(p);
+        fd = safe_close(fd);
+
+        assert_se(q = path_join(t, "linkname"));
+        assert_se(readlinkat_malloc(AT_FDCWD, q, &p) >= 0);
+        assert_se(streq(p, expect));
+        p = mfree(p);
+        assert_se(readlinkat_malloc(INT_MAX, q, &p) >= 0);
+        assert_se(streq(p, expect));
+        p = mfree(p);
+        q = mfree(q);
+}
+
 DEFINE_TEST_MAIN_WITH_INTRO(LOG_INFO, intro);