Do not let rsync() write through symlinks in the destination

With LITE_FOPT_RSYNC_LINKS, rsync() leaves symlinks in the destination.
If the source later has a regular file or a directory by the same name,
the next run follows the old link and writes to whatever it points at,
which can be outside the destination tree.

Remove such a link before copying.

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>

doc/API.md

@@ -251,6 +251,10 @@ Generic Functions
   symlink to a file, or a dangling symlink, is recreated in the
   destination instead.  Symlinks to directories are always followed.
 
+  With `LITE_FOPT_RSYNC_LINKS` a symlink in the destination is never
+  followed.  If one sits where a file or directory from the source
+  goes, it is removed first.
+  
   In releases prior to v2.0 the argument controlling pruning was called
   `delete`, it is now called `opt`.  The APIs are 100% compatible if the
   value `1` was used.

src/rsync.c

@@ -36,7 +36,7 @@
 #include "lite.h"
 
 static int copy(char *src, char *dst, int opt);
-static int mdir(char *buf, size_t buf_len, char *dir, char *name, struct stat *st);
+static int mdir(char *buf, size_t buf_len, char *dir, char *name, struct stat *st, int opt);
 static int prune(char *dst, char **new_files, int new_num);
 static int set_mtime(char *fn, struct stat *st);
 
@@ -69,6 +69,10 @@ static int set_mtime(char *fn, struct stat *st);
  * text, replacing any file or symlink already there.  Symlinks to
  * directories in @p src are always followed.
  *
+ * With ::LITE_FOPT_RSYNC_LINKS a symlink in @p dst is never followed.
+ * If one sits where a file or directory from @p src goes, it is removed
+ * first.
+ *
  * The filter callback, @p filter, if provided, is used to determine
  * what files to include from the source directory when backing up.  If
  * a file is to be skipped the callback should simply return zero.
@@ -113,7 +117,7 @@ int rsync(char *src, char *dst, int opt, int (*filter)(const char *file))
 		if (stat(src, &st))
 			return 1;
 
-		if (mdir(dest, sizeof(dest), dst, ptr, &st))
+		if (mdir(dest, sizeof(dest), dst, ptr, &st, opt))
 			return 1;
 		dst = dest;
 		do_mtim = keep_mtim;
@@ -133,7 +137,7 @@ int rsync(char *src, char *dst, int opt, int (*filter)(const char *file))
 				continue;
 			}
 
-			if (mdir(dst2, sizeof(dst2), dst, files[i], &sb)) {
+			if (mdir(dst2, sizeof(dst2), dst, files[i], &sb, opt)) {
 				result++;
 				continue;
 			}
@@ -166,20 +170,30 @@ int rsync(char *src, char *dst, int opt, int (*filter)(const char *file))
 	return result;
 }
 
+/* Leaves errno alone, a missing @path is not an error to rsync() */
+static int islink(const char *path)
+{
+	int rc, saved_errno = errno;
+	struct stat st;
+
+	rc = !lstat(path, &st) && S_ISLNK(st.st_mode);
+	errno = saved_errno;
+
+	return rc;
+}
+
 static int copy(char *src, char *dst, int opt)
 {
 	char *dest = NULL, *temp = NULL, *file = NULL;
 	int result = 1, saved_errno, made_temp = 0;
 	int copt = opt & LITE_FOPT_KEEP_MTIME;
 
-	if (opt & LITE_FOPT_RSYNC_LINKS)
+	if (!(opt & LITE_FOPT_RSYNC_LINKS)) {
-		copt |= LITE_FOPT_COPYFILE_SYM;
+		copyfile(src, dst, 0, copt);
-
-	copyfile(src, dst, 0, copt);
-	if (errno != EEXIST)
 		return errno ? 1 : 0;
+	}
+	copt |= LITE_FOPT_COPYFILE_SYM;
 
-	/* Replace an existing destination only after the new link is ready. */
 	if (fisdir(dst)) {
 		char *base = strrchr(src, '/');
 
@@ -190,6 +204,21 @@ static int copy(char *src, char *dst, int opt)
 		if (!dest)
 			return 1;
 	}
+
+	/*
+	 * A link in the destination, e.g. from a previous run, may point
+	 * anywhere.  Remove it so copyfile() cannot write through it.
+	 */
+	if (islink(dest) && !islink(src))
+		unlink(dest);
+
+	copyfile(src, dst, 0, copt);
+	if (errno != EEXIST) {
+		result = errno ? 1 : 0;
+		goto exit;
+	}
+
+	/* Replace an existing destination only after the new link is ready. */
 	if (asprintf(&temp, "%s/.rsync-XXXXXX", dirname(strdupa(dest))) < 0)
 		goto exit;
 	if (!mkdtemp(temp))
@@ -218,9 +247,14 @@ exit:
 }
 
 /* Creates dir/name @mode ... skipping / if dir already ends so. */
-static int mdir(char *buf, size_t buf_len, char *dir, char *name, struct stat *st)
+static int mdir(char *buf, size_t buf_len, char *dir, char *name, struct stat *st, int opt)
 {
 	snprintf(buf, buf_len, "%s%s%s", dir, fisslashdir(dir) ? "" : "/", name);
+
+	/* Same as in copy(), do not descend into a stale link. */
+	if ((opt & LITE_FOPT_RSYNC_LINKS) && islink(buf))
+		unlink(buf);
+
 	if (mkdir(buf, st->st_mode)) {
 		if (EEXIST != errno)
 			return 1;

test/copyfile-symlink.c

@@ -15,6 +15,39 @@ static void check_link(const char *path, const char *target)
 	fail_unless(memcmp(buf, target, len) == 0);
 }
 
+/* A link left in the destination by an earlier run must never be written through. */
+static void rsync_stale_links(void)
+{
+	struct stat st;
+	FILE *fp;
+
+	fail_unless(mkdir("src", 0700) == 0 && mkdir("dst", 0700) == 0);
+	fail_unless(touch("victim") == 0);
+	fail_unless(symlink("../victim", "src/a") == 0);
+	fail_unless(symlink("../victims", "src/d") == 0);
+	fail_unless(rsync("src/", "dst/", LITE_FOPT_RSYNC_LINKS, NULL) == 0);
+	check_link("dst/a", "../victim");
+	check_link("dst/d", "../victims");
+
+	/* The source links become a file and a directory, and dst/d stops dangling. */
+	fail_unless(unlink("src/a") == 0 && unlink("src/d") == 0);
+	fp = fopen("src/a", "w");
+	fail_unless(fp && fputs("data", fp) >= 0 && fclose(fp) == 0);
+	fail_unless(mkdir("src/d", 0700) == 0 && touch("src/d/f") == 0);
+	fail_unless(mkdir("victims", 0700) == 0);
+	fail_unless(rsync("src/", "dst/", LITE_FOPT_RSYNC_LINKS, NULL) == 0);
+
+	fail_unless(lstat("dst/a", &st) == 0 && S_ISREG(st.st_mode) && st.st_size == 4);
+	fail_unless(stat("victim", &st) == 0 && st.st_size == 0);
+	fail_unless(lstat("dst/d", &st) == 0 && S_ISDIR(st.st_mode));
+	fail_unless(fexist("dst/d/f") && !fexist("victims/f"));
+
+	fail_unless(unlink("src/a") == 0 && unlink("src/d/f") == 0 && rmdir("src/d") == 0);
+	fail_unless(unlink("dst/a") == 0 && unlink("dst/d/f") == 0 && rmdir("dst/d") == 0);
+	fail_unless(rmdir("src") == 0 && rmdir("dst") == 0);
+	fail_unless(unlink("victim") == 0 && rmdir("victims") == 0);
+}
+
 int main(void)
 {
 	char tmp[] = "/tmp/libite-copylink-XXXXXX";
@@ -106,6 +139,7 @@ int main(void)
 		fail_unless(unlink("protected/link") == 0 && rmdir("protected") == 0);
 	}
 	fail_unless(unlink("copied") == 0 && unlink("link") == 0);
+	rsync_stale_links();
 
 	errno = 0;
 	fail_unless(copyfile("dir", "copied", 0, LITE_FOPT_COPYFILE_SYM) == 0 && errno == EISDIR);