flist: accept the missing-args mode-0 entry in recv_file_entry (#910)

--delete-missing-args (missing_args==2) sends a missing --files-from arg as a
mode-0 entry (IS_MISSING_FILE), the generator's delete signal.  The mode-type
validation in recv_file_entry() rejected mode 0 as an invalid file type,
aborting the transfer with 'invalid file mode 00 ... code 2' before the
generator could act (a regression from 3.4.1).  Allow mode 0 through only when
missing_args==2 (the delete mode -- not --ignore-missing-args, which never
sends a mode-0 entry); all other modes are still rejected.

flist.c

@@ -865,13 +865,18 @@ static struct file_struct *recv_file_entry(int f, struct file_list *flist, int x
 		mode = from_wire_mode(read_int(f));
 		/* Reject modes whose type bits are not one of the standard
 		 * file types; otherwise garbage mode values propagate through
-		 * the file-type checks below unpredictably. */
-		if (!S_ISREG(mode) && !S_ISDIR(mode) && !S_ISLNK(mode)
-		 && !S_ISCHR(mode) && !S_ISBLK(mode)
-		 && !S_ISFIFO(mode) && !S_ISSOCK(mode)) {
+		 * the file-type checks below unpredictably.  mode 0 is the one
+		 * legitimate exception: --delete-missing-args (missing_args==2)
+		 * sends a missing arg as a mode-0 entry (IS_MISSING_FILE), the
+		 * generator's delete signal (#910). */
+		if (mode != 0 || missing_args != 2) {
+		    if (!S_ISREG(mode) && !S_ISDIR(mode) && !S_ISLNK(mode)
+		     && !S_ISCHR(mode) && !S_ISBLK(mode)
+		     && !S_ISFIFO(mode) && !S_ISSOCK(mode)) {
 			rprintf(FERROR, "invalid file mode 0%o for %s [%s]\n",
 				(unsigned)mode, lastname, who_am_i());
 			exit_cleanup(RERR_PROTOCOL);
+		    }
 		}
 	}
 	if (atimes_ndx && !S_ISDIR(mode) && !(xflags & XMIT_SAME_ATIME)) {

testsuite/delete-missing-args-files-from_test.py

@@ -0,0 +1,80 @@
+#!/usr/bin/env python3
+# Functional regression: --delete-missing-args with --files-from aborts the
+# transfer with "invalid file mode 00 ... protocol incompatibility (code 2)"
+# instead of deleting the entries that are missing on the sender.
+#
+# Reported as #910 ("Security fix in flist.c breaks --delete-missing-args with
+# --files-from").
+#
+# Root cause: for a --files-from entry that does not exist on the sender,
+# --delete-missing-args==2 deliberately sends a "missing" file entry with
+# mode == 0 (the generator's signal to delete it on the receiver).  The 3.4.x
+# security mode-validation added to recv_file_entry() (flist.c) rejects mode 0
+# as an invalid file type BEFORE the generator can act on it, so the receiver
+# bails out with a protocol error and nothing is deleted.  Works in 3.4.1.
+#
+# Two scenarios, since a missing FILE and a missing DIRECTORY are sent as
+# distinct mode-0 entries:
+#   * a regular file present on the receiver but absent on the sender, and
+#   * a directory present on the receiver but absent on the sender,
+# both named in --files-from.  Both must be deleted on the receiver.
+#
+# XFAIL until recv_file_entry() accepts the missing-args mode-0 entry again
+# (the accompanying flist.c fix).  Runs at any uid.
+
+import subprocess
+
+from rsyncfns import (
+    SCRATCHDIR, makepath, rmtree, rsync_argv, start_test_daemon, test_fail,
+    test_xfail, write_daemon_conf,
+)
+
+DAEMON_PORT = 12910
+
+mod = SCRATCHDIR / 'recvmod910'    # daemon receive module
+src = SCRATCHDIR / 'src910'
+rmtree(mod)
+rmtree(src)
+makepath(mod / 'ghostdir', src)
+(src / 'keep.txt').write_text("keep-me\n")            # present on sender
+(mod / 'keep.txt').write_text("stale\n")              # will be updated
+(mod / 'ghost.txt').write_text("delete-me-file\n")    # absent on sender -> delete
+(mod / 'ghostdir' / 'inner').write_text("delete-me-dir\n")  # absent on sender -> delete
+
+# --files-from lists one present file plus the two entries that are missing on
+# the sender (a file and a directory) -- those become mode-0 "missing" entries.
+flist = SCRATCHDIR / 'files910.lst'
+flist.write_text("keep.txt\nghost.txt\nghostdir\n")
+
+conf = write_daemon_conf([
+    ('recv', {'path': str(mod), 'read only': 'no'}),
+])
+url = start_test_daemon(conf, DAEMON_PORT)
+
+proc = subprocess.run(
+    rsync_argv('-a', '--delete', '--delete-missing-args',
+               f'--files-from={flist}', f'{src}/', f'{url}recv/'),
+    stdout=subprocess.PIPE, stderr=subprocess.STDOUT, text=True)
+out = proc.stdout or ''
+print(out)
+
+# Bug present: the receiver rejects the mode-0 missing-args entry.
+if 'invalid file mode' in out or (proc.returncode == 2 and (mod / 'ghost.txt').exists()):
+    test_xfail(
+        "#910: --delete-missing-args with --files-from aborts with "
+        "`invalid file mode 00 ... protocol incompatibility (code 2)`.  The "
+        "sender sends mode-0 entries for the missing args (the delete signal), "
+        "but recv_file_entry()'s 3.4.x mode-validation rejects mode 0 before the "
+        "generator can delete them.  To be closed by accepting the "
+        "missing-args mode-0 entry in recv_file_entry().")
+
+# Bug fixed (or absent): both missing args were deleted, the present file kept.
+if proc.returncode != 0:
+    test_fail(f"transfer failed unexpectedly (rc={proc.returncode}); "
+              f"not the #910 mode-00 symptom:\n{out}")
+if (mod / 'ghost.txt').exists():
+    test_fail("missing-arg file ghost.txt was not deleted on the receiver")
+if (mod / 'ghostdir').exists():
+    test_fail("missing-arg directory ghostdir was not deleted on the receiver")
+if not (mod / 'keep.txt').is_file() or (mod / 'keep.txt').read_text() != "keep-me\n":
+    test_fail("present file keep.txt was not delivered/updated correctly")