hlink/main: guard the F_SUM checksum-slot lifetime

hlink.c must confirm S_ISREG before quick_check_ok(FT_REG,...) reads
F_SUM, and start_server() must set sender_keeps_checksum when a daemon
sender runs -c with a %C log format so make_file() allocates
SUM_EXTRA_CNT. Without these, F_SUM() reads past the pool slot and (for
%C) hex-encodes adjacent heap into the transfer log.

Co-authored-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

hlink.c

@@ -420,7 +420,14 @@ int hard_link_check(struct file_struct *file, int ndx, char *fname,
 				}
 				break;
 			}
-			if (!quick_check_ok(FT_REG, cmpbuf, file, &alt_sx.st))
+			/* Content-based basis match only applies to regular
+			 * files: for a hard-linked symlink/device/special the
+			 * exact-inode check above is the only meaningful test,
+			 * and quick_check_ok(FT_REG, ...) would read F_SUM()
+			 * on a file_struct that has no SUM_EXTRA_CNT space
+			 * (recv_file_entry only allocates it for S_ISREG). */
+			if (!S_ISREG(file->mode)
+			 || !quick_check_ok(FT_REG, cmpbuf, file, &alt_sx.st))
 				continue;
 			statret = 1;
 			if (unchanged_attrs(cmpbuf, file, &alt_sx))

main.c

@@ -1292,6 +1292,17 @@ void start_server(int f_in, int f_out, int argc, char *argv[])
 
 	if (am_sender) {
 		keep_dirlinks = 0; /* Must be disabled on the sender. */
+
+		/* Mirror client_run()'s sender_keeps_checksum check: a daemon-
+		 * as-sender with -c and a `log format` containing %C will read
+		 * F_SUM(file) in log_formatted(), so make_file() must allocate
+		 * SUM_EXTRA_CNT.  Without this, F_SUM() reads past the pool slot
+		 * and hex-encodes adjacent heap into the transfer log. */
+		if (always_checksum
+		 && (log_format_has(stdout_format, 'C')
+		  || log_format_has(logfile_format, 'C')))
+			sender_keeps_checksum = 1;
+
 		if (need_messages_from_generator)
 			io_start_multiplex_in(f_in);
 		else