Created attachment 13732 [details]
patch to handle argc == 0

$ bin/rsync --version
rsync  version 3.1.3dev  protocol version 31

this exist in 3.1.2 as well. 

rsyncd reads in arguments from the connecting client. These are parsed in parse_arguments.parse_arguments parses what it can via popt, and increments argv and decrements argc to reflect the consumed values. 

The problem is that do_server_sender is expecting at least 1 argument to use as dir

786     char *dir = argv[0];

When there are no arguments in argv this leads a a NULL deref. 

ASAN:SIGSEGV
=================================================================
==7407==ERROR: AddressSanitizer: SEGV on unknown address 0x000000000000 (pc 0x000000453662 bp 0x7fffffffae90 sp 0x7fffffffae50 T0)
    #0 0x453661 in do_server_sender /home/raj/rsync/rsync/main.c:786
    #1 0x4556a0 in start_server /home/j/rsync/rsync/main.c:1110
    #2 0x4b202a in rsync_module /home/j/rsync/rsync/clientserver.c:1007
    #3 0x4b2a49 in start_daemon /home/j/rsync/rsync/clientserver.c:1135
    #4 0x48f56e in start_accept_loop /home/j/rsync/rsync/socket.c:618
    #5 0x4b320a in daemon_main /home/j/rsync/rsync/clientserver.c:1237
    #6 0x4582da in main /home/j/rsync/rsync/main.c:1627
    #7 0x7ffff64d866f in __libc_start_main (/lib64/libc.so.6+0x2066f)
    #8 0x4047c8 in _start (/home/j/rsync/asan/bin/rsync+0x4047c8)

The attached patch will check for argc == 0 and exit_cleanup.