[erlang-questions] unix domain sockets with abstract namespace: can't use all 108 bytes

Bekes, Andras G Andras.Bekes@REDACTED
Mon Mar 27 18:03:40 CEST 2017

Hi All,

I need to communicate with unix domain sockets using the abstract namespace and the server (which is not under my control) opens the port with all 108 bytes of the path used.
If I understand it correctly, the server/client can decide to indicate the number of actual bytes used in the 3rd parameter to the bind/connect syscalls (this what Erlang does), but it is also possible to use all 108 bytes and fill the unused part with 0 bytes (this is what my server does).

Apparently, I can fill the path with 0 bytes up to 107 only. If I try to pass a 108-byte address to gen_tcp:connect, I get a badarg exception:

> Sun_path_length=108.

> UDS_path = <<"whatever">>.

> Fill_length = 8*(Sun_path_length-byte_size(UDS_path)-1).

> Address = {local,<<0, UDS_path/binary,0:Fill_length>>}.

> Port = 0.

> gen_tcp:connect(Address,Port,[local],infinity).
(<0.236.0>) call gen_tcp:connect({local,<<0,119,104,97,116,101,118,101,114,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,
(<0.236.0>) call gen_tcp:connect1({local,<<0,119,104,97,116,101,118,101,114,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,
(<0.236.0>) call gen_tcp:try_connect([{local,<<0,119,104,97,116,101,118,101,114,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,
(<0.236.0>) returned from gen_tcp:try_connect/6 -> {error,einval}
** exception exit: badarg
     in function  gen_tcp:connect/4 (gen_tcp.erl, line 149)
(<0.236.0>) returned from gen_tcp:connect1/4 -> {error,einval}
(<0.236.0>) exception_from {gen_tcp,connect,4} {exit,badarg}

If I reduce the path to 107 bytes, it works. With strace, I can see the connect syscall:
connect(19, {sa_family=AF_FILE, path=@"whatever"...}, 109) = -1 ECONNREFUSED (Connection refused)
When I strace the official client of this server, the 3rd parameter to the connect syscall is 110 (108 + 2), regardless of the actual length of the path.

Apparently, with Erlang it is not possible to use all 108 bytes. I should only get a badarg error at 109 bytes, not 108.
Seems to me that this is a bug in the Erlang implementation. What do you think?

Thank you very much,

Andras G. Bekes

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