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Re: [PATCH 08/11] sockets: Fix socket_sockaddr_to_address_unix() for abs

From: Eric Blake
Subject: Re: [PATCH 08/11] sockets: Fix socket_sockaddr_to_address_unix() for abstract sockets
Date: Fri, 30 Oct 2020 07:39:07 -0500
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.3.1

On 10/30/20 4:04 AM, Markus Armbruster wrote:

>>> +    addr->u.q_unix.path = g_strdup(su->sun_path);
>> This is wrong on at least Linux, where su->sun_path need not be
>> NUL-terminated (allowing file-system Unix sockets to have one more byte
>> in their name);
> Out of curiosity: is this usage portable?  I tried man pages and SUS, no
> luck.

On Linux, 'man 7 unix' says:

>        When binding a socket to an address, Linux is one  of  the  implementa‐
>        tions  that  appends a null terminator if none is supplied in sun_path.
>        In most cases  this  is  unproblematic:  when  the  socket  address  is
>        retrieved,  it  will  be  one  byte  longer than that supplied when the
>        socket was bound.  However, there is one case where confusing  behavior
>        can  result: if 108 non-null bytes are supplied when a socket is bound,
>        then the addition of the null terminator takes the length of the  path‐
>        name beyond sizeof(sun_path).  Consequently, when retrieving the socket
>        address (for example, via accept(2)), if the input addrlen argument for
>        the  retrieving  call  is specified as sizeof(struct sockaddr_un), then
>        the  returned  address  structure  won't  have  a  null  terminator  in
>        sun_path.
>        In  addition, some implementations don't require a null terminator when
>        binding a socket (the addrlen argument is used to determine the  length
>        of  sun_path)  and when the socket address is retrieved on these imple‐
>        mentations, there is no null terminator in sun_path.

along with advice on using strnlen and/or overallocation to handle
various cases in a cleaner manner, and the caveat that if you always use
a name smaller than sun_path you can avoid the tricky code (at the
expense of one byte less in your namespace).

Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org

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