Re: Why you should avoid world-writable directories

[thorpej () NAS NASA GOV (Jason Thorpe)]

On Thu, 24 Dec 1998 00:50:48 -0800
 Jason Thorpe <thorpej () nas nasa gov> wrote:

> On Wed, 23 Dec 1998 09:28:35 +1100
>  Darren Reed <avalon () coombs anu edu au> wrote:
>
>  > In a way, that is exactly the type of thing he is referring to, BUT,
>  > LOCAL_CREDS must be supplied to be received as opposed to just "looked up"
>  > with getpeeruid() (my understanding anyway).
>
> Yes, they are a control message.  This works well for SOCK_DGRAM, but
> not as well for SOCK_STREAM, since w/ SOCK_STREAM you can connect and
> then never send any data, thus the task wanting the credentials never
> gets them.

Actually, let me clarify this just a little.  I was pretty tired when I
wrote that reply :-)

The credentials are supplied by the kernel.  The kernel prevents the
sender from supplying them, for obvious reasons.  Upon reception of
any datagram on a LOCAL_CREDS-enabled socket, the credentials are
available in a control message.  There is one credentials message per
datagram if the option is enabled.

For stream sockets, the credentials are supplied only once, when the
sender first sends data.  If the sender never sends data, the receiver
never recieves the credentials.

In any case, if you (the receiver) enable LOCAL_CREDS on the socket, the
only work you have to do is the receive of the control message and parsing
of it.  The sending application does no additional work.  It is out of the
loop as far as the sending of credentials is concerned.

I don't really see any value in getpeeruid() since it can _only_ apply
to Unix domain sockets.  Why invent a system call with no other use when
you can just use existing general-purpose interfaces?

Jason R. Thorpe                                       thorpej () nas nasa gov
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