Re: if file exist

["David Thompson" <david.thompson1@worldnet.att.net>]

Mark Biggar <mark.a.biggar@attbi.com> wrote :
> steve_H wrote:
...
> > But the above is not logical. If your function return FALSE, then one
> > does not know if this means the file actually does not exist, or that
> > the function was not able to determine if it exist or not becuase of
> > permission issues.  The user might want to know this.
>
> No, from a computer security point of view, this is exactly what is
> wanted.  A user should see absolutely no difference between "file does
> not exist" and "you don't have permission to see the file".  Otherwise,
> you have introduced a covert information channel.
>
First this only matters if you want/need nondiscretionary controls.
Second it is OK if the third state is not specifically 'file exists but
you don't have access' but rather 'I won't say if the file exists'* --
as in Multics' most-frequent and perhaps most-annoying error
code/message "Insufficient access to return any information".

* assuming there isn't an observable timing difference either

--
- David.Thompson 1 now at worldnet.att.net