Re: Xinetd /tmp race?

[casper () HOLLAND SUN COM (Casper Dik)]

> 1. I don't think that this is safe over NFS.

Not with NFSv2, but it is safe w/ NFSv3.

> The Linux open(2) manpage says:
>
>       O_EXCL When used with O_CREAT, if the file already  exists
>              it  is  an error and the open will fail.  O_EXCL is
>              broken on NFS file systems, programs which rely  on
>              it for performing locking tasks will contain a race
>              condition.  The solution for performing atomic file
>              locking using a lockfile is to create a unique file
>              on the same fs (e.g.,  incorporating  hostname  and
>              pid),  use  link(2)  to make a link to the lockfile
>              and use stat(2) on the unique file to check if  its
>              link  count  has  increased  to  2.  Do not use the
>              return value of the link() call.
>
> 2. The current behaviour is to append to an existing file if it
> exists, or to create a new file if it doesn't. Adding O_EXCL would
> break this behaviour.

> IMHO, a better approach in this case would be to use a directory which
> isn't world-writable e.g. /var/run, /var/log etc.

Seconded.

Casper