Re: Publically writable directories

[fox () mikey convex com (David DeSimone)]

Bill Pemberton wrote:
> You need to open it like so:
>
> fp_handle = open(fname, O_RDWR|O_CREAT|O_EXCL,0600)
>
>
> The open() call does not follow symlinks.

This is not really true; only the final component of the pathname is
checked for symlinking.  From the man page:

          O_EXCL   If O_EXCL and O_CREAT are set, open() will
                   fail if the file exists.  This can be used to
                   implement a simple exclusive access locking
                   mechanism.  If O_EXCL is set and the last com-
                   ponent of the pathname is a symbolic link, the
                   open will fail even if the symbolic link
                   points to a non-existent name.

This protects against someone creating a file with 666 permissions and
hoping that you will open the file and start writing into it.  It also
protects against someone creating a link to a name that does not exist,
hoping that you will create it.  However, it does not stop someone
from creating a symlink further up the path.  But with a path like
"/tmp/file.$$", this isn't much of an issue.

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