A more abstract view of the generalized open() issue

December 12, 2007

Part of the generalized open() problem is that such generalized open()s silently cross what I will call 'security contexts'. Here a security context is both what you can do and where the data is coming from; read versus write versus run a pipe, local files versus remote URLs, and so on.

When something is security sensitive (and open() is here), it should either be explicit about what security context it is operating in, or it should at least be explicit about the fact that it is crossing them. In other words, this is one of the places where you deliberately want to break abstractions, so that there is no possibility of code accidentally operating in the wrong security context.

As a corollary, namespaces that cross security contexts are a danger sign. Firefox has had problems with because its internal files and resources are partly named using URIs in the chrome:// scheme, so lots of things that accept URLs have to be blocked from using them (and sometimes stuff using chrome:// URLs had to be blocked from using other sorts of URLs). And of course there's the great fun to be had because browsers accept 'javascript:' as a scheme in URLs.

(I suspect that these things sneak into languages partly because the language designers did not see them as crossing security boundaries, and in part I think this may be because security boundaries sometimes only become obvious with painful experience. After all, no one deliberately puts security holes in their language or their library.)

Written on 12 December 2007.
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Last modified: Wed Dec 12 23:26:23 2007
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