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Userland filesystems and LD_PRELOAD



I am interested in userland filesystems using system call hooking and
no kernel support (not even Coda or NFS). However in the documentation of
one such filesystem, avfs (http://www.inf.bme.hu/~mszeredi/avfs/) there
reads:

<CLIP>
The 'preload' method works by "overriding" the system calls in the
libc library, and replacing them with versions that can emulate
virtual files.

Since GLIBC 6.1 this "overriding" is made impossible for many
systemcalls, and so this method cannot be used anymore. This change
and its effects have been discussed with the GLIBC developers, and the
result has been that it will remain so in the future.
</CLIP>

So how and why is this overriding made impossible?

I think there would be software who could use this method, if hooking
filesystem calls (just like hooking malloc, DNS and user database) would
be possible. And not all users have root access (so they cannot use
kernel tricks).

- Jani


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