[Bug dynamic-link/30134] DT_AUDIT is ignored for dlopen()ed solib

["fweimer at redhat dot com" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=30134

--- Comment #4 from Florian Weimer <fweimer at redhat dot com> ---
(In reply to Stas Sergeev from comment #2)
> (In reply to Florian Weimer from comment #1)
> > The link editor (ld) is expected to
> > copy audit entries to the main program as DT_DEPAUDIT entries, so that the
> > auditors are loaded at run time. But that only works for explicit linking,
> > it fails with dlopen.
> 
> It is possible to implement DT_AUDIT for
> dlopen(), isn't it?

Maybe in theory, but it's not how the current implementation is structured. It
depends on an auditing flag in RELRO memory for hardening, so that the auditing
(function) pointers cannot be overwritten if the process did not start with
auditing enabled.

We would also have to figure out all the small corner cases and work out what
to do for them. For example, auditing is currently a process-global operation,
and the auditing modules are never unloaded after being created. Adding dlopen
support might change that: auditing could perhaps be restricted to the local
search scope, and dlclose might unload the auditor as well. Or not, it's hard
to tell without some exploration.

We also have hard limits on the number of auditors. The explicit one (namespace
count) is perhaps somewhat easy to overcome, but there is also the static TLS
space consumption by new auditing namespaces.

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