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From: "janderson at rice dot edu" <sourceware-bugzilla@sourceware.org>
To: glibc-bugs@sourceware.org
Subject: [Bug dynamic-link/30127] [rfe]: enable ld audit at run-time
Date: Thu, 02 Mar 2023 16:13:37 +0000
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https://sourceware.org/bugzilla/show_bug.cgi?id=3D30127

--- Comment #31 from Jonathon Anderson <janderson at rice dot edu> ---
(In reply to Stas Sergeev from comment #30)
> Hi Jonathon, so if you are satisfied with the current
> solution (return EINVAL for disallowed audit call-backs),
> then maybe you can give me an ACK? :)
> I am not aware of any problems or a pending review
> comments for the last (v6) version of the patch.

Busy week for me, expect more slow responses. :)

I can't say the current solutions are appealing. You've made it very diffic=
ult
for a dynamically-loaded auditor to accomplish any auditor-like task, outsi=
de
of a hypothetical callback (la_premap) that hasn't been mainstreamed yet. On
top of that, I'm still a bit leery of letting the application observe the
auditors at all (outside of _r_debug).

Before your patches, there are parts of your use case that I am not convinc=
ed
do not have alternative solutions. One doesn't hack Glibc before exploring =
all
other alternatives, so I assume you've tried these but I'd like the context=
 to
know why they don't work:

You mentioned in another ticket
(https://sourceware.org/bugzilla/show_bug.cgi?id=3D30007#c2) that the whole=
 point
of this is to access data objects in the solib. Do you write this solib? If=
 you
do, can you replace these static objects with pointers that are mmap'd into=
 the
right location (e.g. initialized in an init() function or init constructors=
)?
If you don't, these are data structures that (a) part of the public API for=
 the
solib and (b) are certainly identical (alignment padding included) between =
the
64-bit and 32-bit ABI?

Could you override mmap itself for the files you want mapped in the right
locations? If LD_PRELOAD won't fly, newer kernels have syscall user dispatch
[1] for doing this at the syscall level.

You mentioned DT_AUDIT wouldn't work, so I presume the executable is out of
your control, e.g. you're extending a VM with an solib plugin? If so, do you
not have the access to set LD_AUDIT in the environment before launching the
executable? You can always make your auditor "no-op" except when needed.

[1]: https://docs.kernel.org/admin-guide/syscall-user-dispatch.html

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