From: Carlos O'Donell <codonell@redhat.com>
To: Eric Blake <eblake@redhat.com>, libc-help@sourceware.org
Cc: "libguestfs@redhat.com" <libguestfs@redhat.com>
Subject: Re: [Libguestfs] alternatives for hooking dlopen() without LD_LIBRARY_PATH or LD_AUDIT?
Date: Fri, 14 Feb 2020 22:20:00 -0000 [thread overview]
Message-ID: <acc4ddf1-699e-f62d-aba0-421ca4c6b617@redhat.com> (raw)
In-Reply-To: <39d94f06-5793-16b5-1372-df3248924671@redhat.com>
On 2/14/20 4:29 PM, Eric Blake wrote:
> On 2/14/20 1:02 PM, Eric Blake wrote:
>
>> Writing my own dlopen() wrapper directly in nbdkit seems like a
>> non-starter (my override has to come from a shared library before
>> it can replace the shared version that would be imported from -ldl,
>> at least for all subsequent shared library loads that want to bind
>> to the override).
>
> Maybe I spoke too soon. I've tried another approach that looks like
> it will do what I want: put my shim dlopen() in a shared library, but
> link nbdkit against that shared library PRIOR to -ldl (so that name
> lookup for dlopen resolves there first). The shim library in turn
> depends on -ldl so that dlsym(RTLD_NEXT, "dlopen") still lets me get
> to the real dlopen. And by linking it directly into nbdkit, rather
> than into the nbdkit-vddk-plugin.so that gets loaded later, the first
> bound dlopen() in use for all subsequent loads is from my shim. It's
> still a bit less clean than I'd like (it requires tighter coupling
> between nbdkit and nbdkit-vddk-plugin.so than what used to exist),
> but the fact that it works without dlmopen() or LD_LIBRARY_PATH is in
> its favor. I'm now polishing up the experiment, and will post the
> patch when it's ready.
I think that's the best solution you're going to get.
The alternatives (LD_LIBRARY_PATH, direct loader invocation, dlmopen)
all have limitations that aren't helpful to your particular design.
You have design strategies that look like this:
- Move the object higher in the search order at link time (interposition)
- Investigate static link order.
- Investigate dynamic loader search order
- Change what object is searched for
- LD_LIBRARY_PATH, DT_RPATH, DT_RUNPATH, etc.
- LD_AUDIT with la_objsearch()
Your "shim dlopen()" is a case of moving the static link order
around to ensure your shim is used first.
--
Cheers,
Carlos.
next prev parent reply other threads:[~2020-02-14 22:20 UTC|newest]
Thread overview: 15+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-02-14 19:02 Eric Blake
2020-02-14 21:29 ` [Libguestfs] " Eric Blake
2020-02-14 22:20 ` Carlos O'Donell [this message]
2020-02-17 15:04 ` Eric Blake
2020-02-17 15:12 ` Florian Weimer
2020-02-18 21:32 ` Eric Blake
2020-02-21 12:19 ` Florian Weimer
2020-02-21 13:26 ` Eric Blake
2020-02-21 14:51 ` Richard W.M. Jones
2020-02-21 15:00 ` Florian Weimer
2020-02-21 15:40 ` Richard W.M. Jones
2020-02-21 16:02 ` Florian Weimer
2020-02-21 17:42 ` Richard W.M. Jones
2020-02-21 20:28 ` Eric Blake
2020-02-21 20:21 ` Eric Blake
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