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Ensuring symbol resolution order at runtime
- From: "Kevin P. Fleming" <kpfleming at digium dot com>
- To: binutils at sourceware dot org
- Date: Fri, 20 Jan 2012 13:34:08 -0600
- Subject: Ensuring symbol resolution order at runtime
(please keep me in the CC list, as I'm not subscribed. thanks)
I'll ask this question in the context of 'modern' Linux (Ubuntu 11.10,
for example) using ELF binaries and current binutils, but I'm interested
in knowing whether the behavior I desire, if it is in fact how the
static/dynamic linkers operate, is specified by any standards, or is
just 'common practice'.
Scenario 1: An executable is being linked, and the linker is provided
the names of two static libraries. Both of those libraries export a
symbol named "foo", and one of the objects being linked into the
executable refers to that symbol.
In this situation, it is understood that the linker will choose a symbol
"foo" to satisfy the reference from the *first* library specified in the
linker's arguments that provides a symbol "foo".
Scenario 2: An executable is being linked, and the static linker is
provided the names of two shared libraries. Both of those libraries
export a symbol named "foo", and one of the objects being linked into
the executable refers to that symbol.
In this situation, it is understood that the static linker will locate a
symbol "foo" to satisfy the reference from the *first* library specified
in the linker's arguments that provides a symbol "foo". This is fine.
However, at runtime, when the dynamic linker has the responsibility for
loading these shared libraries into the process and resolving symbols,
is there any guarantee that the the *same* "foo" will be chosen by the
dynamic linker?
Scenario 3: An executable is being linked, and the static linker is
provided the name of a shared library. That shared library contains a
reference to another shared library. Both of those libraries export a
symbol named "foo", and one of the objects being linked into the
executable refers to that symbol.
In this situation, the static linker only has the symbol "foo" from the
first library available to satisfy the reference, because it was not
made explicitly aware of the second library that also provides "foo". At
runtime, it seems reasonable to assume that the dynamic linker will load
the first shared library into the process before the second shared
library, and *should* resolve symbol references from the executable into
that library before loading the second shared library into the process.
I'm trying to work on a problem related to multiple shared libraries
linking to libssl.so (from OpenSSL), and need to come up with a way to
ensure that only *one* library calls the initialization functions from
libssl, without requiring any cooperating behavior from the other
libraries that would normally do it as well. My approach so far has been
to 'mask' the real functions from libssl.so in another shared library
that is loaded *first* into the process, thereby ensuring that no matter
what libraries are loaded into the process later, they will not be able
to gain access to the underlying functions from libssl.so (other than
via exploiting dlsym() to do so explicitly).
So, is this approach reasonable, and if so, is it backed up by standards
or other documents that prescribe the linker's behavior for these
scenarios? I'm really not keen on relying on any assumptions about the
dynamic linker's behavior (other than that it will follow the relevant
standards if it claims to do so).
Thanks in advance for any light you can shed on this :-)
--
Kevin P. Fleming
Digium, Inc. | Director of Software Technologies
Jabber: kfleming@digium.com | SIP: kpfleming@digium.com | Skype: kpfleming
445 Jan Davis Drive NW - Huntsville, AL 35806 - USA
Check us out at www.digium.com & www.asterisk.org