Ensuring symbol resolution order at runtime

[Kevin P. Fleming]

On 01/20/2012 03:52 PM, Cary Coutant wrote:

> As long as the runtime loader sees the same libraries that the linker
> saw, yes, there is a guarantee. However, if lib1.so is replaced with a
> library that does not define "foo", then the dynamic loader will bind
> "foo" to the definition in lib2.so without any complaint.

So, to be as specific as I can, not only will the runtime loader see 
those same libraries, but it will look at them in the *same order* that 
the static linker did (presumably because the static linker will list 
them in the linked executable in the order it processed them, and the 
dynamic linker follows that order).

> I'd think a better approach would be to use a static variable in the
> initialization function to protect it from being called twice. Even
> better would be to have libssl's initialization functions actually
> marked as initialization functions so that they run automatically when
> the library is loaded.

No argument here; unfortunately the maintainers of OpenSSL have not seen 
fit to do that, even though this problem has been known about for quite 
some time. We are not the first project to have to work around the 
problem in our own code, and even other projects that use OpenSSL and 
produce their own libraries for consumption (PostgreSQL, for example) 
have made accommodations in their libraries to allow the eventual 
application using the combined libraries to be able to avoid this 
complication. Unfortunately not every library that uses OpenSSL has made 
these accommodations, so we need a 'belt and suspenders' approach that 
we can rely on.

-- 
Kevin P. Fleming
Digium, Inc. | Director of Software Technologies
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