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[Gcl-devel] Re: function addresses and ld.so

From: Andrew Pinski
Subject: [Gcl-devel] Re: function addresses and ld.so
Date: Wed, 13 Aug 2003 20:21:32 -0400

On Wednesday, Aug 13, 2003, at 20:11 US/Eastern, Zack Weinberg wrote:

Richard Henderson <address@hidden> writes:

On Wed, Aug 13, 2003 at 04:35:36PM -0700, David Mosberger wrote:
Wouldn't, e.g., LD_PRELOADing something break this assumption?

Yes.  Or, indeed, just recompiling the library could result
in different PLT offsets within the DSO, even on x86.

This behaviour is completely broken.  It'll never work reliably.

A tactic that _will_ work is to mimic ld.so's PLT stubs.  I'll use
sqrt() as an example:

extern double sqrt(double);
static double stub_sqrt(double);

// this is the value that gets written to the unexec file
double (*ptr_sqrt)(double) = stub_sqrt;

double stub_sqrt(double x)
  ptr_sqrt = sqrt;
  return ptr_sqrt(x);

Generate one of these stubs for every function you care about, and
bob's your uncle.  Just make sure that none of the stubs get called
before the unexec file is written out.

Unfortunately this won't work for variadic functions.

There is no portable way that I know of to make the stub _be_ the PLT
stub, which is kind of a shame, as it would be (marginally) more
efficient and would work for variadics.

Apple uses this exact technic in mach-o (and works with prebinding) but with an asm function to "bootstrap" the c function. The stub is only called once after setting the lazy-pointer which points to the real function.
The mach-o stuff to do this is in the cc tools project of Darwin.

Andrew Pinski

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