Re: Minimal program crashes, depending on linker script

[Paul Pluzhnikov]

Andre Poenitz <poenitz@htwm.de> writes:

> Is there a possibility to 'relink' an existing library such that the
> result of this operaton would be the same as if -Bsymbolic had been
> given at link time?

No. First, the DSO result of a link is considered "final", further
modification is difficult if not impossible.

Second, '-Bsymbolic' doesn't just set DT_SYMBOLIC tag (which I
initially thought was all it did). It also changes the way that
intra-library calls are performed (directly, rather then through PLT).

If you only had one "problem" symbol, you could binary-edit the DSO
(as in emacs hexl-mode), and "retarget" all calls to
"problemSymbol@plt" to go directly to "problemSymbol" instead by
patching its address into the call instruction. Since all direct
calls on x86 are PC-relative, you'd have to compute offset separately
for each call, so this is more tedious if problemSymbol is called
a lot.

Another alternative is to perform equivalent patch at runtime:
find the GOT for the problemSymbol, and simply put correct address
in there. This requires understanding of ELF image in memory and
the process of lazy resolution, but isn't too difficult.

> [Btw, am I the only one thinking that using '-Bsymbolic' should rather
> be the common case instead of the exception?]

Initially, the motivation for current behaviour was to emulate what
users came to expect with archive libraries:

  if you wanted myfprintf.o to be used, you simply linked that
  before libc.a, and magically all calls to fprintf() went your way.

This would have been broken if printf() [which you didn't replace]
called libc.so`fprintf() directly (instead of through PLT), as
would happen with -Bsymbolic.

Another thing that would break if -Bsymbolic was the default,
is LD_PRELOAD, which is often very useful for e.g. debug malloc.

AIX is the only UNIX I know where (effectively) -Bsymbolic is
the default, and they had to do special hack to allow for
user-replaceable malloc. Good luck intercepting/redirecting any
other libc functions.

In fact, their linking model caused so many problems with porting,
that they recently introduced "deferred linking", which effectively
restores the shared library behaviour to that of all other UNIXes.

Win32 also effectively uses the same model, resulting in a whole slew
of complications, such as: you shalt not free() memory in FOO.DLL,
if that memory was malloc()ed in BAR.DLL (unless both were linked
with /MD flag).

Cheers,
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