Re: GNU bash, 3.00.15(1)-release, referenced cmd in cwd executes alternate cmd

[Bob Proulx]

Matthew Woehlke wrote:
> Apparently selectively shadowing libc is non-trivial... any 
> suggestions/hints?

Not so much non-trivial as perhaps non-obvious.  The dynamic loader is
part of libc and so by the time the program tries to use
LD_LIBRARY_PATH it is already too late because it has already loaded
the system libc.

Therefore LD_LIBRARY_PATH is not useful for overriding libc and only
works for non-libc libraries.  To override libc you have to call the
ld.so directly.  Also you don't want LD_LIBRARY_PATH to hang around
causing trouble for other children processes that might need something
still different so you want to use the --library-path option instead,
then there is no lingering smell.

Given the following setup with copies of glibc and other lib files in
a local "sandbox".

  ./mylib/ld-linux.so.2
  ./mylib/libc.so.6
  ./mylibexec/myprog

Then this script in ./mybin/myprog works:

  #!/bin/bash
  MYAPPDIR=$(dirname $(dirname $0))
  exec -a $MYAPPDIR/mybin/myprog $MYAPPDIR/mylib/ld-linux.so.2 --library-path 
$MYAPPDIR/mylib $MYAPPDIR/mylibexec/myprog "$@"

Using the bash specific 'exec -a ALIAS' works well here.  It keeps the
application itself from knowing that it has been overridden.  It does
not need to know, and depending upon the application knowing would
confuse it, such as when it tries to use $(dirname $0) for any reason.

The key points are:

 * Call the ld.so (aka ld-linux.so.2) directly
 * Don't use LD_LIBRARY_PATH (because it messes up later progs)
 * Put the real program off to the side and use a wrapper
 * Use 'exec -a NAME' to hide this from the program itself

Here are pointers to previous messages that I have posted about how to
use ld.so wrapping to get a particular libc.

  http://gcc.gnu.org/ml/gcc-help/2006-07/msg00126.html
  http://svn.haxx.se/users/archive-2005-05/1727.shtml

Hope that helps!
Bob