How to run gdb with LD_PRELOAD?

Question

I have a program using LD_PRELOAD. The program should be run like this, "LD_PRELOAD=/path/to/libfoo.so qemu -U LD_PRELOAD a.out", if without gdb.

Here are what I did while running gdb.

(gdb) set environment LD_PRELOAD=/nfs_home/chenwj/tools/lib/libdbo.so

(gdb) file /nfs_home/chenwj/tools/bin/qemu-i386

(gdb) r -U LD_PRELOAD bzip2_base.i386-m32-gcc44-annotated input.source 1

But gdb gave me the error below

Starting program: /nfs_home/chenwj/tools/bin/qemu-i386 -U LD_PRELOAD bzip2_base.i386-m32-gcc44-annotated input.source 1

bash: open "/bin/bash" failed: Permission denied

During startup program exited with code 66.

Any sugguestion appreciated.

Regards, chenwj

Answer 1

GDB does not invoke your executable directly. Instead, it does

bash -c '/nfs_home/chenwj/tools/bin/qemu-i386  -U LD_PRELOAD bzip2_base.i386-m32-gcc44-annotated input.source 1'

This is done so that bash takes care of I/O redirection (which you are not using).

My guess is that /bin/bash doesn't work when LD_PRELOAD=libdbo.so is in effect, though I don't understand the exact nature of failure.

One way to work around this problem is to create a wrapper executable, implementing C equivalent of this:

export LD_PRELOAD=/nfs_home/chenwj/tools/lib/libdbo.so
exec /nfs_home/chenwj/tools/bin/qemu-i386 "$@"

and debug that executable (without setting LD_PRELOAD). You'll see an extra SIGTRAP when the wrapper execve()s the wrapped qemu-i386, which you should ignore and continue.