I get the following error when I try to run a script I have only execution access for:
uname: symbol lookup error: /home/dumindara/random/sotest/a.out: undefined symbol: dlsym
This is after I have set LD_PRELOAD environment variable to /home/dumindara/random/sotest/a.out.
a.out has a test malloc function, and calls dlsym internally.
I don't get this problem when running ls. Most processes do give this error. Why does this happen and what can I do to make it work?
I assume that your a.out file is a shared object and not a executable and move on...
dlsym() is a function from the libdl library, which usually resides in the libdl.so.2 shared object on modern Linux systems.
I'll hazzard a guess that your a.out shared object is not linked to libdl. That means that when you preload in a simple binary like uname that does not pull in a lot of other libraries, libdl.so.2 may not be pulled in and you get an undefined symbol error.
If, on the other hand, you preload it to a binary that is linked to and finally pulls in libdl.so.2, your shared object works fine.
I'd check with ldd if your own shared object is linked against libdl as it should, and also what libraries are directly or indirectly pulled in when uname and ls run.
EDIT:
I just confirmed this. The way to fix this error is to link your shared object against libdl. Adding -ldl to its LDFLAGS should do the trick.