Modifying a Dynamic Library Without Changing the Source Code

Placing your own code between a program and the libraries it is linked against is easy when you use the LD_PRELOAD environment variable.
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Can you post that code

Flowers's picture

Can you post that code perchance?

I have found interesting

Anonymous's picture

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A little thing you forgot

grib's picture

You wrote:
This command tells glibc...

Hmm, lets see:

$ man ld.so
NAME
ld.so/ld-linux.so - dynamic linker/loader
...
ENVIRONMENT
...
LD_PRELOAD
A whitespace-separated list of additional, user-specified, ELF
shared libraries to be loaded before all others. This can be
used to selectively override functions in other shared
libraries. For setuid/setgid ELF binaries, only libraries in
the standard search directories that are also setgid will be
loaded.
...
$

So, LD_PRELOAD affects the dynamic linker rather than glibc.

It's possible to override

promag's picture

It's possible to override dlopen and dlsym?

Yes

i3839's picture

Yes, it is, but you need to do extra work if you want to call the real ones yourself (there are special linking options which can ahcieve this).

poor man's AOP ?

kevin bedell's picture

This strikes me as describing a fundamental way to implement AOP on theop of the Linux Kernel.

Anyone heaerd of people taking this idea further and actually trying to build an AOP implementation?

re: force a process to a specific CPU

Anonymous's picture

Ahh, the teaser promises binding to a single CPU -- can you post that code perchance?

Thanks!

Nice example. A trick that be

bildrulle's picture

Nice example. A trick that be usefull for many things. But this particular example could just as easily have been achived by running the ltrace command.

Re: Nice example. A trick that be

Anonymous's picture

Ltrace is nice, but the LD_PRELOAD shim can do more things with the shimmed function(s). For example, it might only print out the trace message when certain conditions are met in the parameters. Or, every call to the shimmed function could scan target library internal data structures for corruption. Etc.

thats one of the problems wit

gfdsa's picture

thats one of the problems with open source today, it became "too much", for every thing you want to do, there are a few ways

no problem :)

Anonymous's picture

It's not a problem, to have a lot of options, it's flexibility and last not least, it's freedom.

It's an illusion of freedom

Anonymous's picture

It's an illusion of freedom

I know you

Anonymous's picture

Osama, is that you?

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