Is it possible to LD_PRELOAD a function with different parameters?

Question

Say I replace a function by creating a shared object and using LD_PRELOAD to load it first. Is it possible to have parameters to that function different from the one in original library?

For example, if I replace pthread_mutex_lock, such that instead of parameter pthread_mutex_t it takes pthread_my_mutex_t. Is it possible?

Secondly, besides function, is it possible to change structure declarations using LD_PRELOAD? For example, one may add one more field to a structure.

Answer 1

Although you can arrange to provide your modified pthread_mutex_lock() function, the code will have been compiled to call the standard function. This will lead to problems when the replacement is called with the parameters passed to the standard function. This is a polite way of saying:

• Expect it to crash and burn

Any pre-loaded function must implement the same interface — same name, same arguments in, same values out — as the function it replaces. The internals can be implemented as differently as you need, but the interface must be the same.

Similarly with structures. The existing code was compiled to expect one size for the structure, with one specific layout. You might get away with adding an extra field at the end, but the non-substituted code will probably not work correctly. It will allocate space for the original size of structure, not the enhanced structure, etc. It will never access the extra element itself. It probably isn't quite impossible, but you must have designed the program to handle dynamically changing structure sizes, which places severe enough constraints on when you can do it that the answer "you can't" is probably apposite (and is certainly much simpler).

IMNSHO, the LD_PRELOAD mechanism is for dire emergencies (and is a temporary band-aid for a given problem). It is not a mechanism you should plan to use on anything remotely resembling a regular basis.

Answer 2

The effects of what you're describing are conceptually the same as the effects of providing a mismatching external function at normal link time: undefined behavior.

If you want to do this, rather than playing with fire, why don't you make your replacement function also take pthread_mutex_t * as its argument type, and then just convert the pointer to pthread_my_mutex_t * in the function body? Normally this conversion will take place only at the source level anyway; no code should be generated for it.

Answer 3

LD_PRELOAD does one thing, and one thing only. It arranges for a particular DSO file to be at the front of the list that ld.so uses to look up symbols. It has nothing to do with how the code uses a function or data item once found.

Anything you can do with LD_PRELOAD, you can simulate by just linking the replacement library with -l at the front of the list. If, on the other hand, you can't accomplish a task with that -l, you can't do it with LD_PRELOAD.