What are practical applications of library interpositioning?

Wrapping library calls powers sandboxing/encryption (security), call-counting and malloc-tracing (profiling), fault injection and mocks (testing), and debugging tools like Valgrind, strace, and ltrace.

The common thread is that you sit transparently between a program and the functions it calls, so you can observe, modify, or restrict behavior on an unmodified binary — which is exactly why both defenders and attackers rely on it.

Security:

• Sandboxing: Interpose libc calls to restrict what programs can do
• Transparent encryption: Automatically encrypt network connections
• Access control: Monitor and filter file system access

Monitoring and Profiling:

• Call counting: Track how many times each function is called
• Argument logging: Record what parameters are passed
• Malloc tracing: Detect memory leaks by logging allocations/frees
• Address traces: Generate memory access patterns

Testing:

• Fault injection: Make functions fail to test error handling
• Mock functions: Replace dependencies during unit testing

Debugging:

• Valgrind: Uses interpositioning for memory error detection
• strace: Intercepts system calls
• ltrace: Intercepts library calls

Tip: Tools like AddressSanitizer and ThreadSanitizer use interpositioning to instrument memory operations at runtime.

Go deeper:

• dlsym(3) — RTLD_NEXT wrappers — the one call that lets an interposed function reach the real one underneath it.