AddrMon: A Beginner’s Guide to Memory Address MonitoringMemory is the backbone of every software system. Whether you’re writing firmware for a microcontroller, developing a kernel module, or debugging a user-space application, understanding how memory is accessed and modified is critical. AddrMon (short for Address Monitor) is a technique and a family of tools that help developers observe, trace, and react to reads and writes at specific memory addresses. This guide introduces concepts, practical uses, and simple examples to get you started with AddrMon.
What is AddrMon?
AddrMon is a tool/technique that monitors accesses (reads and/or writes) to specific memory addresses or ranges. It reports when those accesses happen and can optionally trigger actions such as logging, breakpoints, or callbacks. AddrMon can be implemented in hardware (MMU/MPU, debug units), the OS kernel, or user-space via instrumentation and virtual memory tricks.
Why memory address monitoring matters
Memory bugs are often silent and dangerous: race conditions, buffer overflows, use-after-free, and unauthorized accesses can cause crashes, data corruption, or security vulnerabilities. AddrMon helps by:
- Detecting unexpected accesses to sensitive regions (e.g., hardware registers, protected buffers).
- Tracing who/what touched a memory location and when.
- Helping reproduce intermittent bugs by capturing access sequences.
- Enabling fine-grained profiling of memory hotspots.
- Assisting security analysis (detecting tampering or unauthorized reads).
Common implementation approaches
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Hardware-assisted monitoring
- Many processors offer debug units or performance-monitoring units that can detect accesses to physical addresses or address ranges and generate traps or events.
- Memory Protection Units (MPUs) and MMUs can be configured to mark pages non-present or protected so accesses generate exceptions the kernel can catch.
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OS/kernel-level monitoring
- Kernel modules can watch virtual memory mappings, insert page protections, and handle faults to detect accesses.
- Tools like ptrace on Unix let a debugger single-step processes and set watchpoints.
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User-space instrumentation
- Compiler-based instrumentation (address sanitizer, custom instrumentation passes) injects checks around memory operations.
- Binary instrumentation frameworks (Pin, DynamoRIO) can rewrite code to log accesses.
- mprotect tricks: mark pages read-only/none and handle SIGSEGV to detect and emulate accesses.
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Software watchpoints / debug registers
- CPUs often provide a limited number of hardware watchpoints (debug registers) that break on memory read/write/execute at specific addresses.
How AddrMon works in practice: basic patterns
- Watchpoint: set a hardware/software trigger on an address; when accessed, control is transferred to a debugger or handler.
- Page-fault trapping: protect the page(s) containing the address so a fault occurs; the fault handler inspects the instruction and decides whether to allow, log, emulate, or terminate.
- Instrumentation: insert code to record accesses, often including call stacks and timestamps.
- Snapshot + diff: periodically snapshot memory regions and diff to detect changes (less precise but low overhead options exist).
Example scenarios
- Embedded firmware: monitor writes to a peripheral register to verify driver correctness.
- Kernel development: detect which module is touching a sensitive kernel data structure.
- Security research: watch a secret buffer for reads to discover exfiltration paths.
- Debugging race conditions: log the sequence of accesses to a shared buffer and timestamps to reconstruct interleavings.
Simple examples
- Linux user-space using ptrace (conceptual)
- Attach to a process, set a hardware watchpoint via debug registers, resume. When the target accesses the address, your tracer receives a SIGTRAP and can inspect registers and the instruction pointer.
- mprotect + SIGSEGV approach (user-space)
- mprotect the page to PROT_NONE.
- Run the program; on SIGSEGV, examine the faulting address in siginfo_t.
- Log or handle the access, then emulate or allow it (e.g., temporarily restore permissions and single-step).
- Using AddressSanitizer (instrumentation)
- Compile with -fsanitize=address to detect out-of-bounds and use-after-free; while not a precise AddrMon for arbitrary addresses, it provides powerful runtime detection.
Pros and cons (comparison)
| Approach | Pros | Cons |
|---|---|---|
| Hardware watchpoints | Low overhead, precise | Limited quantity, address alignment constraints |
| Page-protection trapping | Works for ranges, flexible | High overhead if many faults; coarse granularity (page-sized) |
| Binary instrumentation | Very flexible, arbitrary data | Performance overhead, complex tooling |
| Compiler instrumentation | Integrates at build time, detailed checks | Requires rebuild, may change program behavior |
| Snapshot/diff | Simple, low intrusion | Low precision, misses short-lived accesses |
Performance and limits
- Hardware watchpoints are fastest but scarce (often 2–8 per core).
- Page-protection methods cause faults and context switches, which are expensive if accesses are frequent.
- Instrumentation overhead depends on how fine-grained and how much logging is done; sampling reduces overhead at the cost of completeness.
- Consider how long you need monitoring and whether you can limit the scope (e.g., specific threads, time windows) to reduce impact.
Practical tips
- Start small: monitor a single address or small range to validate your approach before scaling.
- Prefer hardware watchpoints for short, targeted investigations.
- For intermittent or high-frequency accesses, use sampling or selective instrumentation to avoid overwhelming logs.
- Capture useful context: instruction pointer, call stack, thread id, timestamp, and register values.
- Automate log correlation: use unique markers or structured logs to reconstruct timelines.
- Clean up protections/watchpoints promptly to avoid leaving the system in a degraded state.
Example workflow: find who writes a corrupted buffer
- Reproduce the corruption scenario to determine which buffer and approximate time.
- Choose monitoring method: hardware watchpoint if address fits, else mprotect-based trapping.
- Set the monitor and run the program under test.
- When the monitor triggers, capture the instruction pointer and call stack.
- Map the instruction to source (symbolize) and inspect the write path.
- Fix the bug, re-run under lighter monitoring to confirm.
Tools and resources
- gdb/LLDB: set watchpoints and inspect state.
- perf / hardware performance counters: low-level events and sampling.
- AddressSanitizer / Valgrind: memory error detectors.
- Pin / DynamoRIO / Frida: dynamic binary instrumentation.
- Custom kernel modules or eBPF programs for kernel/OS-level monitoring.
Security considerations
- Monitoring can expose sensitive data (logs should be protected).
- Be careful when modifying page protections in production systems—this can cause instability.
- Attaching debuggers or instrumentation to running processes may change timing and hide concurrency bugs (Heisenbugs).
Summary
AddrMon gives developers a precise way to observe and react to memory accesses. By picking the right implementation—hardware watchpoints for precision, page-trapping for range coverage, or instrumentation for flexibility—you can find and fix elusive memory bugs, profile memory behavior, and harden systems against misuse. Start with a small, focused test, collect targeted evidence (IP, stack, timestamp), and iterate until the root cause is clear.
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