Clang-22 & ASan: Workaround for Sanitizer Errors
Running into unexpected AddressSanitizer (ASan) errors after upgrading to Clang-22? Learn why it happens and discover a simple, effective workaround.
Alex Petrov
A senior C++ developer specializing in performance, tooling, and low-level system design.
There’s always a certain excitement that comes with a new major compiler release. Upgrading your toolchain, like moving to the shiny new Clang-22, promises better optimizations, new language features, and improved diagnostics. But sometimes, the upgrade path has a few unexpected bumps. One such bump developers are encountering is a sudden surge of errors from AddressSanitizer (ASan) on code that was perfectly clean just a version ago.
If you’ve recently switched to Clang-22, enabled ASan for your debug builds, and are now staring at a screen full of `heap-buffer-overflow` errors in what seems like perfectly standard C++ code, you’re not alone. Don’t panic! This post will walk you through why this is happening and provide a simple, effective workaround to get your builds green again.
The Ghost in the Machine: What's Happening with Clang-22 and ASan?
The scenario is frustratingly common. You have a stable codebase that has been passing all your ASan-instrumented tests for months. You update your build environment to use Clang-22, recompile with -fsanitize=address, and suddenly, your CI pipeline is a sea of red. The errors often point to seemingly innocuous operations on standard library containers like std::string or std::vector.
Consider this simple piece of code:
#include <iostream>
#include <string>
#include <vector>
// A function that does some basic string manipulation
void process_user_input(const std::string& input) {
std::string buffer = "User: ";
buffer += input; // This operation might trigger the error
if (buffer.length() > 32) {
std::cout << "Received long input.\n";
}
}
int main() {
process_user_input("Here is some sample data that is long enough to cause a reallocation.");
std::cout << "Processing complete.\n";
return 0;
}
With an older version of Clang, this code runs cleanly under ASan. With Clang-22, however, you might be greeted with an error log that looks something like this:
==12345==ERROR: AddressSanitizer: heap-buffer-overflow on address 0x60c0000001a8 at pc 0x55d8f789e4a1 bp 0x7ffc3f7b2e30 sp 0x7ffc3f7b2e28
READ of size 1 at 0x60c0000001a8 thread T0
#0 0x55d8f789e4a0 in std::__1::basic_string<...>::operator+=(char const*)
#1 0x55d8f789e1d5 in process_user_input(std::__1::basic_string<...> const&) /app/main.cpp:8
#2 0x55d8f789e312 in main /app/main.cpp:16
#3 0x7f1a2b3c80b2 in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x240b2)
0x60c0000001a8 is located 0 bytes to the right of 72-byte region [0x60c000000160,0x60c0000001a8)
allocated by thread T0 here:
#0 0x7f1a2b6d7bc8 in operator new(unsigned long) (/usr/lib/x86_64-linux-gnu/libasan.so.8+0xd7bc8)
#1 0x55d8f789e3f1 in std::__1::basic_string<...>::__init(char const*, unsigned long) /usr/bin/../include/c++/v1/string:2334
...
The error suggests we’re reading one byte past the end of an allocated buffer during a string concatenation. But we know our code is just using the standard library correctly. So, what’s the real cause?
The issue typically stems from a mismatch between the latest version of Clang's C++ standard library (libc++) and AddressSanitizer's container-specific checks. Compilers and standard libraries are constantly evolving. A new version might change the internal memory layout of std::string for optimization purposes (like adjusting the Small String Optimization strategy). ASan has a special, highly-aware feature to detect overflows in containers, but it relies on knowing exactly how those containers manage their memory. When the library's implementation changes, ASan's knowledge can become outdated, leading it to misinterpret a valid memory access as an overflow—a classic false positive.
The Workaround: Taming ASan with `ASAN_OPTIONS`
Fortunately, the LLVM/Clang developers provide a way to configure the sanitizer's behavior at runtime through the ASAN_OPTIONS environment variable. This is our escape hatch.
The most effective workaround for this specific class of false positives is to disable the container overflow detection feature. You can do this by setting the following option:
export ASAN_OPTIONS=detect_container_overflow=0This command tells AddressSanitizer to skip its specialized, aggressive checks for C++ standard containers. The crucial thing to understand is that this does not disable ASan entirely. All of its other powerful features—detecting use-after-free, heap buffer overflows on regular arrays, stack buffer overflows, and memory leaks—remain fully active. You're only toggling off the one feature that is currently causing the false positives.
How to Apply the Workaround
You can set this environment variable in several ways, depending on your workflow:
-
For a single run in your terminal:
ASAN_OPTIONS=detect_container_overflow=0 ./your_sanitized_program -
In a CMake project:
You can set the environment variable for your test runs directly in your
CMakeLists.txt. This is great for ensuring your CI checks pass.# Set this before you define your test targets set(ENV{ASAN_OPTIONS} "detect_container_overflow=0") add_executable(my_app main.cpp) # ... or for CTest ... add_test(NAME MyTest COMMAND my_app) set_property(TEST MyTest PROPERTY ENVIRONMENT "ASAN_OPTIONS=detect_container_overflow=0") -
In a shell script or CI pipeline:
Simply export the variable at the beginning of your script.
#!/bin/bash export ASAN_OPTIONS=detect_container_overflow=0 # Your build and test commands go here ./your_sanitized_program
Is This a Risky Move? Understanding the Trade-offs
It's natural to be wary of disabling any part of a security tool like ASan. Are you just hiding a real bug? In this specific case, the answer is most likely no. When these false positives appear immediately after a toolchain upgrade and are localized to standard container operations, it's a strong signal that the issue lies with the tooling interaction, not your logic.
Disabling detect_container_overflow is a targeted, low-risk mitigation strategy. You lose the specialized safety net for container operations, but you retain the 95% of other checks that make ASan indispensable. The alternative—letting your builds fail or, worse, disabling ASan entirely—is far riskier.
Think of this as a temporary measure. The correct, long-term fix will come from an update to the Clang/LLVM toolchain itself, where ASan's container knowledge is brought back in sync with the latest libc++ implementation. You should:
- Treat it as temporary: Keep the workaround in place to stay productive, but don't forget about it.
- Monitor for updates: Keep an eye on the LLVM bug tracker and the Clang release notes. A search for "ASan Clang-22 container overflow" will likely lead you to the relevant issue.
- Update your toolchain: Once a patch is released (e.g., in Clang-22.0.1 or Clang-23), you should plan to upgrade and remove the
ASAN_OPTIONSworkaround.
Wrapping Up: Moving Forward with Confidence
Upgrading your compiler toolchain is a fundamental part of modern software development, but it's not always a seamless process. Encountering false positives from advanced tools like AddressSanitizer after an upgrade to Clang-22 can be jarring, but it's usually a sign of the tool's own development catching up to compiler and library changes.
The key takeaway is that you don't have to roll back your upgrade or abandon sanitizers. By using ASAN_OPTIONS=detect_container_overflow=0, you can implement a safe, temporary workaround that keeps your development and testing workflows running smoothly. Stay informed, wait for the official patch, and continue to leverage the powerful combination of modern Clang and its best-in-class sanitizers.
Have you run into this issue? Found another solution or have a different experience to share? Drop a comment below—I'd love to hear about it!