UndefinedBehaviorSanitizer — Clang 10 documentation

# Clang 10 documentation

## UndefinedBehaviorSanitizer

- Introduction
- How to build
- Usage
- Available checks
  - Volatile
- Minimal Runtime
- Stack traces and report symbolization
- Logging
- Silencing Unsigned Integer Overflow
- Issue Suppression
  - Disabling Instrumentation with `__attribute__((no_sanitize("undefined")))`
  - Suppressing Errors in Recompiled Code (Blacklist)
  - Runtime suppressions
- Supported Platforms
- Current Status
- Additional Configuration
  - Example
- More Information

## Introduction

UndefinedBehaviorSanitizer (UBSan) is a fast undefined behavior detector. UBSan modifies the program at compile-time to catch various kinds of undefined behavior during program execution, for example:

- Using misaligned or null pointer
- Signed integer overflow
- Conversion to, from, or between floating-point types which would overflow the destination

See the full list of available checks below.

UBSan has an optional run-time library which provides better error reporting. The checks have small runtime cost and no impact on address space layout or ABI.

## How to build

Build LLVM/Clang with CMake.

## Usage

Use `clang++` to compile and link your program with `-fsanitize=undefined` flag. Make sure to use `clang++` (not `ld`) as a linker, so that your executable is linked with proper UBSan runtime libraries. You can use `clang` instead of `clang++` if you’re compiling/linking C code.

    % cat test.cc
    int main(int argc, char **argv) {
      int k = 0x7fffffff;
      k += argc;
      return 0;
    }
    % clang++ -fsanitize=undefined test.cc
    % ./a.out
    test.cc:3:5: runtime error: signed integer overflow: 2147483647 + 1 cannot be represented in type 'int'

You can enable only a subset of checks offered by UBSan, and define the desired behavior for each kind of check:

- `-fsanitize=...`: print a verbose error report and continue execution (default);
- `-fno-sanitize-recover=...`: print a verbose error report and exit the program;
- `-fsanitize-trap=...`: execute a trap instruction (doesn’t require UBSan run-time support).

For example if you compile/link your program as:

    % clang++ -fsanitize=signed-integer-overflow,null,alignment -fno-sanitize-recover=null -fsanitize-trap=alignment

the program will continue execution after signed integer overflows, exit after the first invalid use of a null pointer, and trap after the first use of misaligned pointer.

## Available checks

Available checks are:

> - `-fsanitize=alignment`: Use of a misaligned pointer or creation of a misaligned reference. Also sanitizes assume_aligned-like attributes.
> - `-fsanitize=bool`: Load of a `bool` value which is neither `true` nor `false`.
> - `-fsanitize=builtin`: Passing invalid values to compiler builtins.
> - `-fsanitize=bounds`: Out of bounds array indexing, in cases where the array bound can be statically determined.
> - `-fsanitize=enum`: Load of a value of an enumerated type which is not in the range of representable values for that enumerated type.
> - `-fsanitize=float-cast-overflow`: Conversion to, from, or between floating-point types which would overflow the destination. Because the range of representable values for all floating-point types supported by Clang is [-inf, +inf], the only cases detected are conversions from floating point to integer types.
> - `-fsanitize=float-divide-by-zero`: Floating point division by zero. This is undefined per the C and C++ standards, but is defined by Clang (and by ISO/IEC/IEEE 60559 / IEEE 754) as producing either an infinity or NaN value, so is not included in `-fsanitize=undefined`.
> - `-fsanitize=function`: Indirect call of a function through a function pointer of the wrong type (Darwin/Linux, C++ and x86/x86_64 only).
> - `-fsanitize=implicit-unsigned-integer-truncation`, `-fsanitize=implicit-signed-integer-truncation`: Implicit conversion from integer of larger bit width to smaller bit width, if that results in data loss. That is, if the demoted value, after casting back to the original width, is not equal to the original value before the downcast. The `-fsanitize=implicit-unsigned-integer-truncation` handles conversions between two `unsigned` types, while `-fsanitize=implicit-signed-integer-truncation` handles the rest of the conversions - when either one, or both of the types are signed. Issues caught by these sanitizers are not undefined behavior, but are often unintentional.
> - `-fsanitize=implicit-integer-sign-change`: Implicit conversion between integer types, if that changes the sign of the value. That is, if the the original value was negative and the new value is positive (or zero), or the original value was positive, and the new value is negative. Issues caught by this sanitizer are not undefined behavior, but are often unintentional.
> - `-fsanitize=integer-divide-by-zero`: Integer division by zero.
> - `-fsanitize=nonnull-attribute`: Passing null pointer as a function parameter which is declared to never be null.
> - `-fsanitize=null`: Use of a null pointer or creation of a null reference.
> - `-fsanitize=nullability-arg`: Passing null as a function parameter which is annotated with `_Nonnull`.
> - `-fsanitize=nullability-assign`: Assigning null to an lvalue which is annotated with `_Nonnull`.
> - `-fsanitize=nullability-return`: Returning null from a function with a return type annotated with `_Nonnull`.
> - `-fsanitize=object-size`: An attempt to potentially use bytes which the optimizer can determine are not part of the object being accessed. This will also detect some types of undefined behavior that may not directly access memory, but are provably incorrect given the size of the objects involved, such as invalid downcasts and calling methods on invalid pointers. These checks are made in terms of `__builtin_object_size`, and consequently may be able to detect more problems at higher optimization levels.
> - `-fsanitize=pointer-overflow`: Performing pointer arithmetic which overflows, or where either the old or new pointer value is a null pointer (or in C, when they both are).
> - `-fsanitize=return`: In C++, reaching the end of a value-returning function without returning a value.
> - `-fsanitize=returns-nonnull-attribute`: Returning null pointer from a function which is declared to never return null.
> - `-fsanitize=shift`: Shift operators where the amount shifted is greater or equal to the promoted bit-width of the left hand side or less than zero, or where the left hand side is negative. For a signed left shift, also checks for signed overflow in C, and for unsigned overflow in C++. You can use `-fsanitize=shift-base` or `-fsanitize=shift-exponent` to check only left-hand side or right-hand side of shift operation, respectively.
> - `-fsanitize=signed-integer-overflow`: Signed integer overflow, where the result of a signed integer computation cannot be represented in its type. This includes all the checks covered by `-ftrapv`, as well as checks for signed division overflow (`INT_MIN/-1`), but not checks for lossy implicit conversions performed before the computation (see `-fsanitize=implicit-conversion`). Both of these two issues are handled by `-fsanitize=implicit-conversion` group of checks.
> - `-fsanitize=unreachable`: If control flow reaches an unreachable program point.
> - `-fsanitize=unsigned-integer-overflow`: Unsigned integer overflow, where the result of an unsigned integer computation cannot be represented in its type. Unlike signed integer overflow, this is not undefined behavior, but it is often unintentional. This sanitizer does not check for lossy implicit conversions performed before such a computation (see `-fsanitize=implicit-conversion`).
> - `-fsanitize=vla-bound`: A variable-length array whose bound does not evaluate to a positive value.
> - `-fsanitize=vptr`: Use of an object whose vptr indicates that it is of the wrong dynamic type, or that its lifetime has not begun or has ended. Incompatible with `-fno-rtti`. Link must be performed by `clang++`, not `clang`, to make sure C++-specific parts of the runtime library and C++ standard libraries are present.

You can also use the following check groups:
  - `-fsanitize=undefined`: All of the checks listed above other than `float-divide-by-zero`, `unsigned-integer-overflow`, `implicit-conversion`, and the `nullability-*` group of checks.
  - `-fsanitize=undefined-trap`: Deprecated alias of `-fsanitize=undefined`.
  - `-fsanitize=implicit-integer-truncation`: Catches lossy integral conversions. Enables `implicit-signed-integer-truncation` and `implicit-unsigned-integer-truncation`.
  - `-fsanitize=implicit-integer-arithmetic-value-change`: Catches implicit conversions that change the arithmetic value of the integer. Enables `implicit-signed-integer-truncation` and `implicit-integer-sign-change`.
  - `-fsanitize=implicit-conversion`: Checks for suspicious behavior of implicit conversions. Enables `implicit-unsigned-integer-truncation`, `implicit-signed-integer-truncation`, and `implicit-integer-sign-change`.
  - `-fsanitize=integer`: Checks for undefined or suspicious integer behavior (e.g. unsigned integer overflow). Enables `signed-integer-overflow`, `unsigned-integer-overflow`, `shift`, `integer-divide-by-zero`, `implicit-unsigned-integer-truncation`, `implicit-signed-integer-truncation`, and `implicit-integer-sign-change`.
  - `-fsanitize=nullability`: Enables `nullability-arg`, `nullability-assign`, and `nullability-return`. While violating nullability does not have undefined behavior, it is often unintentional, so UBSan offers to catch it.

### Volatile

The `null`, `alignment`, `object-size`, and `vptr` checks do not apply to pointers to types with the `volatile` qualifier.

## Minimal Runtime

There is a minimal UBSan runtime available suitable for use in production environments. This runtime has a small attack surface. It only provides very basic issue logging and deduplication, and does not support `-fsanitize=function` and `-fsanitize=vptr` checking.

To use the minimal runtime, add `-fsanitize-minimal-runtime` to the clang command line options. For example, if you’re used to compiling with `-fsanitize=undefined`, you could enable the minimal runtime with `-fsanitize=undefined -fsanitize-minimal-runtime`.

## Stack traces and report symbolization

If you want UBSan to print symbolized stack trace for each error report, you will need to:

1. Compile with `-g` and `-fno-omit-frame-pointer` to get proper debug information in your binary.
2. Run your program with environment variable `UBSAN_OPTIONS=print_stacktrace=1`.
3. Make sure `llvm-symbolizer` binary is in `PATH`.

## Logging

The default log file for diagnostics is “stderr”. To log diagnostics to another file, you can set `UBSAN_OPTIONS=log_path=...`.

## Silencing Unsigned Integer Overflow

To silence reports from unsigned integer overflow, you can set `UBSAN_OPTIONS=silence_unsigned_overflow=1`. This feature, combined with `-fsanitize-recover=unsigned-integer-overflow`, is particularly useful for providing fuzzing signal without blowing up logs.

## Issue Suppression

UndefinedBehaviorSanitizer is not expected to produce false positives. If you see one, look again; most likely it is a true positive!

### Disabling Instrumentation with `__attribute__((no_sanitize("undefined")))`

You disable UBSan checks for particular functions with `__attribute__((no_sanitize("undefined")))`. You can use all values of `-fsanitize=` flag in this attribute, e.g. if your function deliberately contains possible signed integer overflow, you can use `__attribute__((no_sanitize("signed-integer-overflow")))`.

This attribute may not be supported by other compilers, so consider using it together with `#if defined(__clang__)`.

### Suppressing Errors in Recompiled Code (Blacklist)

UndefinedBehaviorSanitizer supports `src` and `fun` entity types in Sanitizer special case list, that can be used to suppress error reports in the specified source files or functions.

### Runtime suppressions

Sometimes you can suppress UBSan error reports for specific files, functions, or libraries without recompiling the code. You need to pass a path to suppression file in a `UBSAN_OPTIONS` environment variable.

    UBSAN_OPTIONS=suppressions=MyUBSan.supp

You need to specify a check you are suppressing and the bug location. For example:

    signed-integer-overflow:file-with-known-overflow.cpp
    alignment:function_doing_unaligned_access
    vptr:shared_object_with_vptr_failures.so

There are several limitations:

- Sometimes your binary must have enough debug info and/or symbol table, so that the runtime could figure out source file or function name to match against the suppression.
- It is only possible to suppress recoverable checks. For the example above, you can additionally pass `-fsanitize-recover=signed-integer-overflow,alignment,vptr`, although most of UBSan checks are recoverable by default.
- Check groups (like `undefined`) can’t be used in suppressions file, only fine-grained checks are supported.

## Supported Platforms

UndefinedBehaviorSanitizer is supported on the following operating systems:

- Android
- Linux
- NetBSD
- FreeBSD
- OpenBSD
- macOS
- Windows

The runtime library is relatively portable and platform independent. If the OS you need is not listed above, UndefinedBehaviorSanitizer may already work for it, or could be made to work with a minor porting effort.

## Current Status

UndefinedBehaviorSanitizer is available on selected platforms starting from LLVM 3.3. The test suite is integrated into the CMake build and can be run with `check-ubsan` command.

## Additional Configuration

UndefinedBehaviorSanitizer adds static check data for each check unless it is in trap mode. This check data includes the full file name. The option `-fsanitize-undefined-strip-path-components=N` can be used to trim this information. If `N` is positive, file information emitted by UndefinedBehaviorSanitizer will drop the first `N` components from the file path. If `N` is negative, the last `N` components will be kept.

### Example

For a file called `/code/library/file.cpp`, here is what would be emitted:

- Default (No flag, or `-fsanitize-undefined-strip-path-components=0`): `/code/library/file.cpp`
- `-fsanitize-undefined-strip-path-components=1`: `code/library/file.cpp`
- `-fsanitize-undefined-strip-path-components=2`: `library/file.cpp`
- `-fsanitize-undefined-strip-path-components=-1`: `file.cpp`
- `-fsanitize-undefined-strip-path-components=-2`: `library/file.cpp`

## More Information

- From LLVM project blog: What Every C Programmer Should Know About Undefined Behavior
- From John Regehr’s Embedded in Academia blog: A Guide to Undefined Behavior in C and C++