Sanitizer special case list


This document describes the way to disable or alter the behavior of sanitizer tools for certain source-level entities by providing a special file at compile-time.

Goal and usage

User of sanitizer tools, such as AddressSanitizer, ThreadSanitizer or MemorySanitizer may want to disable or alter some checks for certain source-level entities to:

  • speedup hot function, which is known to be correct;

  • ignore a function that does some low-level magic (e.g. walks through the thread stack, bypassing the frame boundaries);

  • ignore a known problem.

To achieve this, user may create a file listing the entities they want to ignore, and pass it to clang at compile-time using -fsanitize-ignorelist flag. See Clang Compiler User’s Manual for details.


$ cat foo.c
#include <stdlib.h>
void bad_foo() {
  int *a = (int*)malloc(40);
  a[10] = 1;
int main() { bad_foo(); }
$ cat ignorelist.txt
# Ignore reports from bad_foo function.
$ clang -fsanitize=address foo.c ; ./a.out
# AddressSanitizer prints an error report.
$ clang -fsanitize=address -fsanitize-ignorelist=ignorelist.txt foo.c ; ./a.out
# No error report here.


Ignorelists consist of entries, optionally grouped into sections. Empty lines and lines starting with “#” are ignored.

Section names are regular expressions written in square brackets that denote which sanitizer the following entries apply to. For example, [address] specifies AddressSanitizer while [cfi-vcall|cfi-icall] specifies Control Flow Integrity virtual and indirect call checking. Entries without a section will be placed under the [*] section applying to all enabled sanitizers.

Entries contain an entity type, followed by a colon and a regular expression, specifying the names of the entities, optionally followed by an equals sign and a tool-specific category, e.g. fun:*ExampleFunc=example_category. The meaning of * in regular expression for entity names is different - it is treated as in shell wildcarding. Two generic entity types are src and fun, which allow users to specify source files and functions, respectively. Some sanitizer tools may introduce custom entity types and categories - refer to tool-specific docs.

# Lines starting with # are ignored.
# Turn off checks for the source file (use absolute path or path relative
# to the current working directory):
# Turn off checks for this main file, including files included by it.
# Useful when the main file instead of an included file should be ignored.
# Turn off checks for a particular functions (use mangled names):
# Extended regular expressions are supported:
# Shell like usage of * is supported (* is treated as .*):
# Specific sanitizer tools may introduce categories.
# Sections can be used to limit ignorelist entries to specific sanitizers
# Section names are regular expressions
# Entries without sections are placed into [*] and apply to all sanitizers

mainfile is similar to applying -fno-sanitize= to a set of files but does not need plumbing into the build system. This works well for internal linkage functions but has a caveat for C++ vague linkage functions.

C++ vague linkage functions (e.g. inline functions, template instantiations) are deduplicated at link time. A function (in an included file) ignored by a specific mainfile pattern may not be the prevailing copy picked by the linker. Therefore, using mainfile requires caution. It may still be useful, e.g. when patterns are picked in a way to ensure the prevailing one is ignored. (There is action-at-a-distance risk.)

mainfile can be useful enabling a ubsan check for a large code base when finding the direct stack frame triggering the failure for every failure is difficult.