Clarify that ASan is incompatible with -D_FORTIFY_SOURCE #977
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yugr
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Sep 4, 2025
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| Applications that incorrectly use `malloc_usable_size`[^malloc_usable_size] to use the additional size reported by the function may abort at runtime. This is a bug in the application because the additional size reported by `malloc_usable_size` is not generally safe to dereference and is for diagnostic uses only. The correct fix for such issues is to avoid using `malloc_usable_size` as the glibc manual specifically states that it is for diagnostic purposes *only* [^malloc_usable_size]. On many Linux systems these incorrect uses can be detected by running `readelf -Ws <path>` on the ELF binaries and searching for `malloc_usable_size@GLIBC`[^kpyrd23]. If avoiding `malloc_usable_size` is not possible, one may call `realloc` to resize the block to its usable size and to benefit from `_FORTIFY_SOURCE=3`. | ||
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| Additionally, `_FORTIFY_SOURCE` is currently incompatible with [AddressSanitizer](#-fsanitize=address) (and other sanitizers[^Ostapenko16]) as they do not support source fortification. As a result, sanitizers can misbehave on binaries with source fortification enabled (they either produces false negatives or false positives). Consequently we do not recommend enabling `_FORTIFY_SOURCE` for instrumented test builds where sanitizers are used. |
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Fortification is enabled by default on most modern distros so maybe mention that it sometimes needs to be explicitly disabled ?
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Addressed in a0e9336.
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Distros do things differently, and sometimes people compile things themselves (for many reasons). But noting this particular footgun could be really helpful.
How about appending this to the proposed paragraph starting with "Additionally"?:
Note that many distributions enable
_FORTIFY_SOURCEby default.
Signed-off-by: Thomas Nyman <[email protected]>
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yugr
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Sep 5, 2025
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Fixes #956 (together with #918)