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ubsan: Reintroduce signed overflow sanitizer
In order to mitigate unexpected signed wrap-around[1], bring back the signed integer overflow sanitizer. It was removed in commit 6aaa31a ("ubsan: remove overflow checks") because it was effectively a no-op when combined with -fno-strict-overflow (which correctly changes signed overflow from being "undefined" to being explicitly "wrap around"). Compilers are adjusting their sanitizers to trap wrap-around and to detecting common code patterns that should not be instrumented (e.g. "var + offset < var"). Prepare for this and explicitly rename the option from "OVERFLOW" to "WRAP" to more accurately describe the behavior. To annotate intentional wrap-around arithmetic, the helpers wrapping_add/sub/mul_wrap() can be used for individual statements. At the function level, the __signed_wrap attribute can be used to mark an entire function as expecting its signed arithmetic to wrap around. For a single object file the Makefile can use "UBSAN_SIGNED_WRAP_target.o := n" to mark it as wrapping, and for an entire directory, "UBSAN_SIGNED_WRAP := n" can be used. Additionally keep these disabled under CONFIG_COMPILE_TEST for now. Link: KSPP/linux#26 [1] Cc: Miguel Ojeda <[email protected]> Cc: Nathan Chancellor <[email protected]> Cc: Peter Zijlstra <[email protected]> Cc: Hao Luo <[email protected]> Reviewed-by: Marco Elver <[email protected]> Reviewed-by: Justin Stitt <[email protected]> Signed-off-by: Kees Cook <[email protected]>
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