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    SubjectRe: [RFC] Mitigating unexpected arithmetic overflow


    On May 15, 2024 12:36:36 AM PDT, Peter Zijlstra <peterz@infradead.org> wrote:
    >On Wed, May 08, 2024 at 04:47:25PM -0700, Linus Torvalds wrote:
    >> For example, the most common case of overflow we've ever had has very
    >> much been array indexing. Now, sometimes that has actually been actual
    >> undefined behavior, because it's been overflow in signed variables,
    >> and those are "easy" to find in the sense that you just say "no, can't
    >> do that". UBSAN finds them, and that's good.
    >
    >We build with -fno-strict-overflow, which implies -fwrapv, which removes
    >the UB from signed overflow by mandating 2s complement.

    I am a broken record. :) This is _not_ about undefined behavior.

    This is about finding a way to make the intent of C authors unambiguous. That overflow wraps is well defined. It is not always _desired_. C has no way to distinguish between the two cases.

    -Kees

    --
    Kees Cook

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