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Re: diffutils-3.7.41-7475 exercises undefined behaviour

From: Jim Meyering
Subject: Re: diffutils-3.7.41-7475 exercises undefined behaviour
Date: Wed, 28 Jul 2021 22:27:41 -0700

On Wed, Jul 28, 2021 at 2:09 AM Bruno Haible <bruno@clisp.org> wrote:
> Hi Jim,
> > Thanks again. Even though I never reproduced the issue, I've pushed
> > the following, which should resolve it:
> >
> > https://git.sv.gnu.org/cgit/diffutils.git/commit/?id=v3.7-42-g1aa7ccf
> No, the patch does not fully resolve the issue. I still get
>   Conditional jump or move depends on uninitialised value(s)
>      at 0x402256: block_compare (cmp.c:622)
>      by 0x402256: cmp (cmp.c:465)
>      by 0x402256: main (cmp.c:361)
> The reason is that valgrind is silent when you copy uninitialized values
> to a different memory locations, and complains only when these uninitialized
> values effectively have an effect. [1]
> Here, the original uninitialized values are buf1[read0] and buf0[read1].
> The attached patch fixes it (and does not introduce regressions in
> "make check" without valgrind).

Nice. Thank you. I've reverted my own change and applied/pushed yours
with only a trivial comment change.

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