Re: [PATCH] wrapper: avoid UB in macOS

From: Junio C Hamano <gitster@pobox.com>
Date: Jun 19, 2019, 14:40 UTC
Message-ID: <xmqqk1dhr63n.fsf@gitster-ct.c.googlers.com>
In-Reply-To: <20190616184003.17236-1-carenas@gmail.com>

Carlo Marcelo Arenas Belón  <carenas@gmail.com> writes:

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> 0620b39b3b ("compat: add a mkstemps() compatibility function", 2009-05-31)
> included a function based on code from libiberty which would result in
> undefined behaviour in platforms where timeval's tv_usec is a 32-bit signed
> type as shown by:
>
> wrapper.c:505:31: runtime error: left shift of 594546 by 16 places cannot be represented in type '__darwin_suseconds_t' (aka 'int')

I had to scratch my head wondering what an UB was (spell it out as "undefined behaviour" if that is what you wanted to say).

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> interestingly the version of this code from gcc never had this bug and the
> code had a cast that would had prevented the issue (at least in 64-bit
> platforms) but was misapplied.
>
> change the cast to uint64_t so it also works in 32-bit platforms.

Yup, regardless of the platform, that's the "right" type to use, certainly more correct than "size_t", as value is declared to be u64 in this function.

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>  	gettimeofday(&tv, NULL);
> -	value = ((size_t)(tv.tv_usec << 16)) ^ tv.tv_sec ^ getpid();
> +	value = ((uint64_t)tv.tv_usec << 16) ^ tv.tv_sec ^ getpid();
>  	filename_template = &pattern[len - 6 - suffix_len];
>  	for (count = 0; count < TMP_MAX; ++count) {
>  		uint64_t v = value;

Thanks.