Re: [PATCH 02/13] msvc: avoid using minus operator on unsigned types

[Johannes Schindelin <Johannes.Schindelin@gmx.de>]

Hi Denton,

On Thu, 26 Sep 2019, Denton Liu wrote:

> Hi Dscho,
>
> On Thu, Sep 26, 2019 at 01:30:10AM -0700, Johannes Schindelin via GitGitGadget wrote:
> > From: Johannes Schindelin <johannes.schindelin@gmx.de>
> >
> > MSVC complains about this with `-Wall`, which can be taken as a sign
> > that this is indeed a real bug. The symptom is:
> >
> > 	C4146: unary minus operator applied to unsigned type, result
> > 	still unsigned
> >
> > Let's avoid this warning in the minimal way, e.g. writing `-1 -
> > <unsigned value>` instead of `-<unsigned value> - 1`.
>
> [...]
>
> > ---
> >  read-cache.c  | 4 ++--
> >  sha1-lookup.c | 2 +-
> >  2 files changed, 3 insertions(+), 3 deletions(-)
> >
> > diff --git a/read-cache.c b/read-cache.c
> > index c701f7f8b8..11f3357216 100644
> > --- a/read-cache.c
> > +++ b/read-cache.c
> > @@ -1276,7 +1276,7 @@ static int add_index_entry_with_check(struct index_state *istate, struct cache_e
> >  	 */
> >  	if (istate->cache_nr > 0 &&
> >  		strcmp(ce->name, istate->cache[istate->cache_nr - 1]->name) > 0)
> > -		pos = -istate->cache_nr - 1;
> > +		pos = -1 - istate->cache_nr;
>
> I've been thinking about this and I'm still not certain that this 100%
> correct from a language-lawyer perspective.
>
> If we do `-1 - istate->cache_nr`, then the unsignedness of
> istate->cache_nr takes over and the whole expression is a very large
> unsigned number.
>
> Then, when we assign to `int pos`, we are converting an unsigned number
> which is out of the range of the signed number. According to a
> StackOverflow post citing the C99 standard[1]:
>
> 	Otherwise, the new type is signed and the value cannot be
> 	represented in it; either the result is implementation-defined
> 	or an implementation-defined signal is raised.
>
> I'm sure that most platforms that we support will handle it sanely but
> could we write this as
>
> 	pos = -1 - (int) istate->cache_nr;
>
> to be doubly sure that no funny business will happen?

I guess we should use `signed_add_overflows()` to make extra certain
that it does what we want it to do, kind of like `st_add()`. Or just do
the check explicitly, like so:

	if (istate->cache_nr > INT_MAX)
		die("overflow: -1 - %u", istate->cache_nr);
	pos = -1 - istate->cache_nr;
}
>
> >  	else
> >  		pos = index_name_stage_pos(istate, ce->name, ce_namelen(ce), ce_stage(ce));
> >
> > @@ -1894,7 +1894,7 @@ static size_t estimate_cache_size(size_t ondisk_size, unsigned int entries)
> >  	/*
> >  	 * Account for potential alignment differences.
> >  	 */
> > -	per_entry += align_padding_size(sizeof(struct cache_entry), -sizeof(struct ondisk_cache_entry));
> > +	per_entry += align_padding_size(per_entry, 0);
> >  	return ondisk_size + entries * per_entry;
> >  }
> >
> > diff --git a/sha1-lookup.c b/sha1-lookup.c
> > index 796ab68da8..c819687730 100644
> > --- a/sha1-lookup.c
> > +++ b/sha1-lookup.c
> > @@ -97,7 +97,7 @@ int sha1_pos(const unsigned char *sha1, void *table, size_t nr,
> >  			lo = mi + 1;
> >  		mi = lo + (hi - lo) / 2;
> >  	} while (lo < hi);
> > -	return -lo-1;
> > +	return -1 - lo;
>
> Same thing here.

This is even more critical, as `lo` has the type `size_t`:

	if (lo > INT_MAX)
		die("overflow: -1 - %"PRIuMAX, (uintmax_t)lo);
	return -1 - lo;
>
What do you think?
Dscho

> [1]: https://stackoverflow.com/questions/50605/signed-to-unsigned-conversion-in-c-is-it-always-safe
>
> >  }
> >
> >  int bsearch_hash(const unsigned char *sha1, const uint32_t *fanout_nbo,
> > --
> > gitgitgadget
> >
>