Re: [PATCH v2 15/17] midx: use 64-bit multiplication for chunk sizes

Derrick Stolee [off-list ref] writes:

On 2/4/2021 7:00 PM, Junio C Hamano wrote:

quoted

"Derrick Stolee via GitGitGadget" [off-list ref] writes:

quoted

From: Derrick Stolee <redacted>

When calculating the sizes of certain chunks, we should use 64-bit
multiplication always. This allows us to properly predict the chunk
sizes without risk of overflow.

Signed-off-by: Derrick Stolee <redacted>
---
 midx.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

This one I find somewhat questionable for multiple reasons.

 * the fourth parameter of add_chunk() is of size_t, not uint64_t;
   shouldn't the multiplication be done in type size_t instead?

This is probably appropriate because we will truncate to size_t if
it is smaller than uint64_t.

In other words, if size_t turns out to be too small, doing
multiplication in uint64_t would not help at all and add_chunk() API
needs its parameter types updated [*].

    side note: I really wish that the language and the compiler
    helped us so that we didn't have to do this---after all, our
    function prototype says the result will be passed as a certain
    type, so it would be nice if the arithmetic to compute that
    result were automatically carried out in a way not to cause
    truncation.

quoted

 * these mutiplications were introduced in "midx: use chunk-format
   API in write_midx_internal()"; that step should use the
   arithmetic with cast (if necessary) from the start, no?

I wanted to isolate these changes specifically so we could be
careful about the multiplications and not be distracted by them
when converting to the chunk-format API. The multiplications were
"moved" by that patch, not "introduced".

Hmph, I somehow had an impression that they did not have truncation
issue in the original context, but perhaps I was wrong.  OK.

I did a full inspection of all multiplications in midx.c and
found a few more instances of possible overflow. Two are on the
read side, but they require the object lookup chunk to have size
4gb or larger. This is not _that_ far off from possibility! My
multi-pack-index for the Windows repository is currently ~1.6 GB
(in total, including the other chunks).

Thanks.