From: Johannes Schindelin <johannes.schindelin@gmx.de>

On platforms where `unsigned long` and `size_t` differ in bit size, we want to clamp the buffers we pass to zlib to the former's size, as per d05d666977 (git-zlib: handle data streams larger than 4GB, 2026-05-08).

The logic introduced in that commit performs a clamping to the bits, though, which fails to do what is needed here: If too many bytes are available in the buffers, we need to clamp to the maximum value of an `unsigned long`. Otherwise, we ask zlib to use too small buffers, in the worst case using 0 as the size (think: a value whose 32 lowest bits are all zero).

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
---
    zlib: properly clamp to uLong
    
    I re-read this logic earlier this week... and I am quite convinced that
    it needs to be fixed.

Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-2153%2Fdscho%2Ffix-ulong-clamping-for-zlib-v1
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-2153/dscho/fix-ulong-clamping-for-zlib-v1
Pull-Request: https://github.com/gitgitgadget/git/pull/2153

 git-zlib.c | 13 +++++++++----
 1 file changed, 9 insertions(+), 4 deletions(-)

The original in js/objects-larger-than-4gb-on-windows says things like:

+ s->z.total_in = (uLong)(s->total_in & ULONG_MAX_VALUE); + s->z.total_out = (uLong)(s->total_out & ULONG_MAX_VALUE);

Your patch ...

... is an obvious fix for that.

Because we now clamp (not "taking lower bits of") s->total_out to a value between 0..4GB and store it in s->z.total_out in pre-call, let zlib do its thing that increments s->z.total_out modulo 4GB, and we clamp the s->total_out (before incrementing) the same way in post_call here, both sides of "!=" above even out. But the comment before this comparison that claims that "we ... verify only the low bits match" is a bit off the reality, I suspect.

Hi Junio,

On Thu, 18 Jun 2026, Junio C Hamano wrote:

Technically, the range is 0..(4GB-1), but yes, that's exactly the idea.

If we clamped bit-wise, i.e. to the lower bits as is currently done, we would _also_ stay within that range, but we'd restrict the total size unnecessarily in most cases (i.e. in all cases where `total_out` isn't one less than an exact multiple of 4GB). In the worst case, we'd restrict to 0 bytes, in which case we would run into an infinite loop because zlib has no space to work with and we'd try again and again to whittle away a chunk of that large input.

I am afraid that the comment is still true. The thing is, we're trying to compare the _real_ `total_out + bytes_produced` to zlib's necessarily restricted `total_out` (we cannot change the data type of that attribute of `struct z_stream_s`, it's not ours to change, it'll remain `uLong` because zlib made the same mistake as Git to choose that imprecise data type for memory size calculations). The sum `total_out + bytes_produced` is of type `size_t`, the attribute `s->z.total_out` is of type `uLong`.

Therefore, we still need to clamp bit-wise, as the _real_ `total_out + bytes_produced` may very well exceed the maximal value of `s->z.total_out`, and the zlib operation will _still_ have produced the expected number of bytes, i.e. that sanity check should _pass_.

If anything, we _could_ consider dropping that masking of `s->z.total_out` to the maximal `unsigned long` value, seeing as `s->z.total_out` _is_ of that data type and therefore cannot reasonably exceed that. But then, there might emerge a zlib variant in the future that recapitulates Git's effort to use `size_t` where `size_t` is due, and compiling/linking against _that_ zlib variant would need this mask, otherwise the sanity check could fail for completely bogus reasons.

So: The comment is still correct, even with the adjusted logic.

Ciao, Johannes