[PATCH] erofs: fix unsigned underflow in z_erofs_lz4_handle_overlap()

[Junrui Luo]

On Thu, Apr 09, 2026 at 06:56:42PM +0800, Gao Xiang wrote:
> Can you share your initial crafted image binary
> with `gzip -9 | base64` encoding here?

$ gzip -9 < /tmp/erofs-test/test.erofs | base64
H4sIAJGR12kCA+3SPUoDQRgG4MkmkkZk8QRbRFIIi9hbpEjrHQI5ghfwCN5BLCzTGtLbBI+gdilS
Jo1CnIm7GEXFxhT6PDDwfrs73/ywIQD/1ePD4r7Ou6ETsrq4mu7XcWfj++Pb58nJU/9iPNtbjhan
04/9GtX4qVYc814WDqt6FaX5s+ZwXXeq52lndT6IuVvlblytLMvh4Gzwaf90nsvz2DF/21+20T/l
dgp5s1jXRaN4t/8izsy/OUB6e/Qa79r+JwAAAAAAAL52vQVuGQAAAP6+my1wywAAAAAAAADwu14A
TsEYtgBQAAA=

In QEMU:
$ mount -t erofs -o cache_strategy=disabled test.erofs /mnt
$ dd if=/mnt/data of=/dev/null bs=4096 count=1

> I think the proper place to fix this is in
> z_erofs_map_sanity_check().
 
I will resend with the check in
z_erofs_map_sanity_check() instead if the reproducer is acceptable.

Thanks,
Junrui Luo