In the Linux kernel, the following vulnerability has been resolved:

erofs/zmap.c: Fix incorrect offset calculation

Effective offset to add to length was being incorrectly calculated,
which resulted in iomap->length being set to 0, triggering a WARN_ON
in iomap_iter_done().

Fix that, and describe it in comments.

This was reported as a crash by syzbot under an issue about a warning
encountered in iomap_iter_done(), but unrelated to erofs.

C reproducer: https://syzkaller.appspot.com/text?tag=ReproC&x=1037a6b2880000
Kernel config: https://syzkaller.appspot.com/text?tag=KernelConfig&x=e2021a61197ebe02
Dashboard link: https://syzkaller.appspot.com/bug?extid=a8e049cd3abd342936b6

https://git.kernel.org/stable/c/2144859229c1e74f52d3ea067338d314a83a8afb

https://git.kernel.org/stable/c/6acd87d50998ef0afafc441613aeaf5a8f5c9eff

https://git.kernel.org/stable/c/9f31d8c889d9a4e47bfcc6c4537d0c9f89fe582c