CVE-2024-57952: Revert "libfs: fix infinite directory reads for offset dir"

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

Revert "libfs: fix infinite directory reads for offset dir"

The current directory offset allocator (based on mtreealloccyclic) stores the next offset value to return in octx->nextoffset. This mechanism typically returns values that increase monotonically over time. Eventually, though, the newly allocated offset value wraps back to a low number (say, 2) which is smaller than other already- allocated offset values.

Yu Kuai <yukuai3@huawei.com> reports that, after commit 64a7ce76fb90 ("libfs: fix infinite directory reads for offset dir"), if a directory's offset allocator wraps, existing entries are no longer visible via readdir/getdents because offsetreaddir() stops listing entries once an entry's offset is larger than octx->nextoffset. These entries vanish persistently -- they can be looked up, but will never again appear in readdir(3) output.

The reason for this is that the commit treats directory offsets as monotonically increasing integer values rather than opaque cookies, and introduces this comparison:

if (dentry2offset(dentry) >= lastindex) {

On 64-bit platforms, the directory offset value upper bound is 2^63 - 1. Directory offsets will monotonically increase for millions of years without wrapping.

On 32-bit platforms, however, LONGMAX is 2^31 - 1. The allocator can wrap after only a few weeks (at worst).

Revert commit 64a7ce76fb90 ("libfs: fix infinite directory reads for offset dir") to prepare for a fix that can work properly on 32-bit systems and might apply to recent LTS kernels where shmem employs the simpleoffset mechanism.