Re: [RFC PATCH] ovl: keep merged and impure readdir caches separate

Hi Amir,

After a lot of debug and traces I found another bug in
ovl_iterate_merged(): err is set from PTR_ERR(cache) before the
IS_ERR(cache) check, so on success err holds the truncated cache
pointer.

Claude generated this: 
  getdents64
  └── iterate_dir(outer_overlay_file)
      └── ovl_iterate_merged                                    [OUTER]
          │
          ├── cache = ovl_cache_get(dentry):                    [OUTER ovl_cache_get]
          │   │   cache_local = kzalloc(...)
          │   │   res = ovl_dir_read_merged(...)
          │   │   │
          │   │   └── iterate_dir(inner_overlay_file)
          │   │       └── ovl_iterate_merged                    [INNER]
          │   │           ├── cache_inner = ovl_cache_get(...)  valid 0xFFFF8881_CAC4D940
          │   │           ├── err = PTR_ERR(cache_inner)        = -893068992 (low32 of ptr)
          │   │           ├── IS_ERR(cache_inner) → FALSE
          │   │           └── return err;                       ← leaked stale int
          │   │
          │   └── return ERR_PTR(res);                          res = -893068992 (int)
          │                                                     sign-extended →
          │                                                     (void *)0xFFFFFFFF_CAC4D940
          │
          ├── err = PTR_ERR(cache);                             err = -893068992
          ├── if (IS_ERR(cache))   ← FALSE: IS_ERR only trips on top 4K
          │                                  (errno window);
          │                                  0xFFFFFFFF_CAC4D940 sits below
          │       return err;                                    ← skipped
          ├── od->cache = cache;   ★ corrupted pointer stored
          └── ovl_seek_cursor(od, ctx->pos)
                  list_for_each(p, &od->cache->entries)
                  p = *(&od->cache->entries) on bad pointer     ★ PAGE FAULT

Sent it as a separate patch: "[PATCH] ovl: keep err zero after
successful ovl_cache_get()".

Let me know what you think.

Regards,
Nirmoy