bcachefs: question about fixing use-after-free in bch2_extent_ptr_to_text

Hi Kent,

I'm investigating syzbot report 564efbe31172fe908429 and have 
successfully reproduced the crash with the C reproducer.

Link: https://syzkaller.appspot.com/bug?extid=564efbe31172fe908429

The issue: bch2_extent_ptr_to_text() crashes when called on a corrupted
extent pointer (bucket 27 < first_bucket 1024). The validation already
detected this:
  "invalid bkey: pointer before first bucket (27 < 1024), deleting"

But bch2_extent_ptr_to_text() is still called afterward (for debug 
output) and crashes in dev_ptr_stale_rcu() when it tries to access 
bucket metadata at an invalid offset.

Looking at the code in fs/bcachefs/extents.c:1247, the function calls
dev_ptr_stale_rcu() without checking if the bucket number is valid.

Should I add bounds checking like this:

  if (b < ca->mi.first_bucket || b >= ca->mi.nbuckets) {
      prt_str(out, " invalid");
  } else {
      int stale = dev_ptr_stale_rcu(ca, ptr);
      ...
  }

Or is there a better approach? I want to make sure I'm fixing this
correctly since you reviewed my previous dirent patch.

Thanks,
Nirbhay Sharma