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Added phase bits to cksum tags

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This carves out two more bits in cksum tags to store the "phase" of the
rbyd block (maybe the name is too fancy, this is just the lowest 2 bits
of the block address):

  LFSR_TAG_CKSUM        0x300p  v-11 ---- ---- -pqq
                                                ^ ^
                                                | '-- phase bits
                                                '---- perturb bit

The intention here is to catch mrootanchors that are "out-of-phase",
i.e. they've been shifted by a small number of blocks.

This can happen if we find the wrong mrootanchor (after, say, a magic
scan), and risks filesystem corruption:

                formatted
  .-----------------'-----------------.
                          mounted
           .-----------------'-----------------.
  .--------+--------+--------+--------+ ...
  |(erased)| mroot  |
  |        | anchor |                   ...
  |        |        |
  '--------+--------+--------+--------+ ...

Including the lower 2 bits of the block address in cksum tags avoids
this, for up to a 3 block shift (the maximum number of redund
mrootanchors).

---

Note that cksum tags really are the only place we could put these bits.
Anywhere else and they would interfere with the canonical cksum, which
would break error correction. By definition these need to be different
per block.

We include these phase bits in every cksum tag (because it's easier),
but these don't really say much about mdirs that are not the
mrootanchor. Non-anchor mdirs can have arbitrary block addresses,
therefore arbitrary phase bits.

You _might_ be able to do something interesting if you sort the rbyd
addresses and use the index as the phase bits, but that would add quite
a bit of code for questionable benefit...

You could argue this adds noise to our cksums, but:

1. 2 bits seems like a really small amount of noise
2. our cksums are just crc32cs
3. the phase bits humorously never change when you rewrite a block

---

As with any feature this adds code, but only a small amount. I think
it's worth the extra protection:

           code          stack          ctx
  before: 35792           2368          636
  after:  35824 (+0.1%)   2368 (+0.0%)  636 (+0.0%)

Also added test_mount_incompat_out_of_phase to test this.

The dbg scripts _don't_ error (block mismatch seems likely when
debugging), but dbgrbyd.py at least adds phase mismatch notes in
-l/--log mode.
This commit is contained in:
Christopher Haster
2025-04-29 17:31:12 -05:00
parent 97f8eeb9e9
commit de7564e448
9 changed files with 141 additions and 56 deletions
Download Patch File Download Diff File Expand all files Collapse all files

16
scripts/dbgbmap.py
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@@ -67,8 +67,9 @@ TAG_B = 0x0000
TAG_R = 0x2000
TAG_LE = 0x0000
TAG_GT = 0x1000
TAG_CKSUM = 0x3000 ## 0x300p v-11 ---- ---- ---p
TAG_P = 0x0001
TAG_CKSUM = 0x3000 ## 0x300p v-11 ---- ---- -pqq
TAG_PHASE = 0x0003
TAG_PERTURB = 0x0004
TAG_NOTE = 0x3100 ## 0x3100 v-11 ---1 ---- ----
TAG_ECKSUM = 0x3200 ## 0x3200 v-11 --1- ---- ----
TAG_GCKSUMDELTA = 0x3300 ## 0x3300 v-11 --11 ---- ----
@@ -346,7 +347,7 @@ def tagrepr(tag, weight=None, size=None, *,
return '%s%sattr 0x%02x%s%s' % (
'shrub' if tag & TAG_SHRUB else '',
's' if tag & 0x100 else 'u',
((tag & 0x100) >> 1) ^ (tag & 0xff),
((tag & 0x100) >> 1) + (tag & 0xff),
' w%d' % weight if weight else '',
' %s' % size if size is not None else '')
# alt pointers
@@ -362,9 +363,10 @@ def tagrepr(tag, weight=None, size=None, *,
else '')
# checksum tags
elif (tag & 0x7f00) == TAG_CKSUM:
return 'cksum%s%s%s%s' % (
'p' if not tag & 0xfe and tag & TAG_P else '',
' 0x%02x' % (tag & 0xff) if tag & 0xfe else '',
return 'cksum%s%s%s%s%s' % (
'q%d' % (tag & 0x3),
'p' if tag & TAG_PERTURB else '',
' 0x%02x' % (tag & 0xff) if tag & 0xf8 else '',
' w%d' % weight if weight else '',
' %s' % size if size is not None else '')
# note tags
@@ -684,7 +686,7 @@ class Rbyd:
gcksumdelta = gcksumdelta_
gcksumdelta_ = None
# update perturb bit
perturb = tag & TAG_P
perturb = bool(tag & TAG_PERTURB)
# revert to data cksum and perturb
cksum__ = cksum_ ^ (0xfca42daf if perturb else 0)