> > Ah, that makes sense to me now. Would you please include the detailed
> > explaination in commit log as well?
>
> ...and maybe a sample output of a seek_data/seek_hole scan between a
> gocryptfs file and the ext4fs underneath it? I'm still trying to wrap
> my head around what the problem here is.
>
> It might also help to describe where the 32 bytes of overhead goes --
> are you interleaving the overhead inline with 4k of encrypted content?
Yes, it's all inline, the file format is like this [2]:
file header ... 18 bytes
data block 1 ... 16 bytes block header (IV)
1-4096 bytes user data
16 bytes block trailer (MAC)
[more data blocks...]
I am attaching the SEEK_DATA/SEEK_HOLE trace below [1]. One seek goes like this:
(1) gocryptfs gets a seek() call via FUSE
(2) translate plaintext to ciphertext offset
(3) call ext4 seek
(4) translate back to plaintext offset and return
Actually Eryu's first comment had me start thinking about this, and
what gocryptfs
does is a little stupid, because the offsets that get returned to userspace may
point in the middle of a block, but gocryptfs reads or writes in
blocks of 4128 bytes (= 4096 bytes of user data)
(*except at EOF), so it may as well round up at seek already.
This would also mean that the offsets returned to userspace get
aligned to 4096 bytes,
and generic/286 would just work as is. Another filesystem not aligning
to 4096 bytes may hit
the underflow, but it won't be gocryptfs :)
Thanks, Jakob
[1]:
gocryptfs seek( 0, SEEK_DATA) -> translate -> ext4 seek(
0, SEEK_DATA) = 0 -> translate -> return 0
gocryptfs seek( 0, SEEK_HOLE) -> translate -> ext4 seek(
0, SEEK_HOLE) = 1060864 -> translate -> return 1052622
gocryptfs seek( 1052622, SEEK_DATA) -> translate -> ext4 seek(
1060864, SEEK_DATA) = 5283840 -> translate -> return 5242862
gocryptfs seek( 5242862, SEEK_HOLE) -> translate -> ext4 seek(
5283840, SEEK_HOLE) = 6344704 -> translate -> return 6295502
gocryptfs seek( 6295502, SEEK_DATA) -> translate -> ext4 seek(
6344704, SEEK_DATA) = 10567680 -> translate -> return 10485742
gocryptfs seek( 10485742, SEEK_HOLE) -> translate -> ext4 seek(
10567680, SEEK_HOLE) = 11628544 -> translate -> return 11538382
gocryptfs seek( 11538382, SEEK_DATA) -> translate -> ext4 seek(
11628544, SEEK_DATA) = 15851520 -> translate -> return 15728622
gocryptfs seek( 15728622, SEEK_HOLE) -> translate -> ext4 seek(
15851520, SEEK_HOLE) = 16912384 -> translate -> return 16781262
gocryptfs seek( 16781262, SEEK_DATA) -> translate -> ext4 seek(
16912384, SEEK_DATA) = 21135360 -> translate -> return 20971502
gocryptfs seek( 20971502, SEEK_HOLE) -> translate -> ext4 seek(
21135360, SEEK_HOLE) = 22196224 -> translate -> return 22024142
gocryptfs seek( 22024142, SEEK_DATA) -> translate -> ext4 seek(
22196224, SEEK_DATA) = 26419200 -> translate -> return 26214382
gocryptfs seek( 26214382, SEEK_HOLE) -> translate -> ext4 seek(
26419200, SEEK_HOLE) = 27480064 -> translate -> return 27267022
gocryptfs seek( 27267022, SEEK_DATA) -> translate -> ext4 seek(
27480064, SEEK_DATA) = 31703040 -> translate -> return 31457262
gocryptfs seek( 31457262, SEEK_HOLE) -> translate -> ext4 seek(
31703040, SEEK_HOLE) = 32763904 -> translate -> return 32509902
gocryptfs seek( 32509902, SEEK_DATA) -> translate -> ext4 seek(
32763904, SEEK_DATA) = 36986880 -> translate -> return 36700142
gocryptfs seek( 36700142, SEEK_HOLE) -> translate -> ext4 seek(
36986880, SEEK_HOLE) = 38047744 -> translate -> return 37752782
gocryptfs seek( 37752782, SEEK_DATA) -> translate -> ext4 seek(
38047744, SEEK_DATA) = 42270720 -> translate -> return 41943022
gocryptfs seek( 41943022, SEEK_HOLE) -> translate -> ext4 seek(
42270720, SEEK_HOLE) = 43331584 -> translate -> return 42995662
gocryptfs seek( 42995662, SEEK_DATA) -> translate -> ext4 seek(
43331584, SEEK_DATA) = 47554560 -> translate -> return 47185902
gocryptfs seek( 47185902, SEEK_HOLE) -> translate -> ext4 seek(
47554560, SEEK_HOLE) = 48615424 -> translate -> return 48238542
gocryptfs seek( 48238542, SEEK_DATA) -> translate -> ext4 seek(
48615424, SEEK_DATA) = 52838400 -> translate -> return 52428782
gocryptfs seek( 52428782, SEEK_HOLE) -> translate -> ext4 seek(
52838400, SEEK_HOLE) = 53899264 -> translate -> return 53481422
gocryptfs seek( 53481422, SEEK_DATA) -> translate -> ext4 seek(
53899264, SEEK_DATA) = 58122240 -> translate -> return 57671662
gocryptfs seek( 57671662, SEEK_HOLE) -> translate -> ext4 seek(
58122240, SEEK_HOLE) = 59183104 -> translate -> return 58724302
gocryptfs seek( 58724302, SEEK_DATA) -> translate -> ext4 seek(
59183104, SEEK_DATA) = 63406080 -> translate -> return 62914542
gocryptfs seek( 62914542, SEEK_HOLE) -> translate -> ext4 seek(
63406080, SEEK_HOLE) = 64466944 -> translate -> return 63967182
gocryptfs seek( 63967182, SEEK_DATA) -> translate -> ext4 seek(
64466944, SEEK_DATA) = 68689920 -> translate -> return 68157422
gocryptfs seek( 68157422, SEEK_HOLE) -> translate -> ext4 seek(
68689920, SEEK_HOLE) = 69750784 -> translate -> return 69210062
gocryptfs seek( 69210062, SEEK_DATA) -> translate -> ext4 seek(
69750784, SEEK_DATA) = 73973760 -> translate -> return 73400302
gocryptfs seek( 73400302, SEEK_HOLE) -> translate -> ext4 seek(
73973760, SEEK_HOLE) = 75034624 -> translate -> return 74452942
gocryptfs seek( 74452942, SEEK_DATA) -> translate -> ext4 seek(
75034624, SEEK_DATA) = 79257600 -> translate -> return 78643182
gocryptfs seek( 78643182, SEEK_HOLE) -> translate -> ext4 seek(
79257600, SEEK_HOLE) = 80318464 -> translate -> return 79695822
gocryptfs seek( 79695822, SEEK_DATA) -> translate -> ext4 seek(
80318464, SEEK_DATA) = 84541440 -> translate -> return 83886062
gocryptfs seek( 83886062, SEEK_HOLE) -> translate -> ext4 seek(
84541440, SEEK_HOLE) = 85602304 -> translate -> return 84938702
gocryptfs seek( 84938702, SEEK_DATA) -> translate -> ext4 seek(
85602304, SEEK_DATA) = 89825280 -> translate -> return 89128942
gocryptfs seek( 89128942, SEEK_HOLE) -> translate -> ext4 seek(
89825280, SEEK_HOLE) = 90886144 -> translate -> return 90181582
gocryptfs seek( 90181582, SEEK_DATA) -> translate -> ext4 seek(
90886144, SEEK_DATA) = 95109120 -> translate -> return 94371822
gocryptfs seek( 94371822, SEEK_HOLE) -> translate -> ext4 seek(
95109120, SEEK_HOLE) = 96169984 -> translate -> return 95424462
gocryptfs seek( 95424462, SEEK_DATA) -> translate -> ext4 seek(
96169984, SEEK_DATA) = 100392960 -> translate -> return 99614702
gocryptfs seek( 99614702, SEEK_HOLE) -> translate -> ext4
seek(100392960, SEEK_HOLE) = 101453824 -> translate -> return
100667342
gocryptfs seek(100667342, SEEK_DATA) -> translate -> ext4
seek(101453824, SEEK_DATA) = 105676800 -> translate -> return
104857582
gocryptfs seek(104857582, SEEK_HOLE) -> translate -> ext4
seek(105676800, SEEK_HOLE) = 106733586 -> translate -> return
105906176
[2]: https://github.com/rfjakob/gocryptfs/blob/master/Documentation/file-format.md
