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brainplot

Member

Registered: 2018-01-27

Posts: 35

[SOLVED] Confusion regarding cryptsetup output on encrypted swap part

Hello there,

I was configuring an encrypted swap partition using dm-crypt, as described here.
The partition I'm trying to use for my swap space is the following:

/dev/nvme0n1p2: PARTUUID="6588817b-63c8-47cc-8306-6672adeafeb1"

Such partition is roughly 4GiB in size, as confirmed by "lsblk" (N.B I am aware my swap is not mounted yet):

NAME        MAJ:MIN RM   SIZE RO TYPE  MOUNTPOINTS
nvme0n1     259:5    0 465.8G  0 disk
├─nvme0n1p1 259:6    0     1G  0 part  /boot
├─nvme0n1p2 259:7    0     4G  0 part
│ └─swap    254:1    0     4G  0 crypt
└─nvme0n1p3 259:8    0 460.8G  0 part
  └─root    254:0    0 460.7G  0 crypt /

I'm re-encrypting it upon every boot with a fresh key thanks to the following line in my /etc/crypttab:

# <name>    <device>                                             <password>          <options>
swap        PARTUUID=6588817b-63c8-47cc-8306-6672adeafeb1        /dev/urandom        swap,cipher=aes-xts-plain64,size=512,sector-size=4096

At this point I was trying to inspect my swap volume mapper with the following command and I am confused by the sizes reported in its output:

# cryptsetup status swap

/dev/mapper/swap is active.
type:    PLAIN
cipher:  aes-xts-plain64
keysize: 512 [bits]
key location: dm-crypt
device:  /dev/nvme0n1p2
sector size:  4096 [bytes]
offset:  0 [512-byte units] (0 [bytes])
size:    8392704 [512-byte units] (34376515584 [bytes])
mode:    read/write

Specifically, it says 8392704 [512-byte units] which adds up to the exact size in bytes of my swap partition, multiplying 8392704 by 512.

However, the other size - 34376515584 bytes - doesn't mean anything to me.
It's 32 GiB but that doesn't correspond to anything on my system. Initially I thought it could have been the amount of memory in my system but I have 64GB of RAM - not 32. So what gives?

Obviously I tried searching around and reading man pages but I could not find the answer to my question.

Last edited by brainplot (2025-12-30 13:29:44)

frostschutz

Member

Registered: 2013-11-15

Posts: 1,590

Re: [SOLVED] Confusion regarding cryptsetup output on encrypted swap part

probably a bug? says 512 byte units but multiplies with 4096. some math isn't mathing somewhere, in the cryptsetup status code.

https://gitlab.com/cryptsetup/cryptsetu … L1022-1025

the [512-byte units] part of the message is hardcoded, then multiplies with a variable sector size, which is incorrect if the size given was actually in 512-byte sectors regardless of specified sector size

Last edited by frostschutz (2025-12-29 23:37:32)

dimich

Member

From: Kharkiv, Ukraine

Registered: 2009-11-03

Posts: 449

Re: [SOLVED] Confusion regarding cryptsetup output on encrypted swap part

Yep, looks like they mistakenly multiply devmapper device size in 512-byte units by device-specific sector size, which can be 4096 bytes. Deserves to be reported as a bug.

Last edited by dimich (2025-12-30 07:44:31)

brainplot

Member

Registered: 2018-01-27

Posts: 35

Re: [SOLVED] Confusion regarding cryptsetup output on encrypted swap part

That makes sense. I reported it upstream.

Thank you frostschutz for taking the time to look through the code. I mentioned your finding in the issue.