Ubuntu Manpages

composefs-dump

textual file format for composefs content

Both the composefs-info and the mkcompose commands support generation/consumptions of a textual descriptions of the contents of a composefs image. This can be used to inspect or modify an image, or to generate an image without having to have a local directory with the files in it.

The file format is very simple, with one file per line, first with a 11 fixed fields, followed by a variable number of extended attributes for the file. It is recommended, but not required to include a trailing newline for the last file.

Fields are separated by a single space, and lines by a single newline. Extended attributes further use '=' to separate key from value. Therefore all these characters, as well as non-printable characters are escaped in the fields ('=' only in xattr fields). Also, back-slashes have to be escaped as they are used as the escape mechanism.

Escapes are of the form \xXY which escapes a single byte using two hex digits. For example \x00 is the zero byte and \xff is the 255 byte. Optionally, these custom escapes are supported:

*\\*
: backslash.

\n
: newline.

\r
: carriage return.

\t
: tab

Optional fields that are not set contain '-', and if a field actually has that particular value it is escaped.

The fixed fields on a line are (all numbers in base 10 unless otherwise specified):

The full, absolute path of the file in the image. Any directories used as prefix in the path must have been in the file before this line.

The size of the file. This is ignored for directories.

The st_mode stat field the file in octal, which includes both the
permissions and the file type.

Additionally, if the file is a hardlink, then this field will
start with a single '@' character, and the payload field points
to the target file. Note that all other fields are typically
filled out for a hardlink as the target, but for generation
of a new file we ignore all the fields except the payload.

The st_nlink stat field.

The owner uid.

The owner gid.

The st_rdev stat field.

The modification time in seconds and nanoseconds since the unix
epoch, separated by '.'. Note this is not a float, "1.1" means
one second and one nanosecond.

The payload of the file. For symbolic links this means the symlink targets. For regular files this is the relative pathname for the backing files. For hardlinks (see MODE), this is the path of another file in this file that this is a hardlink of.

Small files can inline the actual content in the composefs image. This contains an escaped version of the content. This must match the size specified in SIZE

A fs-verity digest for the file (only used for regular files, and not if CONTENT is set) that will be validated against backing files when used.

After the fixed fields comes the xattrs, escaped and space-separated in the form KEY=VALUE. Note that '=' must be escaped in KEY.

/ 4096 40755 4 1000 1000 0 1695372970.944925700 - - - security.selinux=unconfined_u:object_r:unlabeled_t:s0\x00
/a\x20dir\x20w\x20space 27 40755 2 1000 1000 0 1694598852.869646118 - - - security.selinux=unconfined_u:object_r:unlabeled_t:s0\x00
/a-dir 45 40755 2 1000 1000 0 1674041780.601887980 - - - security.selinux=unconfined_u:object_r:unlabeled_t:s0\x00
/a-dir/a-file 259 100644 1 1000 1000 0 1695368732.385062094 35/d02f81325122d77ec1d11baba655bc9bf8a891ab26119a41c50fa03ddfb408 - 35d02f81325122d77ec1d11baba655bc9bf8a891ab26119a41c50fa03ddfb408 security.selinux=unconfined_u:object_r:unlabeled_t:s0\x00
/a-hardlink 259 @100644 1 1000 1000 0 1695368732.385062094 /a-dir/a-file - 35d02f81325122d77ec1d11baba655bc9bf8a891ab26119a41c50fa03ddfb408 security.selinux=unconfined_u:object_r:unlabeled_t:s0\x00
/inline.txt 10 100644 1 1000 1000 0 1697019909.446146440 - some-text\n - security.selinux=unconfined_u:object_r:unlabeled_t:s0\x00

composefs-info(1), mkcomposefs(1)

composefs upstream ⟨https://github.com/containers/composefs⟩