make_ext4fs to generate filesystem [LWN.net]

make_ext4fs to generate filesystem

Posted Oct 9, 2020 2:58 UTC (Fri) by djwong (subscriber, #23506)
In reply to: make_ext4fs to generate filesystem by jhhaller
Parent article: The ABI status of filesystem formats

Yeah... mke2fs -d takes care of slurping a directory tree into the new filesystem now.

Also, that's a very interesting man page link-- now I've learned where the mkfs.xfs "protofile" format comes from! The manpage for mkfs.xfs even features a very similar example file.

Er, thanks!

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make_ext4fs to generate filesystem

Posted Oct 9, 2020 15:05 UTC (Fri) by jhhaller (guest, #56103) [Link] (1 responses)

The -d option would still require root access to allow creating device file entries which may not match the host's devices.
Setting the correct file ownership is another aspect requiring root access. The protofile addresses both of these issues,
as it overwrites the filesystem owner, permissions, and allows creating device files. I doubt mkfs was originally
concerned with CI systems, but creating filesystem images on a non-native host was likely an early concern.

make_ext4fs to generate filesystem

Posted Oct 12, 2020 16:36 UTC (Mon) by mebrown (subscriber, #7960) [Link]

The yocto build system has a utility called 'pseudo'.

It uses an LD_PRELOAD library such that anything doing a mknod() libc call is saved to a database instead of invoking the underlying mknod() call, and instead just creates a regular placeholder file.

Then when you run mke2fs, similarly the LD_PRELOAD then hooks stat() such that mke2fs reading the placeholder file actually reads the details from the database instead of the filesystem.

Seems a little hacky at first, but seems to work well enough in practice.