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[release-1.5 backport] libct: Enforce nr_inodes=2 to fix Focal mount errors #5355

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Merged
rata merged 2 commits into from
Jun 30, 2026
Merged

[release-1.5 backport] libct: Enforce nr_inodes=2 to fix Focal mount errors #5355

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a94d089
libct: Enforce nr_inodes=2 to fix Focal mount errors
lifubang Jun 29, 2026
b678b1f
libct: add a fallback for nr_inodes=2
lifubang Jun 29, 2026
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13 changes: 12 additions & 1 deletion libcontainer/rootfs_linux.go
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Original file line number Diff line number Diff line change
Expand Up @@ -1323,7 +1323,18 @@ func verifyDevNull(f *os.File) error {

// maskDir mounts a read-only tmpfs on top of the specified path.
func maskDir(path, mountLabel string) error {
return mount("tmpfs", path, "tmpfs", unix.MS_RDONLY, label.FormatMountLabel("nr_blocks=1,nr_inodes=1", mountLabel))
// On most kernels `nr_inodes=1` works fine. However, Ubuntu 20.04 (Focal) with
// the official 5.4 kernel carries a private patch in mm/shmem.c that rejects
// "nr_inodes<2", so let's keep `nr_inodes=2` here!
// For reference, search for "case Opt_nr_inodes" in:
// https://git.launchpad.net/~ubuntu-kernel/ubuntu/+source/linux/+git/focal/plain/mm/shmem.c?h=Ubuntu-5.4.0-216.236
err := mount("tmpfs", path, "tmpfs", unix.MS_RDONLY, label.FormatMountLabel("nr_blocks=1,nr_inodes=2", mountLabel))
// We don't know whether some kernels will fail with "nr_inodes=2",
// so let's fall back to mount a tmpfs without this option.
if errors.Is(err, unix.EINVAL) {
err = mount("tmpfs", path, "tmpfs", unix.MS_RDONLY, label.FormatMountLabel("nr_blocks=1", mountLabel))
}
return err
}

// maskPaths masks the top of the specified paths inside a container to avoid
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