diff --git a/snapshots/overlay/overlayutils/check.go b/snapshots/overlay/overlayutils/check.go index 5813394c3..75cf2ed8c 100644 --- a/snapshots/overlay/overlayutils/check.go +++ b/snapshots/overlay/overlayutils/check.go @@ -24,6 +24,8 @@ import ( "path/filepath" "syscall" + "golang.org/x/sys/unix" + kernel "github.com/containerd/containerd/contrib/seccomp/kernelversion" "github.com/containerd/containerd/log" "github.com/containerd/containerd/mount" @@ -201,11 +203,95 @@ func NeedsUserXAttr(d string) (bool, error) { // SupportsIDMappedMounts tells if this kernel supports idmapped mounts for overlayfs // or not. +// +// This function returns error whether the kernel supports idmapped mounts +// for overlayfs or not, i.e. if e.g. -ENOSYS may be returned as well as -EPERM. +// So, caller should check for (true, err == nil), otherwise treat it as there's +// no support from the kernel side. func SupportsIDMappedMounts() (bool, error) { // Fast path fiveDotNineteen := kernel.KernelVersion{Kernel: 5, Major: 19} if ok, err := kernel.GreaterEqualThan(fiveDotNineteen); err == nil && ok { return true, nil } - return false, nil + + // Do slow path, because idmapped mounts may be backported to older kernels. + uidMap := syscall.SysProcIDMap{ + ContainerID: 0, + HostID: 666, + Size: 1, + } + gidMap := syscall.SysProcIDMap{ + ContainerID: 0, + HostID: 666, + Size: 1, + } + td, err := os.MkdirTemp("", "ovl-idmapped-check") + if err != nil { + return false, fmt.Errorf("failed to create check directory: %w", err) + } + defer func() { + if err := os.RemoveAll(td); err != nil { + log.L.WithError(err).Warnf("failed to remove check directory %s", td) + } + }() + + for _, dir := range []string{"lower", "upper", "work", "merged"} { + if err = os.Mkdir(filepath.Join(td, dir), 0755); err != nil { + return false, fmt.Errorf("failed to create %s directory: %w", dir, err) + } + } + defer func() { + if err = os.RemoveAll(td); err != nil { + log.L.WithError(err).Warnf("failed remove overlay check directory %s", td) + } + }() + + if err = os.Lchown(filepath.Join(td, "upper"), uidMap.HostID, gidMap.HostID); err != nil { + return false, fmt.Errorf("failed to chown upper directory %s: %w", filepath.Join(td, "upper"), err) + } + + lowerDir := filepath.Join(td, "lower") + uidmap := fmt.Sprintf("%d:%d:%d", uidMap.ContainerID, uidMap.HostID, uidMap.Size) + gidmap := fmt.Sprintf("%d:%d:%d", gidMap.ContainerID, gidMap.HostID, gidMap.Size) + + usernsFd, childProcCleanUp, err := mount.GetUsernsFD(uidmap, gidmap) + if err != nil { + return false, err + } + defer childProcCleanUp() + + if err = mount.IDMapMount(lowerDir, lowerDir, usernsFd); err != nil { + return false, fmt.Errorf("failed to remap lowerdir %s: %w", lowerDir, err) + } + defer func() { + if err = unix.Unmount(lowerDir, 0); err != nil { + log.L.WithError(err).Warnf("failed to unmount lowerdir %s", lowerDir) + } + }() + + opts := fmt.Sprintf("index=off,lowerdir=%s,upperdir=%s,workdir=%s", lowerDir, filepath.Join(td, "upper"), filepath.Join(td, "work")) + if err = unix.Mount("", filepath.Join(td, "merged"), "overlay", uintptr(unix.MS_RDONLY), opts); err != nil { + return false, fmt.Errorf("failed to mount idmapped overlay to %s: %w", filepath.Join(td, "merged"), err) + } + defer func() { + if err = unix.Unmount(filepath.Join(td, "merged"), 0); err != nil { + log.L.WithError(err).Warnf("failed to unmount overlay check directory %s", filepath.Join(td, "merged")) + } + }() + + // NOTE: we can't just return true if mount didn't fail since overlay supports + // idmappings for {lower,upper}dir. That means we need to check merged directory + // to make sure it completely supports idmapped mounts. + st, err := os.Stat(filepath.Join(td, "merged")) + if err != nil { + return false, fmt.Errorf("failed to stat %s: %w", filepath.Join(td, "merged"), err) + } + if stat, ok := st.Sys().(*syscall.Stat_t); !ok { + return false, fmt.Errorf("incompatible types after stat call: *syscall.Stat_t expected") + } else if int(stat.Uid) != uidMap.HostID || int(stat.Gid) != gidMap.HostID { + return false, fmt.Errorf("bad mapping: expected {uid: %d, gid: %d}; real {uid: %d, gid: %d}", uidMap.HostID, gidMap.HostID, int(stat.Uid), int(stat.Gid)) + } + + return true, nil }