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userns: detect initial namespace by inode #240

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userns: detect initial namespace by inode #240

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13 changes: 13 additions & 0 deletions userns/userns_linux.go
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Original file line number Diff line number Diff line change
Expand Up @@ -5,8 +5,12 @@ import (
"fmt"
"os"
"sync"
"syscall"
)

// See PROC_USER_INIT_INO in https://github.com/torvalds/linux/blob/v7.1/include/uapi/linux/nsfs.h#L50.
const procUserInitIno = 0xEFFFFFFD
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var inUserNS = sync.OnceValue(runningInUserNS)

// runningInUserNS detects whether we are currently running in a user namespace.
Expand All @@ -17,6 +21,15 @@ var inUserNS = sync.OnceValue(runningInUserNS)
// [libcontainer/runc]: https://github.com/opencontainers/runc/blob/3778ae603c706494fd1e2c2faf83b406e38d687d/libcontainer/userns/userns_linux.go#L12-L49
// [lcx/incus]: https://github.com/lxc/incus/blob/e45085dd42f826b3c8c3228e9733c0b6f998eafe/shared/util.go#L678-L700
func runningInUserNS() bool {
var st syscall.Stat_t
if err := syscall.Stat("/proc/self/ns/user", &st); err == nil {
return st.Ino != procUserInitIno
} else if !os.IsNotExist(err) {
return false

@AkihiroSuda AkihiroSuda Aug 3, 2026

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This should still fall back to the legacy method?

@aleivag aleivag Aug 3, 2026

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This should still fall back to the legacy method?

I can do this... but i think this might be the wrong way of doing it... consider thisL If the file "/proc/self/ns/user" is there, it means a modern kernel, so if stat fails for any other reason than "file does not exists" it mens that you are still on a modern kernel but something unexpected is wrong with /proc, permissions, mount state, or namespace access, its safer to assume that you are not in the init namespace than to fallback in the old criteria (specially if you plan to remove this line once older kernel are not supported) ...

}

// TODO: Remove this fallback once Linux kernels older than 3.8 are no
// longer supported.
file, err := os.Open("/proc/self/uid_map")
if err != nil {
// This kernel-provided file only exists if user namespaces are supported.
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