Introduction: The Inevitable Arithmetic of Time

Time is broken.

Well, not time itself, but our ability to count it. If you’re running a 32-bit system, your world ends at 03:14:07 UTC on January 19, 2038. That’s when the signed 32-bit integer we use to count seconds since 1970 overflows.

Boom. It’s 1901. Your embedded device bricks itself. Your infrastructure collapses.

This is the Year 2038 problem (Y2038). Unlike Y2K, which was mostly media hype and bad string parsing, Y2038 is an arithmetic inevitability. It is baked into the binary interface of the OS.

To fix this, the Linux kernel underwent a massive, multi-year refactoring. The goal? Drag 32-bit hardware kicking and screaming into a 64-bit future.

The function get_timespec64 is the gatekeeper of this transition. Let’s audit it.

The Data Structure Landscape: A Tale of Three Structs

To understand why this is hard, you have to understand the mess of data structures we’re dealing with.

1. The Legacy: struct timespec

This is the old school struct. On 32-bit systems, it’s 8 bytes.

struct timespec {
    time_t tv_sec;  /* 32-bit signed integer (DOOMED) */
    long   tv_nsec; /* 32-bit signed integer */
};

2. The Solution: struct __kernel_timespec

This is the new “wire format” for the User API (UAPI). It forces 64-bit seconds, even on 32-bit chips.

struct __kernel_timespec {
    __kernel_time64_t tv_sec;   /* 64-bit seconds (SAFE) */
    long long         tv_nsec;  /* 64-bit nanoseconds */
};

3. The Internal Standard: struct timespec64

Inside the kernel, we don’t want to do 64-bit math for nanoseconds on a 32-bit CPU. That’s slow. So we use a hybrid.

struct timespec64 {
    time64_t tv_sec;  /* 64-bit seconds */
    long     tv_nsec; /* Native long (32-bit or 64-bit) */
};

So get_timespec64 has to translate between these three worlds without breaking ABI compatibility for billions of existing binaries. No pressure.

Line-by-Line Analysis of get_timespec64

Let’s look at the code. It’s in kernel/time/time.c.

int get_timespec64(struct timespec64 *ts, const struct __kernel_timespec __user *uts)

First off, notice __user. This pointer is untrusted. It points to user memory, which might be garbage, unmapped, or malicious.

Local Stack Allocation

{
    struct __kernel_timespec kts;
    int ret;

We allocate a local copy kts on the kernel stack. This is standard security practice. We copy data in before we validate it to prevent Time-of-Check to Time-of-Use (TOCTOU) attacks.

The Copy Barrier

    ret = copy_from_user(&kts, uts, sizeof(kts));
    if (ret)
        return -EFAULT;

copy_from_user is the magic barrier. It handles page faults, checks permissions, and ensures the user isn’t trying to trick the kernel into reading its own secrets. If it fails, we bail.

Transferring Seconds

    ts->tv_sec = kts.tv_sec;

This is the easy part. 64-bit to 64-bit copy. Y2038 solved.

The Padding Sanitization Logic (The “Fun” Part)

Here is where it gets nasty.

    /* Zero out the padding for 32 bit systems or in compat mode */
    if (IS_ENABLED(CONFIG_64BIT_TIME) &&
        (!IS_ENABLED(CONFIG_64BIT) || in_compat_syscall()))
        kts.tv_nsec &= 0xFFFFFFFFUL;

What is this doing?

On a 32-bit system (or a 32-bit app on a 64-bit kernel), the user sees tv_nsec as 32 bits. But the kernel reads 64 bits. The upper 32 bits are padding.

If the user app didn’t initialize that padding (which it shouldn’t have to), it contains stack garbage. If we read that garbage as part of a 64-bit integer, 500ns becomes 18 quintillion + 500ns.

This line forcibly masks out the garbage. It says: “I know you’re a 32-bit app, so I’m ignoring the upper bits.” It’s a hack, but a necessary one to keep legacy apps alive.

Transferring Nanoseconds

    ts->tv_nsec = kts.tv_nsec;
    return 0;
}

Finally, we copy the sanitized nanoseconds. Success.

Conclusion: The Guardian of the Epoch

get_timespec64 isn’t just a memcpy. It’s a sophisticated adaptation layer that keeps the Linux ecosystem from imploding in 2038.

It absorbs the complexity of ABI transitions, handles alignment nightmares, and sanitizes input so that the kernel doesn’t choke on garbage data.

To be honest, it’s impressive. It’s the kind of low-level plumbing that no one appreciates until it breaks. But thanks to this function, when the clock ticks past 03:14:07 UTC on January 19, 2038, Linux will just keep ticking.

At least, I hope so.