How do ext2/3/4 filesystems deal with 64 bit time_t?

Question

I am working on small ext2 filesystem image manipulation tool (like listing directories, adding and extracting files without the need to mount it). I've just came into a problem with Unix timestamp fields. They are all 32 bit in ext filesystems. As we know, 32 bit Unix timestamps won't work after year 2038 anymore. Most software can easily deal with this problem by just changing definition of time_t to 64 bit. But it is not that easy for filesystems. They need to be compatible with existing implementations yet they need to be updated from time to time. How exactly ext filesystems do that? Particularly fields like s_mtime, s_wtime, s_lastcheck, i_atime, i_ctime, i_mtime and i_dtime.

score 3 · Accepted Answer · answered Feb 23 '20 at 03:14

If you look at ext4's inode structure, you'll see:

struct ext4_inode {
    ...
    __le32  i_ctime;    /* Inode Change time */
    ...
    __le32  i_ctime_extra;  /* extra Change time      (nsec << 2 | epoch) */
    ...
};

You won't find i_ctime_extra being used (*sigh*). Instead, you'll find:

#define EXT4_INODE_GET_XTIME(xtime, inode, raw_inode)               \
do {                                        \
    (inode)->xtime.tv_sec = (signed)le32_to_cpu((raw_inode)->xtime);    \
    if (EXT4_FITS_IN_INODE(raw_inode, EXT4_I(inode), xtime ## _extra)) {    \
        ext4_decode_extra_time(&(inode)->xtime,             \
                       raw_inode->xtime ## _extra);     \
        }                               \
    else                                    \
        (inode)->xtime.tv_nsec = 0;                 \
} while (0)


#define EXT4_EINODE_GET_XTIME(xtime, einode, raw_inode)                \
do {                                           \
    if (EXT4_FITS_IN_INODE(raw_inode, einode, xtime))              \
        (einode)->xtime.tv_sec =                       \
            (signed)le32_to_cpu((raw_inode)->xtime);           \
    else                                       \
        (einode)->xtime.tv_sec = 0;                    \
    if (EXT4_FITS_IN_INODE(raw_inode, einode, xtime ## _extra))        \
        ext4_decode_extra_time(&(einode)->xtime,               \
                       raw_inode->xtime ## _extra);        \
    else                                       \
        (einode)->xtime.tv_nsec = 0;                       \
} while (0)

And if you look at usages, you'll see that in-memory, a 64 bit integer is used, the separation only exists on disk.

So basically they are not using full 64 bit integers (on disk), but instead, 34 bits for seconds and 30 bits for nanoseconds in inodes. And 40 bit seconds count in superblock, without nanoseconds. Quite messy but will do the job (till year 2446). And it seems like ext2 and ext3 are stuck with 32 bit times. — , Feb 23 '20 at 20:57

How do ext2/3/4 filesystems deal with 64 bit time_t?

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