I found two problems in the grub_datetime2unixtime() function.

Problem 1:
The grub_datetime2unixtime() function returns a 64-bit integer unixtime as the second argument, but the variable used for internal calculation is a 32-bit integer variable.

Problem 2:
Year exceeding the unixtime 32-bit integer value is regarded as an error.


include/grub/datetime.h:

 54 static inline int
 55 grub_datetime2unixtime (const struct grub_datetime *datetime, grub_int64_t *nix)
 56 {
 57   grub_int32_t ret; <= Problem 1: 32bit integer
...snip...
 80   if (datetime->year > 2038 || datetime->year < 1901) <= Problem 2: Year exceeding the unixtime 32-bit integer value is regarded as an error
 81     return 0;
...snip...
122   *nix = ret; <= Assign 32-bit integer value to 64-bit integer variable
123   return 1;

Due to these problems, the "grub-probe --device DEVICE --target=fs" command cannot recognize the FAT file system if there are directories or files dated after January 19, 2038 in the / directory of the FAT file system.

Steps to Reproduce:

Run on Fedora 36 Workstation Live image.

# fallocate -l 100m fat32.img
# LOOPDEV=$(losetup --show -f fat32.img)
# mkdosfs -F 32 $LOOPDEV
# mkdir -p /mnt/fat32
# mount $LOOPDEV /mnt/fat32
# mkdir /mnt/fat32/EFI
# touch -d "2040-01-01" /mnt/fat32/EFI
# grub2-probe --version
grub2-probe (GRUB) 2.06
# grub2-probe --device $LOOPDEV --target=fs
grub2-probe: error: ../grub-core/kern/fs.c:120:unknown filesystem.

Fedora 37 Workstation Live image does not reproduce any problems because commit e43f3d93b28cce852c110c7a8e40d8311bcd8bb1 (fs/fat: Don't error when mtime is 0) is applied.