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Re: [PATCH] fat: Allow out-of-range FAT modification timestamps

From: Heinrich Schuchardt
Subject: Re: [PATCH] fat: Allow out-of-range FAT modification timestamps
Date: Wed, 25 Aug 2021 08:51:09 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.13.0

On 8/16/21 4:59 PM, Tomasz Kramkowski via Grub-devel wrote:
20def1a3c introduced support for file modification times to allow
comparison of file ages on EFI systems. This patch used
grub_datetime2unixtime which uses a 32 bit unix timestamp and as a
result did not allow the full range of times that FAT timestamps do.

In some situations a file with a timestamp of 1970-01-01 gets
transferred to a FAT partition, the timestamp ends up as 2098-01-01
because of FAT's use of the 1980-01-01 DOS epoch and lack of negative

Since 2098 is after 2038, this date cannot fit in a 32 bit timestamp.

Ideally grub should use 64 bit timestamps but I have not investigated
what kind of work would be required to support this.

Field mtime of struct grub_dirhook_info is already 64bit. See commit
81f1962393f4 ("fs: Use 64-bit type for filesystem timestamp"). Function
grub_datetime2unixtime needs to be fixed: The 2037 check should be
removed and the code has to be adjusted to correctly treat year 2100.

grub_unixtime2datetime already handles 64bit timestamps but ignores that
year 2100 is not a leap year:

grub_unixtime2datetime(4107585600) = 2100-02-29 12:00:00

This fixes bug #60565.

Reported-by: Naïm Favier <>
Tested-by: Naïm Favier <>
Signed-off-by: Tomasz Kramkowski <>
  grub-core/fs/fat.c | 7 ++++---
  1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/grub-core/fs/fat.c b/grub-core/fs/fat.c
index dd82e4ee3..34589d7db 100644
--- a/grub-core/fs/fat.c
+++ b/grub-core/fs/fat.c
@@ -1020,16 +1020,17 @@ grub_fat_dir (grub_device_t device, const char *path, 
grub_fs_dir_hook_t hook,
        info.mtimeset = grub_exfat_timestamp (grub_le_to_cpu32 
+      if (info.mtimeset == 0)
+       grub_dprintf("exfat", "invalid modification timestamp for %s\n", path);

According to the commit message there is nothing invalid about the
modification timestamp. It is a GRUB deficiency that it cannot handle
the same date range as FAT and exFAT.

So the message could be:
"GRUB cannot handle the modification timestamp for %s".

        if (ctxt.dir.attr & GRUB_FAT_ATTR_VOLUME_ID)
        info.mtimeset = grub_fat_timestamp (grub_le_to_cpu16 (ctxt.dir.w_time),
                                          grub_le_to_cpu16 (ctxt.dir.w_date),
        if (info.mtimeset == 0)
-       grub_error (GRUB_ERR_OUT_OF_RANGE,
-                   "invalid modification timestamp for %s", path);
+       grub_dprintf("fat", "invalid modification timestamp for %s\n", path);

I suggest to use grub_error() for both messages.

Best regards



        if (hook (ctxt.filename, &info, hook_data))

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