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

[Heinrich Schuchardt]

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
> timestamps.
>
> 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 <n+grub@monade.li>
> Tested-by: Naïm Favier <n+grub@monade.li>
> Signed-off-by: Tomasz Kramkowski <tk@the-tk.com>
> ---
>   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 
> (ctxt.entry.type_specific.file.m_time),
>                                             
> ctxt.entry.type_specific.file.m_time_tenth,
>                                             &info.mtime);
> +      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".

>   #else
>         if (ctxt.dir.attr & GRUB_FAT_ATTR_VOLUME_ID)
>         continue;
>         info.mtimeset = grub_fat_timestamp (grub_le_to_cpu16 (ctxt.dir.w_time),
>                                           grub_le_to_cpu16 (ctxt.dir.w_date),
>                                           &info.mtime);
> -#endif
>         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

Heinrich

> +#endif
>
>         if (hook (ctxt.filename, &info, hook_data))
>         break;