Re: [PATCH v2 2/2] fat: Support file modification times

[David Michael]

On Fri, Mar 6, 2020 at 7:19 AM Daniel Kiper <address@hidden> wrote:
> On Tue, Mar 03, 2020 at 02:41:08PM -0500, David Michael wrote:
> > This allows comparing file ages on EFI system partitions.
> >
> > Signed-off-by: David Michael <address@hidden>
> > ---
> >
> > Changes since v1:
> >   - Added the previous patch to help support exfat
> >   - Added exfat timestamp conversion + setting
> >   - Switched to datetime variable name for consistency with the header
> >   - Switched to tabs-for-alignment for consistency in the file
> >
> >  grub-core/fs/fat.c | 47 ++++++++++++++++++++++++++++++++++++++++++++++
> >  1 file changed, 47 insertions(+)
> >
> > diff --git a/grub-core/fs/fat.c b/grub-core/fs/fat.c
> > index dc493add2..bacf9e60f 100644
> > --- a/grub-core/fs/fat.c
> > +++ b/grub-core/fs/fat.c
> > @@ -26,6 +26,7 @@
> >  #include <grub/err.h>
> >  #include <grub/dl.h>
> >  #include <grub/charset.h>
> > +#include <grub/datetime.h>
> >  #ifndef MODE_EXFAT
> >  #include <grub/fat.h>
> >  #else
> > @@ -730,6 +731,28 @@ grub_fat_iterate_dir_next (grub_fshelp_node_t node,
> >    return grub_errno ? : GRUB_ERR_EOF;
> >  }
> >
> > +static int
> > +grub_exfat_timestamp (grub_uint32_t field, grub_uint8_t msec, grub_int32_t 
> > *nix) {
> > +  struct grub_datetime datetime = {
> > +    .year   = (field >> 25) + 1980,
> > +    .month  = (field & 0x01E00000) >> 21,
> > +    .day    = (field & 0x001F0000) >> 16,
> > +    .hour   = (field & 0x0000F800) >> 11,
> > +    .minute = (field & 0x000007E0) >>  5,
> > +    .second = (field & 0x0000001F) * 2 + (msec >= 100 ? 1 : 0),
> > +  };
> > +
> > +  /* The conversion below allows seconds=60, so don't trust its 
> > validation.  */
>
> 60 seconds is a valid value in case of leap second. Hence, the question
> is: Can 60 seconds be represented properly in exFAT somehow? OK, this
> does not happen often. So, if we want ignore that case then at least
> I would like to have an explanation that we ignore it due to...

I enforced the 0-59 range because that is what is declared valid in
the spec.  See 11.3.5 in ECMA-107[1] and 7.4.8 for exfat[2].

> > +  if ((field & 0x1F) > 29)
> > +    return 0;
>
> You silently ignore this error. Should not you spit something to the
> console in this case? Or maybe at least set grub_errno? This way
> user will know that result of comparison should not be trusted...

The functions also rely on the grub_datetime2unixtime field
validations, which do not print errors.  I can add a general
grub_error if info.mtimeset is zero so it warns of all failures.

Thanks.

David

[1] https://www.ecma-international.org/publications/files/ECMA-ST/Ecma-107.pdf
[2] https://docs.microsoft.com/en-us/windows/win32/fileio/exfat-specification