Re: [PATCH 3/4] luks2: set up dummy sector size during scan

[Fabian Vogt <fvogt@suse.de>]

Hi,

(I *finally* got to this topic again...)

Am Sonntag, 8. August 2021, 16:20:12 CET schrieb Patrick Steinhardt:
> On Fri, Aug 06, 2021 at 12:51:10PM +0800, Michael Chang via Grub-devel wrote:
> [snip]
> > diff --git a/grub-core/disk/cryptodisk.c b/grub-core/disk/cryptodisk.c
> > index 90f82b2d3..c2bb2b6eb 100644
> > --- a/grub-core/disk/cryptodisk.c
> > +++ b/grub-core/disk/cryptodisk.c
> > @@ -1040,6 +1040,7 @@ grub_cryptodisk_cheat_mount (const char *sourcedev, const char *cheat)
> >    grub_cryptodisk_t dev;
> >    grub_cryptodisk_dev_t cr;
> >    grub_disk_t source;
> > +  unsigned int cheat_sector_size;
> >  
> >    /* Try to open disk.  */
> >    source = grub_disk_open (sourcedev);
> > @@ -1062,6 +1063,25 @@ grub_cryptodisk_cheat_mount (const char *sourcedev, const char *cheat)
> >      if (!dev)
> >        continue;
> >  
> > +    /* Use the sector size and count of the cheat device */
> > +    dev->cheat_fd = grub_util_fd_open (cheat, GRUB_UTIL_FD_O_RDONLY);
> > +    if (!GRUB_UTIL_FD_IS_VALID (dev->cheat_fd))
> > +      {
> > +        grub_free (dev);
> > +        return grub_errno;
> > +      }
> > +    dev->total_sectors = grub_util_get_fd_size (dev->cheat_fd, cheat, &cheat_sector_size);
> > +    if (dev->total_sectors == -1)
> > +      {
> > +        grub_util_fd_close (dev->cheat_fd);
> > +        grub_free (dev);
> > +        return grub_errno;
> > +      }
> 
> We may want to print error messages for both error cases. Otherwise it's
> hard to tell why cheat mounting might've failed.

Agreed.

> > +    dev->log_sector_size = cheat_sector_size;
> > +    dev->total_sectors >>= dev->log_sector_size;
> > +    grub_util_fd_close (dev->cheat_fd);
> > +    dev->cheat_fd = GRUB_UTIL_FD_INVALID;
> 
> Wouldn't it make more sense to just use a separate variable for the FD?
> Feels kind of weird to put it into `dev->cheat_fd`, only to clone and
> unset the member variable immediately after we're done again.

A bit, yes, but the alternative is having a variable "int cheat_fd" which makes
it more confusing IMO. I'm fine with either option.

> In any case, thanks for the patch. It does look a lot more sensible
> compared to what I'd been doing. Do you want to keep owning the patch
> and try to upstream it, or shall I pick it up as part of this patch
> series? I'd of course retain author information.

Feel free to incorporate it, or I can refine it and resend as v2.
(I'll most likely get to the latter only in the next year though)

It looks like we have a third patch (series) for this feature meanwhile:
[PATCH 0/2] Have LUKS2 cryptomounts be useable with grub-probe

I CC'd the author, let's try to coordinate.

Thanks,
Fabian

> Patrick