Re: [PATCH] efi/efivars: Create efivars mount point in the registration of efivars abstraction

[joeyli]

Hi Ard,

On Thu, Sep 24, 2020 at 12:47:46PM +0200, Ard Biesheuvel wrote:
> On Thu, 24 Sep 2020 at 10:28, Lee, Chun-Yi <joeyli.kernel@xxxxxxxxx> wrote:
> >
> > This patch moved the logic of creating efivars mount point to the
> > registration of efivars abstraction. It's useful for userland to
> > determine the availability of efivars filesystem by checking the
> > existence of mount point.
> >
> > The 'efivars' platform device be created on generic EFI runtime services
> > platform, so it can be used to determine the availability of efivarfs.
> > But this approach is not available for google gsmi efivars abstraction.
> >
> > This patch be tested on Here on qemu-OVMF and qemu-uboot.
> >
> > Cc: Ard Biesheuvel <ardb@xxxxxxxxxx>
> > Cc: Matthias Brugger <mbrugger@xxxxxxxx>
> > Cc: Fabian Vogt <fvogt@xxxxxxxx>
> > Cc: Ilias Apalodimas <ilias.apalodimas@xxxxxxxxxx>
> > Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
> > Cc: Arthur Heymans <arthur@xxxxxxxxxxxx>
> > Cc: Patrick Rudolph <patrick.rudolph@xxxxxxxxxxxxx>
> > Signed-off-by: "Lee, Chun-Yi" <jlee@xxxxxxxx>
> > ---
> 
> I take it this is v3 of [0]? If so, please explain how it deviates
> from v2. If it doesn't deviate from v2, it is better to continue the
> discussion in the other thread.
> 
> For the sake of discussion, it helps to clarify the confusing nomenclature:
> 
> a) 'efivars abstraction' - an internal kernel API that exposes EFI
> variables, and can potentially be backed by an implementation that is
> not EFI based (i.e., Google gsmi)
> 
> b) efivars.ko module, built on top of the efivars abstraction, which
> exposes EFI variables (real ones or gsmi ones) via the deprecated
> sysfs interface
> 
> c) efivarfs filesystem, also built on top of the efivars abstraction,
> which exposes EFI variables (real ones or gsmi ones) via a special
> filesystem independently of sysfs.
> 
> Of course, the sysfs mount point we create for efivarfs is not called
> 'efivarfs' but 'efivars'. The sysfs subdirectory we create for
> efivars.ko is called 'vars'. Sigh.
>

Thanks for your clarification. It's useful to me!
 
> 
> In this patch, you create the mount point for c) based on whether a)
> gets registered (which occurs on systems with EFI Get/SetVariable
> support or GSMI), right? So, to Greg's point, wouldn't it be easier to
> simply check whether efivarfs is listed in /proc/filesystems?
>

Yes, I think that Greg's suggestion is good enough for a userland tool
to detect the availability of efivarfs. You can ignore my patch. 

Thanks for your help!
Joey Lee
 
> It also helps if you could clarify what the actual use case is, rather
> than saying that it is generally useful.
> 
> 
> 
> 
> 
> [0] https://lore.kernel.org/linux-efi/20200825160719.7188-1-jlee@xxxxxxxx/
> 
> >  drivers/firmware/efi/efi.c  |  7 -------
> >  drivers/firmware/efi/vars.c | 17 +++++++++++++++++
> >  2 files changed, 17 insertions(+), 7 deletions(-)
> >
> > diff --git a/drivers/firmware/efi/efi.c b/drivers/firmware/efi/efi.c
> > index 3aa07c3b5136..23c11a2a3f4d 100644
> > --- a/drivers/firmware/efi/efi.c
> > +++ b/drivers/firmware/efi/efi.c
> > @@ -405,13 +405,6 @@ static int __init efisubsys_init(void)
> >         if (error)
> >                 goto err_remove_group;
> >
> > -       /* and the standard mountpoint for efivarfs */
> > -       error = sysfs_create_mount_point(efi_kobj, "efivars");
> > -       if (error) {
> > -               pr_err("efivars: Subsystem registration failed.\n");
> > -               goto err_remove_group;
> > -       }
> > -
> >         if (efi_enabled(EFI_DBG) && efi_enabled(EFI_PRESERVE_BS_REGIONS))
> >                 efi_debugfs_init();
> >
> > diff --git a/drivers/firmware/efi/vars.c b/drivers/firmware/efi/vars.c
> > index 973eef234b36..6fa7f288d635 100644
> > --- a/drivers/firmware/efi/vars.c
> > +++ b/drivers/firmware/efi/vars.c
> > @@ -1179,6 +1179,8 @@ int efivars_register(struct efivars *efivars,
> >                      const struct efivar_operations *ops,
> >                      struct kobject *kobject)
> >  {
> > +       int error;
> > +
> >         if (down_interruptible(&efivars_lock))
> >                 return -EINTR;
> >
> > @@ -1191,6 +1193,19 @@ int efivars_register(struct efivars *efivars,
> >
> >         up(&efivars_lock);
> >
> > +       /* and the standard mountpoint for efivarfs */
> > +       if (efi_kobj) {
> > +               error = sysfs_create_mount_point(efi_kobj, "efivars");
> > +               if (error) {
> > +                       if (down_interruptible(&efivars_lock))
> > +                               return -EINTR;
> > +                       __efivars = NULL;
> > +                       up(&efivars_lock);
> > +                       pr_err("efivars: Subsystem registration failed.\n");
> > +                       return error;
> > +               }
> > +       }
> > +
> >         return 0;
> >  }
> >  EXPORT_SYMBOL_GPL(efivars_register);
> > @@ -1222,6 +1237,8 @@ int efivars_unregister(struct efivars *efivars)
> >
> >         pr_info("Unregistered efivars operations\n");
> >         __efivars = NULL;
> > +       if (efi_kobj)
> > +               sysfs_remove_mount_point(efi_kobj, "efivars");
> >
> >         rv = 0;
> >  out:
> > --
> > 2.16.4
> >