linux-kernel - Re: [PATCH v2 1/1] efivarfs: fix statfs() on efivarfs

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Message-ID: <CAMj1kXG6g=obpUgptBvQwYtikaDk6biQiTrW6-tJAfhuDCXx0g@mail.gmail.com>
Date:   Mon, 11 Sep 2023 09:57:17 +0200
From:   Ard Biesheuvel <ardb@...nel.org>
To:     Heinrich Schuchardt <heinrich.schuchardt@...onical.com>
Cc:     Anisse Astier <anisse@...ier.eu>, Jeremy Kerr <jk@...abs.org>,
        Anisse Astier <an.astier@...teo.com>,
        linux-efi@...r.kernel.org, linux-kernel@...r.kernel.org,
        Christian Brauner <brauner@...nel.org>
Subject: Re: [PATCH v2 1/1] efivarfs: fix statfs() on efivarfs

On Mon, 11 Sept 2023 at 09:46, Heinrich Schuchardt
<heinrich.schuchardt@...onical.com> wrote:
>
> On 9/11/23 08:45, Ard Biesheuvel wrote:
> > On Sun, 10 Sept 2023 at 22:42, Heinrich Schuchardt
> > <heinrich.schuchardt@...onical.com> wrote:
> >>
> >> On 9/10/23 20:53, Anisse Astier wrote:
> >>> Hi Heinrich,
> >>>
> >>> On Sun, Sep 10, 2023 at 06:54:45AM +0200, Heinrich Schuchardt wrote:
> >>>> Some firmware (notably U-Boot) provides GetVariable() and
> >>>> GetNextVariableName() but not QueryVariableInfo().
> >>>
> >>>   From a quick search, it seems u-boot, does support QueryVariableInfo, is
> >>> it on a given version ?
> >>>
> >>> https://elixir.bootlin.com/u-boot/v2023.07.02/source/lib/efi_loader/efi_variable.c#L391
> >>
> >> QueryVariableInfo() and SetVariable() are available before
> >> ExitBootServices(), i.e. in Linux' EFI stub.
> >>
> >> ExitBootServices() results in calling efi_variables_boot_exit_notify()
> >> which disables these services during the UEFI runtime.
> >>
> >>>
> >>>>
> >>>> With commit d86ff3333cb1 ("efivarfs: expose used and total size") the
> >>>> statfs syscall was broken for such firmware.
> >>>
> >>> Could you be more specific ? What breaks, and what regressed ? I imagine
> >>> it could be some scripts running df, but maybe you had something else in
> >>> mind ?
> >>
> >> Some more details can be found in
> >> https://bugs.launchpad.net/ubuntu/+source/linux-meta-riscv/+bug/2034705.
> >>
> >> Though EFI variables are exposed via GetVariable() and
> >> GetNextVariableName() the efivar command refuses to display variables
> >> when statfs() reports an error.
> >>
> >>>
> >>>>
> >>>> If QueryVariableInfo() does not exist or returns an error, just report the
> >>>> file-system size as 0 as statfs_simple() previously did.
> >>>
> >>> I considered doing this [2] , but we settled on returning an error
> >>> instead for clarity:
> >>> https://lore.kernel.org/linux-efi/20230515-vorgaben-portrait-bb1b4255d31a@brauner/
> >>>
> >>> I still think it would be a good idea if necessary.
> >>
> >> We should never break user APIs.
> >>
> >
> > Indeed.
> >
> >>>
> >>> On the approach, I prefer what Ard proposed, to fall back to the old
> >>> approach. I think the difference in block size could also be a good
> >>> marker that something wrong is happening:
> >>> https://lore.kernel.org/linux-efi/CAMj1kXEkNSoqG4zWfCZ8Ytte5b2SzwXggZp21Xt17Pszd-q0dg@mail.gmail.com/
> >>
> >> This will allow user code making assumptions based on block size:
> >> If block size > 1, assume setting variables is possible.
> >>
> >> We should really avoid this.
> >>
> >
> > I agree that having different block sizes depending on which code path
> > is taken is not great. But that is the situation we are already in,
> > given that older kernels will always report PAGE_SIZE. And actually,
> > PAGE_SIZE does not make sense either - PAGE_SIZE could be larger than
> > 4k on ARM for instance, so the efivarfs block size will be dependent
> > on the page size of the kernel you happened to boot.
> >
> > So I think we should go with the below:
> >
> > --- a/fs/efivarfs/super.c
> > +++ b/fs/efivarfs/super.c
> > @@ -32,10 +32,16 @@ static int efivarfs_statfs(struct dentry *dentry,
> > struct kstatfs *buf)
> >          u64 storage_space, remaining_space, max_variable_size;
> >          efi_status_t status;
> >
> > -       status = efivar_query_variable_info(attr, &storage_space,
> > &remaining_space,
> > -                                           &max_variable_size);
> > -       if (status != EFI_SUCCESS)
> > -               return efi_status_to_err(status);
> > +       /* Some UEFI firmware does not implement QueryVariableInfo() */
> > +       storage_space = remaining_space = 0;
> > +       if (efi_rt_services_supported(EFI_RT_SUPPORTED_QUERY_VARIABLE_INFO)) {
> > +               status = efivar_query_variable_info(attr, &storage_space,
> > +                                                   &remaining_space,
> > +                                                   &max_variable_size);
> > +               if (status != EFI_SUCCESS && status != EFI_UNSUPPORTED)
> > +                       pr_warn_ratelimited("query_variable_info()
> > failed: 0x%lx\n",
> > +                                           status);
>
> Adding a warning here is helpful. The else branch would be:
>
> +               } else {
> +                       buf->f_blocks   = storage_space;
> +                       buf->f_bfree    = remaining_space;
> +               }
>

The else branch is redundant if we leave the assignments of f_blocks
and f_bfree where they were before.