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

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Message-ID: <27e76823-5464-4fd7-844d-7ed273a8a902@canonical.com>
Date:   Mon, 11 Sep 2023 09:47:43 +0200
From:   Heinrich Schuchardt <heinrich.schuchardt@...onical.com>
To:     Ard Biesheuvel <ardb@...nel.org>
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 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;
+		}

Best regards

Heinrich

> +       }
> 
>          /*
>           * This is not a normal filesystem, so no point in pretending
> it has a block