Re: [PATCH v4 1/5] efi: Export boot-services code and data as debugfs-blobs

[Ard Biesheuvel]

On 27 April 2018 at 10:13, Hans de Goede <hdegoede@xxxxxxxxxx> wrote:
> Hi,
>
>
> On 26-04-18 23:35, Ard Biesheuvel wrote:
>>
>> On 26 April 2018 at 23:02, Hans de Goede <hdegoede@xxxxxxxxxx> wrote:
>>>
>>> Hi,
>>>
>>>
>>> On 26-04-18 18:51, Ard Biesheuvel wrote:
>>>>
>>>>
>>>> On 26 April 2018 at 14:06, Hans de Goede <hdegoede@xxxxxxxxxx> wrote:
>>>>>
>>>>>
>>>>> Sometimes it is useful to be able to dump the efi boot-services code
>>>>> and
>>>>> data. This commit adds these as debugfs-blobs to /sys/kernel/debug/efi,
>>>>> but only if efi=debug is passed on the kernel-commandline as this
>>>>> requires
>>>>> not freeing those memory-regions, which costs 20+ MB of RAM.
>>>>>
>>>>> Reviewed-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
>>>>> Signed-off-by: Hans de Goede <hdegoede@xxxxxxxxxx>
>>>>> ---
>>>>> Changes in v4:
>>>>> -Add new EFI_BOOT_SERVICES flag and use it to determine if the
>>>>> boot-services
>>>>>    memory segments are available (and thus if it makes sense to
>>>>> register
>>>>> the
>>>>>    debugfs bits for them)
>>>>>
>>>>> Changes in v2:
>>>>> -Do not call pr_err on debugfs call failures
>>>>> ---
>>>>>    arch/x86/platform/efi/efi.c    |  1 +
>>>>>    arch/x86/platform/efi/quirks.c |  4 +++
>>>>>    drivers/firmware/efi/efi.c     | 53
>>>>> ++++++++++++++++++++++++++++++++++
>>>>>    include/linux/efi.h            |  1 +
>>>>>    4 files changed, 59 insertions(+)
>>>>>
>>>>> diff --git a/arch/x86/platform/efi/efi.c b/arch/x86/platform/efi/efi.c
>>>>> index 9061babfbc83..568b7ee3d323 100644
>>>>> --- a/arch/x86/platform/efi/efi.c
>>>>> +++ b/arch/x86/platform/efi/efi.c
>>>>> @@ -208,6 +208,7 @@ int __init efi_memblock_x86_reserve_range(void)
>>>>>                efi.memmap.desc_version);
>>>>>
>>>>>           memblock_reserve(pmap, efi.memmap.nr_map *
>>>>> efi.memmap.desc_size);
>>>>> +       set_bit(EFI_BOOT_SERVICES, &efi.flags);
>>>>>
>>>>
>>>> I think it would be better if the flag conveys whether boot services
>>>> regions are being preserved, because they will always exist when
>>>> EFI_BOOT is set.
>>>> The name should then reflect that as well, e.g.,
>>>> EFI_PRESERVE_BS_REGIONS.
>>>
>>>
>>>
>>> Ok, I will rename the flag to EFI_PRESERVE_BS_REGIONS for v5
>>> (I'm going to wait a bit with sending out v5 to give others a change
>>>   to comment on v4).
>>>
>>>>>           return 0;
>>>>>    }
>>>>> diff --git a/arch/x86/platform/efi/quirks.c
>>>>> b/arch/x86/platform/efi/quirks.c
>>>>> index 36c1f8b9f7e0..16bdb9e3b343 100644
>>>>> --- a/arch/x86/platform/efi/quirks.c
>>>>> +++ b/arch/x86/platform/efi/quirks.c
>>>>> @@ -376,6 +376,10 @@ void __init efi_free_boot_services(void)
>>>>>           int num_entries = 0;
>>>>>           void *new, *new_md;
>>>>>
>>>>> +       /* Keep all regions for /sys/kernel/debug/efi */
>>>>> +       if (efi_enabled(EFI_DBG))
>>>>> +               return;
>>>>> +
>>>>
>>>>
>>>>
>>>> Why is this only necessary when EFI_DBG is enabled? How are you
>>>> ensuring that the firmware is still in memory when the subsequent
>>>> patches start relying on that?
>>>
>>>
>>>
>>> The 2nd patch in this series makes init/main.c call
>>> efi_check_for_embedded_firmwares() before efi_free_boot_services(),
>>> efi_check_for_embedded_firmwares() then walks the dmi_system_id-s
>>> "registered" (its a static list) by drivers and if their is a dmi_match
>>> searches for the firmware described by the dmi_system_id.driver_data
>>> ptr. If a firmware gets found it gets memdup-ed, so that we do not
>>> have to keep all of the boot-services code around.
>>>
>>
>> Right, thanks for clearing that up.
>>
>> So that means that preserving the boot regions is really only
>> necessary if you want to inspect them via debugfs, and the firmware
>> loader does not rely on that. I missed that part.
>>
>> That means the only reason we have the new flag is to ensure that the
>> shared debugfs code only exposes the boot services regions if they
>> were preserved to begin with by the arch code, right?
>
>
> Mostly right, since we have the flag now anyways I'm also using
> it as a condition to call efi_check_for_embedded_firmwares(),
> efi_check_for_embedded_firmwares() needs to happen after mm_init() (*)
> and on non x86 the boot-services code/data is long gone then so
> there is nothing for efi_check_for_embedded_firmwares() to look at.
>

Ah, of course, yes that was the whole point.

>> If so, after the flag rename:
>>
>> Acked-by: Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx>
>
>
> I assume that the "mostly right" is good enough and I'm going to
> add your Acked-by for the next version. Let me know if you've
> any objections against that.
>

Yes that's fine.