Re: [PATCH v3 09/10] crash_dump: Disallow configfs/crash_dm_crypt_key/reuse if crash hotplug supported

[Sourabh Jain <sourabhjain@linux.ibm.com>]

On 06/08/26 11:03, Coiby Xu wrote:
> On Wed, Aug 05, 2026 at 05:39:28PM +0530, Sourabh Jain wrote:
>>
>>
>> On 29/07/26 09:06, Coiby Xu wrote:
>>> If crash hotplug is supported, dm-crypt keys saved to reserved memory
>>> will be taken care of automatically. Thus it doesn't make sense to use
>>> configfs/crash_dm_crypt_key/reuse. Reserving image->dm_crypt_keys_addr
>>> is also unnecessary. Currently x86_64 and ppc64le have implemented
>>> crash hotplug feature.
>>>
>>> Also update the doc accordingly. Note two doc issues are fixed as well.
>>>
>>> Fixes: 9ebfa8dcaea7 ("crash_dump: reuse saved dm crypt keys for 
>>> CPU/memory hot-plugging")
>>> Signed-off-by: Coiby Xu <coiby.xu@gmail.com>
>>> ---
>>>  Documentation/admin-guide/kdump/kdump.rst | 16 ++++++++++------
>>>  kernel/crash_dump_dm_crypt.c              | 23 ++++++++++++++++++++---
>>>  2 files changed, 30 insertions(+), 9 deletions(-)
>>>
>>> diff --git a/Documentation/admin-guide/kdump/kdump.rst 
>>> b/Documentation/admin-guide/kdump/kdump.rst
>>> index 7587caadbae1..0bf2eb100a05 100644
>>> --- a/Documentation/admin-guide/kdump/kdump.rst
>>> +++ b/Documentation/admin-guide/kdump/kdump.rst
>>> @@ -577,9 +577,10 @@ with /sys/kernel/config/crash_dm_crypt_keys for 
>>> setup,
>>>  1. Tell the first kernel what logon keys are needed to unlock the 
>>> disk volumes,
>>>      # Add key #1
>>> -    mkdir 
>>> /sys/kernel/config/crash_dm_crypt_keys/7d26b7b4-e342-4d2d-b660-7426b0996720
>>> +    VOL1_UUID=7d26b7b4-e342-4d2d-b660-7426b0996720
>>> +    mkdir /sys/kernel/config/crash_dm_crypt_keys/$VOL1_UUID
>>>      # Add key #1's description
>>> -    echo cryptsetup:7d26b7b4-e342-4d2d-b660-7426b0996720 > 
>>> /sys/kernel/config/crash_dm_crypt_keys/description
>>> +    echo cryptsetup:$VOL1_UUID > 
>>> /sys/kernel/config/crash_dm_crypt_keys/$VOL1_UUID/description
>>>      # how many keys do we have now?
>>>      cat /sys/kernel/config/crash_dm_crypt_keys/count
>>> @@ -591,15 +592,18 @@ with /sys/kernel/config/crash_dm_crypt_keys 
>>> for setup,
>>>      cat /sys/kernel/config/crash_dm_crypt_keys/count
>>>      2
>>> -    # To support CPU/memory hot-plugging, reuse keys already saved 
>>> to reserved
>>> -    # memory
>>> -    echo true > /sys/kernel/config/crash_dm_crypt_key/reuse
>>> -
>>>  2. Load the dump-capture kernel
>>>  3. After the dump-capture kerne get booted, restore the keys to 
>>> user keyring
>>>     echo yes > /sys/kernel/crash_dm_crypt_keys/restore
>>> +For CPU/memory hot-plugging, you can reuse keys already saved to 
>>> reserved
>>> +memory before reloading the kdump image,
>>> +    echo true > /sys/kernel/config/crash_dm_crypt_keys/reuse
>>> +
>>> +Note if crash hotplug is supported, this API is totally unnecessary 
>>> thus will
>>> +be disabled automatically.
>>> +
>>>  Contact
>>>  =======
>>> diff --git a/kernel/crash_dump_dm_crypt.c 
>>> b/kernel/crash_dump_dm_crypt.c
>>> index b5b78656cf06..46d6b31c3ca4 100644
>>> --- a/kernel/crash_dump_dm_crypt.c
>>> +++ b/kernel/crash_dump_dm_crypt.c
>>> @@ -310,6 +310,15 @@ static ssize_t config_keys_reuse_show(struct 
>>> config_item *item, char *page)
>>>      return sysfs_emit(page, "%d\n", is_dm_key_reused);
>>>  }
>>> +static bool crash_hotplug_support(struct kimage *image)
>>> +{
>>> +#ifdef CONFIG_CRASH_HOTPLUG
>>> +    return image->hotplug_support;
>>
>> I don't think it is good idea to access kexec_crash_image properties 
>> without
>> holding the kexec lock.
>
> Thanks for raising the concern! Let's discuss this issue in [PATCH v3 
> 03/10].
>
>>
>> Also, would it make sense to move this API somewhere else?
>
> Can you help me understand why to move this API? Is it because using
> ifdef in .c file is discouraged?

This API is used to verify crash hotplug support, which is not directly 
related to crash_dm_crypt_keys.
It also seems to be generally useful.

- Sourabh Jain

>
>>
>> - Sourabh Jain
> [...]
>