[PATCH v5 1/3] powerpc: make fadump resilient with memory add/remove events

[Sourabh Jain]

Hello Aneesh,

On 22/11/23 17:50, Aneesh Kumar K V wrote:
> On 11/22/23 4:05 PM, Sourabh Jain wrote:
>> Hello Michael,
>>
>>
>> On 22/11/23 10:47, Michael Ellerman wrote:
>>> Aneesh Kumar K V <aneesh.kumar at linux.ibm.com> writes:
>>> ...
>>>> I am not sure whether we need to add all the complexity to enable supporting different fadump kernel
>>>> version. Is that even a possible use case with fadump? Can't we always assume that with fadump the
>>>> crash kernel and fadump kernel will be same version?
>>> How sure are we of that?
>>>
>>> Don't we go through grub when we boot into the 2nd kernel. And so
>>> couldn't it choose to boot a different kernel, for whatever reason.
>>>
>>> I don't think we need to support different pt_reg / cpumask sizes, but
>>> requiring the exact same kernel version is too strict I think.
>> Agree.
>>> But maybe I'm wrong. Would be good to hear what distro folks think.
>> How about checking fadump crash info header compatibility in the following way?
>>
>> static bool is_fadump_header_compatible(struct fadump_crash_info_header *fdh)
>> {
>>      if (fdh->magic_number == FADUMP_CRASH_INFO_MAGIC_OLD) {
>>          pr_err("Old magic number, can't process the dump.");
>>          return false;
>>      }
>>
>>      if (fdh->magic_number != FADUMP_CRASH_INFO_MAGIC) {
>>          pr_err("Fadump header is corrupted.");
>>          return false;
>>      }
>>
>>      /*
>>       * If the kernel version of the first/crashed kernel and the second/fadump
>>       * kernel is not same, then only collect the dump if the size of all
>>       * non-primitive type members of the fadump header is the same across kernels.
>>       */
>>      if (strcmp(fdh->kernel_version, init_uts_ns.name.release)) {
>>          if (fdh->pt_regs_sz != sizeof(struct pt_regs) || fdh->cpu_mask_sz != sizeof(struct cpumask)) {
>>              pr_err("Fadump header size mismatch.\n")
>>              return false;
>>          } else
>>              pr_warn("Kernel version mismatch; dump data is unreliable.\n");
>>      }
>>
> You also want a fdh->version check?

Even though we don't have any action against an fdh->version right now, 
I think I should
check the fadump header version. Currently, if the version doesn't 
match, it means the
header is corrupted.

> I am still not sure you need the kernel_version check. IMHO that is too strict
> and can hit that check where you have kexec kernel which is not installed in /boot crashing?

If the kernel versions mismatch, we still collect the dump if the 
`pt_regs` and `cpu_mask`
sizes are the same across the kernels. The kernel version check is just 
to warn users that
the collected dump may be unreliable.

Should I remove the kernel_version filed from fadump crash info header 
and remove the
the kernel version check while processing the kernel dump?

Thanks,
Sourabh Jain