[PATCH 2/2] scripts/gdb: Support getting current task struct in UML

[Jan Kiszka]

On 15.02.23 16:55, Glenn Washburn wrote:
> On Wed, 15 Feb 2023 08:45:20 +0100
> Jan Kiszka <jan.kiszka at siemens.com> wrote:
> 
>> On 15.02.23 00:35, Glenn Washburn wrote:
>>> A running x86 UML kernel reports with architecture "i386:x86-64" as
>>> it is a sub-architecture. However, a difference with bare-metal x86
>>> kernels is in how it manages tasks and the current task struct. To
>>> identify that the inferior is a UML kernel and not bare-metal, the
>>> symbol "uml_kmalloc" is checked for. If it exists, then do the UML
>>> specific way of getting the current task struct.
>>>
>>> Signed-off-by: Glenn Washburn <development at efficientek.com>
>>> ---
>>>  scripts/gdb/linux/cpus.py | 8 ++++++--
>>>  1 file changed, 6 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/scripts/gdb/linux/cpus.py b/scripts/gdb/linux/cpus.py
>>> index 289f0bf4d2a1..90908653f6c7 100644
>>> --- a/scripts/gdb/linux/cpus.py
>>> +++ b/scripts/gdb/linux/cpus.py
>>> @@ -163,8 +163,12 @@ def get_current_task(cpu):
>>>      task_ptr_type = task_type.get_type().pointer()
>>>  
>>>      if utils.is_target_arch("x86"):
>>> -        var_ptr = gdb.parse_and_eval("&current_task")
>>> -        return per_cpu(var_ptr, cpu).dereference()
>>> +        if gdb.lookup_global_symbol("uml_kmalloc"):
>>> +            var_ptr = gdb.parse_and_eval("(struct task_struct
>>> *)cpu_tasks[0].task")
>>
>> Can't we use an UML indicator that is related to the topic? Why not
>> cpu_tasks itself?
> 
> Yes, I like that idea, and makes a lot of sense seeing as then we're
> sure cpu_tasks exists right before using it. The one thing that I like
> about using a symbol with a "uml_" prefix is that it makes it obvious
> that this branch is for UML kernels. I ultimately decided against a
> comment because using uml_kmalloc seemed self-documenting. Do you think
> a comment is warranted if I make the suggested change?

"uml" is clear, "kmalloc" is not so much again. No perfect fit, but I
would leave a commit in any case.

Jan

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