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

[Glenn Washburn]

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?

Glenn

> > +            return var_ptr.dereference()
> > +        else:
> > +            var_ptr = gdb.parse_and_eval("&current_task")
> > +            return per_cpu(var_ptr, cpu).dereference()
> >      elif utils.is_target_arch("aarch64"):
> >          current_task_addr = gdb.parse_and_eval("$SP_EL0")
> >          if((current_task_addr >> 63) != 0):
> 
> Jan
>