RE: [PATCH] scripts/gdb: document lx_current is only supported by x86

[Song Bao Hua (Barry Song)]

> -----Original Message-----
> From: Jan Kiszka [mailto:jan.kiszka@xxxxxxxxxxx]
> Sent: Tuesday, February 23, 2021 8:27 PM
> To: Song Bao Hua (Barry Song) <song.bao.hua@xxxxxxxxxxxxx>;
> kieran.bingham@xxxxxxxxxxxxxxxx; corbet@xxxxxxx; linux-doc@xxxxxxxxxxxxxxx
> Cc: linux-kernel@xxxxxxxxxxxxxxx; linuxarm@xxxxxxxxxxxxx
> Subject: Re: [PATCH] scripts/gdb: document lx_current is only supported by x86
> 
> On 22.02.21 22:18, Song Bao Hua (Barry Song) wrote:
> >
> >
> >> -----Original Message-----
> >> From: Kieran Bingham [mailto:kieran.bingham@xxxxxxxxxxxxxxxx]
> >> Sent: Tuesday, February 23, 2021 12:06 AM
> >> To: Song Bao Hua (Barry Song) <song.bao.hua@xxxxxxxxxxxxx>; corbet@xxxxxxx;
> >> linux-doc@xxxxxxxxxxxxxxx; jan.kiszka@xxxxxxxxxxx
> >> Cc: linux-kernel@xxxxxxxxxxxxxxx; linuxarm@xxxxxxxxxxxxx
> >> Subject: Re: [PATCH] scripts/gdb: document lx_current is only supported by
> x86
> >>
> >> Hi Barry
> >>
> >> On 21/02/2021 21:35, Barry Song wrote:
> >>> lx_current depends on the per_cpu current_task which exists on x86 only:
> >>>
> >>> arch$ git grep current_task | grep -i per_cpu
> >>> x86/include/asm/current.h:DECLARE_PER_CPU(struct task_struct *,
> >> current_task);
> >>> x86/kernel/cpu/common.c:DEFINE_PER_CPU(struct task_struct *,
> current_task)
> >> ____cacheline_aligned =
> >>> x86/kernel/cpu/common.c:EXPORT_PER_CPU_SYMBOL(current_task);
> >>> x86/kernel/cpu/common.c:DEFINE_PER_CPU(struct task_struct *,
> current_task)
> >> = &init_task;
> >>> x86/kernel/cpu/common.c:EXPORT_PER_CPU_SYMBOL(current_task);
> >>> x86/kernel/smpboot.c:	per_cpu(current_task, cpu) = idle;
> >>>
> >>> On other architectures, lx_current() will lead to a python exception:
> >>> (gdb) p $lx_current().pid
> >>> Python Exception <class 'gdb.error'> No symbol "current_task" in current
> >> context.:
> >>> Error occurred in Python: No symbol "current_task" in current context.
> >>>
> >>> To avoid more people struggling and wasting time in other architectures,
> >>> document it.
> >>>
> >>> Cc: Jan Kiszka <jan.kiszka@xxxxxxxxxxx>
> >>> Signed-off-by: Barry Song <song.bao.hua@xxxxxxxxxxxxx>
> >>> ---
> >>>  Documentation/dev-tools/gdb-kernel-debugging.rst |  2 +-
> >>>  scripts/gdb/linux/cpus.py                        | 10 ++++++++--
> >>>  2 files changed, 9 insertions(+), 3 deletions(-)
> >>>
> >>> diff --git a/Documentation/dev-tools/gdb-kernel-debugging.rst
> >> b/Documentation/dev-tools/gdb-kernel-debugging.rst
> >>> index 4756f6b3a04e..1586901b683c 100644
> >>> --- a/Documentation/dev-tools/gdb-kernel-debugging.rst
> >>> +++ b/Documentation/dev-tools/gdb-kernel-debugging.rst
> >>> @@ -114,7 +114,7 @@ Examples of using the Linux-provided gdb helpers
> >>>      [     0.000000] BIOS-e820: [mem
> 0x000000000009fc00-0x000000000009ffff]
> >> reserved
> >>>      ....
> >>>
> >>> -- Examine fields of the current task struct::
> >>> +- Examine fields of the current task struct(supported by x86 only)::
> >>>
> >>>      (gdb) p $lx_current().pid
> >>>      $1 = 4998
> >>> diff --git a/scripts/gdb/linux/cpus.py b/scripts/gdb/linux/cpus.py
> >>> index 008e62f3190d..f382762509d3 100644
> >>> --- a/scripts/gdb/linux/cpus.py
> >>> +++ b/scripts/gdb/linux/cpus.py
> >>> @@ -156,6 +156,13 @@ Note that VAR has to be quoted as string."""
> >>>
> >>>  PerCpu()
> >>>
> >>> +def get_current_task(cpu):
> >>> +    if utils.is_target_arch("x86"):
> >>> +         var_ptr = gdb.parse_and_eval("&current_task")
> >>> +         return per_cpu(var_ptr, cpu).dereference()
> >>> +    else:
> >>> +        raise gdb.GdbError("Sorry, obtaining the current task is not yet
> "
> >>> +                           "supported with this arch")
> >>
> >> I've wondered in the past how we should handle the architecture specific
> >> layers.
> >>
> >> Perhaps we need to have an interface of functionality to implement on
> >> each architecture so that we can create a per-arch set of helpers.
> >>
> >> or break it up into arch specific subdirs at least...
> >>
> >>
> >>>  class LxCurrentFunc(gdb.Function):
> >>>      """Return current task.
> >>> @@ -167,8 +174,7 @@ number. If CPU is omitted, the CPU of the current context
> >> is used."""
> >>>          super(LxCurrentFunc, self).__init__("lx_current")
> >>>
> >>>      def invoke(self, cpu=-1):
> >>> -        var_ptr = gdb.parse_and_eval("&current_task")
> >>> -        return per_cpu(var_ptr, cpu).dereference()
> >>> +        return get_current_task(cpu)
> >>>
> >>
> >> And then perhaps we simply shouldn't even expose commands which can not
> >> be supported on those architectures?
> >
> > I feel it is better to tell users this function is not supported on its arch
> > than simply hiding the function.
> >
> > If we hide it, users still have many chances to try it as they have got
> > information of lx_current from google or somewhere.
> > They will try, if it turns out the lx_current is not in the list and an
> > error like  "invalid data type for function to be called", they will
> > probably suspect their gdb/python environment is not set up correctly,
> > and continue to waste time in checking their environment.
> > Finally they figure out this function is not supported by its arch so it is
> > not exposed. But they have wasted a couple of hours before knowing that.
> >
> > It seems it is more friendly to directly tell users this is not supported
> > on its arch explicitly and clearly than reporting a "invalid data type
> > for function to be called.
> >
> >>
> >> Is it easy to disable this command if it's not supportable on the
> >> architecture?
> >>
> >
> > TBH, I'm not a python expert. I don't know how to do that in an elegant
> > way :-)  on the other hand, it seems lx_current isn’t a command like
> > lx-dmesg. Lx_current is actually similar with lx_per_cpu, we use gdb's
> > print(p) command to show its content.
> >
> >> Presumably you are working on non-x86, have you investigated adding this
> >> support for your architecture (arm/arm64?)?
> >
> > Yes. I've thought about it. But It would be quite trivial to bring up
> > this function on arm64.
> >
> > arch/arm64/include/asm/current.h:
> > static __always_inline struct task_struct *get_current(void)
> > {
> > 	unsigned long sp_el0;
> >
> > 	asm ("mrs %0, sp_el0" : "=r" (sp_el0));
> >
> > 	return (struct task_struct *)sp_el0;
> > }
> >
> > We have to read a special register named sp_el0 and convert it to
> > task_struct while we are running in kernel mode. In gdb I can do
> > it by:
> > (gdb)p/x $SP_EL0
> > $20 = 0xffffffc011492400
> > (gdb)p ((struct task_struct *0xffffffc011492400))->pid
> > $21 = 0
> >
> > What is more complex is that if we are running in user mode(EL0), this
> > register doesn't describe current task any more. so we have to
> > differentiate the modes of processor and make sure it only returns
> > current task while we are running in EL1(processor's kernel mode).
> 
> Is all information needed for this available via gdb?

I can't read current EL level from gdb as CurrentEL is not readable
from EL0 as shown on the ARMv8 manual C5.2.1:
"CurrentEL, Current Exception Level" section "Accessibility". 
Trying to run it in Linux userland raises SIGILL.

But a workaround I can do is that while running in kernel, SP_EL0
is a value like 0xffffxxxx xxxxxxxx; otherwise, it would be a value
like 0x0000xxxx xxxxxxxx.

So I could actually implement lx_current on arm64 by:

p/x $SP_EL0
if value > 0xffff00000000000
   task_struct_addr = value
else
   userspace, no current

the problem is that I don't know how to read the register and
transfer address into task_struct in gdb/scripts. Would you
like to share some example code if you have?

> 
> >
> >>
> >> The fact you have run the command implies it would be useful for you ?
> >>
> >
> > Yes. I think it is a common requirement to get current task. lx_current
> > convenience function can help everyone. Since there is a document saying
> > this command exists, everyone using scripts/gdb would like to try it
> > I guess.
> >
> > The simplest way would be adding current_task per_cpu variable for other
> > arch, but I believe hardly arch maintainers will accept it as its only
> > benefit is bringing up the lx_current. So 99.9% no maintainer wants it.
> >
> > Thus, for the time being, I moved to just stop people from wasting time
> > like what I had done with a couple of hours.
> >
> 
> I agree with the warning, also as potential motivation to add support
> for other archs.
> 

Yep.

> Jan
> 
> --
> Siemens AG, T RDA IOT
> Corporate Competence Center Embedded Linux

Thanks
Barry