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Subject: Re: [PATCH] scripts/gdb: fix list_for_each
To:     Jan Kiszka <jan.kiszka@siemens.com>, <linux-kernel@vger.kernel.org>
CC:     Kieran Bingham <kbingham@kernel.org>
References: <3b382958-9f1d-a3d2-a239-09ba084227e6@amazon.com>
 <2516a051-306f-670b-1f9e-d46fc577c7f8@siemens.com>
From:   "Prekas, George" <prekageo@amazon.com>
Message-ID: <109fe98d-4143-cfd3-b145-8d5fee189f63@amazon.com>
Date:   Tue, 22 Sep 2020 12:17:00 -0500
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On 9/22/2020 9:32 AM, Jan Kiszka wrote:
 >
 > On 22.09.20 16:28, George Prekas wrote:
 >> If the next pointer is NULL, list_for_each gets stuck in an infinite
 >> loop.
 >>
 >> Signed-off-by: George Prekas <prekageo@amazon.com>
 >> ---
 >>   scripts/gdb/linux/lists.py | 2 ++
 >>   1 file changed, 2 insertions(+)
 >>
 >> diff --git a/scripts/gdb/linux/lists.py b/scripts/gdb/linux/lists.py
 >> index c487ddf09d38..424a91c1aa8b 100644
 >> --- a/scripts/gdb/linux/lists.py
 >> +++ b/scripts/gdb/linux/lists.py
 >> @@ -27,6 +27,8 @@ def list_for_each(head):
 >>           raise TypeError("Must be struct list_head not {}"
 >>                              .format(head.type))
 >>
 >> +    if head['next'] == 0:
 >> +        return
 >>       node = head['next'].dereference()
 >>       while node.address != head.address:
 >>           yield node.address
 >
 > Obviously, infinite loops are bad and should be avoided. But NULL is
 > bug, isn't it? Shouldn't we report such a corruption?
 >

Hi Jan,

Is it a bug? Or does it mean that the list is empty?

Let me give some background. If you do the following:

$ qemu-system-x86_64 -nographic -m 1024 -kernel 
build/arch/x86/boot/bzImage -s -S < /dev/null > /dev/null &
$ gdb -q build/vmlinux -ex 'target remote localhost:1234' -iex 'set 
auto-load safe-path /' -ex 'lx-symbols'

You will see:

loading vmlinux
scanning for modules in /home/ubuntu/linux-5.8.10
no module object found for ''

And the last line repeats forever. This happens because modules.next == 
NULL. This is the Python stack trace:

   File ".../symbols.py", line 174, in invoke
     self.load_all_symbols()
   File ".../symbols.py", line 161, in load_all_symbols
     [self.load_module_symbols(module) for module in module_list]
   File ".../symbols.py", line 161, in <listcomp>
     [self.load_module_symbols(module) for module in module_list]
   File ".../modules.py", line 30, in module_list
     for module in lists.list_for_each_entry(modules, module_ptr_type, 
"list"):
   File ".../lists.py", line 41, in list_for_each_entry
     for node in list_for_each(head):
   File ".../lists.py", line 31, in list_for_each
     traceback.print_stack()

This patch tries to fix the above problem.

George