linux-kernel - Re: [PATCH] scripts/gdb: Add command to check list consistency

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Date:	Thu, 23 Apr 2015 14:10:42 +0200
From:	Jan Kiszka <jan.kiszka@...mens.com>
To:	Thiébaud Weksteen <thiebaud@...steen.fr>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] scripts/gdb: Add command to check list consistency

On 2015-04-22 09:32, Thiébaud Weksteen wrote:
> Add a gdb script to verify the consistency of lists.
> 
> Signed-off-by: Thiébaud Weksteen <thiebaud@...steen.fr>
> ---
>  scripts/gdb/linux/lists.py | 78 ++++++++++++++++++++++++++++++++++++++++++++++
>  scripts/gdb/vmlinux-gdb.py |  1 +
>  2 files changed, 79 insertions(+)
>  create mode 100644 scripts/gdb/linux/lists.py
> 
> diff --git a/scripts/gdb/linux/lists.py b/scripts/gdb/linux/lists.py
> new file mode 100644
> index 0000000..d512f62
> --- /dev/null
> +++ b/scripts/gdb/linux/lists.py
> @@ -0,0 +1,78 @@
> +#
> +# gdb helper commands and functions for Linux kernel debugging
> +#
> +#  list tools
> +#
> +# Authors:
> +#  Thiebaud Weksteen <thiebaud@...steen.fr>

Who is the copyright holder, also you, your employer etc.? Please
clarify, ideally by just having a copyright line like in the other scripts.

> +#
> +# This work is licensed under the terms of the GNU GPL version 2.
> +#
> +
> +import gdb
> +
> +from linux import utils
> +
> +list_head = utils.CachedType("struct list_head")
> +
> +def check_list(head):

In general - applies to internal naming as well as the command itself -
I would prefer "list check". That would create a namespace under which
we may add further list helpers later on.

> +    nb = 0
> +    list_head_ptr_type = list_head.get_type().pointer()
> +    c = head.cast(list_head_ptr_type)

Why casting? Is it a common use case to not have the list head as a
variable at hand? The user could still do

lx_list_check (struct list_head *)0x...

when needed.

> +    try:
> +        gdb.write("Starting with: {} {}\n".format(c, c.dereference()))
> +    except gdb.MemoryError:
> +        gdb.write('head is not accessible\n')
> +        return
> +    while True:
> +        p = c['prev']
> +        n = c['next']
> +        try:
> +            if p['next'] != c:
> +                gdb.write('prev.next != current: current={current} '
> +                          'prev={p} prev.next={pnext}\n'.format(
> +                          current=c,
> +                          p=p,
> +                          pnext=p['next']
> +                          ))

This is not pep8-conforming. Please fix (even if the exiting scripts
have some issues as well - just noticed...).

> +                return
> +        except gdb.MemoryError:
> +            gdb.write('prev is not accessible: current={current} '
> +                      'current.prev={p}\n'.format(
> +                      current=c,
> +                      p=p
> +                      ))
> +            return
> +        try:
> +            if n['prev'] != c:
> +                gdb.write('next.prev != current: current={current} '
> +                          'next={n}, next.prev={nprev}\n'.format(
> +                          current=c,
> +                          n=n,
> +                          nprev=n['prev']
> +                          ))
> +                return
> +        except gdb.MemoryError:
> +            gdb.write('next is not accessible: current={current} '
> +                      'current.next={n}\n'.format(
> +                      current=c,
> +                      n=n
> +                      ))
> +            return
> +        c = n
> +        nb += 1
> +        if c == head:
> +            gdb.write("list is consistent: {} node(s)\n".format(nb))
> +            return
> +
> +class LxChkList(gdb.Command):
> +    """Verify a list consistency"""
> +
> +    def __init__(self):
> +        super(LxChkList, self).__init__("lx-check-list", gdb.COMMAND_DATA)
> +
> +    def invoke(self, arg, from_tty):
> +        argv = gdb.string_to_argv(arg)
> +        check_list(gdb.parse_and_eval(argv[0]))
> +
> +LxChkList()
> diff --git a/scripts/gdb/vmlinux-gdb.py b/scripts/gdb/vmlinux-gdb.py
> index 4848928..ce82bf5 100644
> --- a/scripts/gdb/vmlinux-gdb.py
> +++ b/scripts/gdb/vmlinux-gdb.py
> @@ -28,3 +28,4 @@ else:
>      import linux.dmesg
>      import linux.tasks
>      import linux.cpus
> +    import linux.lists
> 

Looks useful! As indicated, I could imagine extending this module by
further helpers, for the other list types or for walking a list,
executing some gdb command for each element.

Thanks,
Jan

-- 
Siemens AG, Corporate Technology, CT RTC ITP SES-DE
Corporate Competence Center Embedded Linux
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