linux-kernel - Re: [PATCH 2/2] docs: disable KASLR when debugging kernel

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Date:   Fri, 7 Jul 2017 16:32:34 +0800
From:   Zhouyi Zhou <zhouzhouyi@...il.com>
To:     Kieran Bingham <kbingham@...nel.org>
Cc:     Jonathan Corbet <corbet@....net>,
        "J. Kiszka" <jan.kiszka@...mens.com>,
        Kieran Bingham <kieran@...uared.org.uk>,
        akpm@...ux-foundation.org, linux-doc@...r.kernel.org,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/2] docs: disable KASLR when debugging kernel

Hi Kieran,
    Thanks for your review and invaluable advise, I will prepare a new
version immediately.

On Fri, Jul 7, 2017 at 4:05 PM, Kieran Bingham <kbingham@...nel.org> wrote:
> Hi Zhouyi
>
> Thankyou for the patch,
>
> On 07/07/17 08:14, Zhouyi Zhou wrote:
>> commit 6807c84652b0 ("x86: Enable KASLR by default") enables KASLR
>> by default on x86. While KASLR will confuse gdb which resolve kernel
>> symbol address from symbol table of vmlinux. We should turn off KASLR for
>> kernel debugging.
> Yes, this is something I had come across and certainly should be documented.
>
>> Signed-off-by: Zhouyi Zhou <zhouzhouyi@...il.com>
>> ---
>>  Documentation/dev-tools/gdb-kernel-debugging.rst | 6 ++++--
>>  1 file changed, 4 insertions(+), 2 deletions(-)
>>
>> diff --git a/Documentation/dev-tools/gdb-kernel-debugging.rst b/Documentation/dev-tools/gdb-kernel-debugging.rst
>> index 5e93c9b..fe2edcc 100644
>> --- a/Documentation/dev-tools/gdb-kernel-debugging.rst
>> +++ b/Documentation/dev-tools/gdb-kernel-debugging.rst
>> @@ -31,12 +31,13 @@ Setup
>>    CONFIG_DEBUG_INFO_REDUCED off. If your architecture supports
>>    CONFIG_FRAME_POINTER, keep it enabled.
>>
>> -- Install that kernel on the guest.
>> +- Install that kernel on the guest, turn off KASLR by adding "nokaslr" to
>> +  the kernel command line .
> Is KASLR available on *all* supported architectures?
>
> If not, then perhaps this should be "turn off KASLR if necessary by ..."
> But I don't think that's a big deal really.
>
> Reviewed-by: Kieran Bingham <kbingham@...nel.org>
>
>
>>    Alternatively, QEMU allows to boot the kernel directly using -kernel,
>>    -append, -initrd command line switches. This is generally only useful if
>>    you do not depend on modules. See QEMU documentation for more details on
>> -  this mode.
>> +  this mode. In this case, you should build the kernel with
>> +  CONFIG_RANDOMIZE_BASE disabled.
>>
>>  - Enable the gdb stub of QEMU/KVM, either
>>
>
> --
> Kieran