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

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Date:   Fri, 7 Jul 2017 09:05:29 +0100
From:   Kieran Bingham <kbingham@...nel.org>
To:     Zhouyi Zhou <zhouzhouyi@...il.com>, corbet@....net,
        jan.kiszka@...mens.com, kieran@...uared.org.uk,
        akpm@...ux-foundation.org, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] docs: disable KASLR when debugging kernel

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