[PATCH v2 2/2] docs: disable KASLR when debugging kernel

[PATCH v2 2/2] docs: disable KASLR when debugging kernel

[Zhouyi Zhou]

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. 

Signed-off-by: Zhouyi Zhou <zhouzhouyi@gmail.com>
Reviewed-by: Kieran Bingham <kbingham@kernel.org>
---
 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..60fec6d 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 if necessary by adding
+  "nokaslr" to the kernel command line.
   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 if the architecture supports KASLR.
 
 - Enable the gdb stub of QEMU/KVM, either
 
-- 
1.9.1

Re: [PATCH v2 2/2] docs: disable KASLR when debugging kernel

[Jan Kiszka]

On 2017-07-07 10:51, 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. 
> 
> Signed-off-by: Zhouyi Zhou <zhouzhouyi@gmail.com>
> Reviewed-by: Kieran Bingham <kbingham@kernel.org>
> ---
>  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..60fec6d 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 if necessary by adding
> +  "nokaslr" to the kernel command line.
>    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 if the architecture supports KASLR.
>  
>  - Enable the gdb stub of QEMU/KVM, either
>  
> 

FWIW: Acked-by: Jan Kiszka <jan.kiszka@siemens.com>

Who's taking this? Jon?

Jan

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

Re: [PATCH v2 2/2] docs: disable KASLR when debugging kernel

[Jonathan Corbet]

On Thu, 13 Jul 2017 08:53:59 +0200
Jan Kiszka <jan.kiszka@siemens.com> wrote:

> FWIW: Acked-by: Jan Kiszka <jan.kiszka@siemens.com>
> 
> Who's taking this? Jon?

I can take them once the merge window closes.

Thanks,

jon