linux-kernel - Re: [PATCH v6 21/21] scripts/gdb: Add basic documentation

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Date:	Wed, 30 Oct 2013 12:17:25 +0100
From:	Borislav Petkov <bp@...en8.de>
To:	Jan Kiszka <jan.kiszka@...mens.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org,
	Jason Wessel <jason.wessel@...driver.com>,
	kgdb-bugreport@...ts.sourceforge.net,
	Andi Kleen <andi@...stfloor.org>,
	Tom Tromey <tromey@...hat.com>,
	Ben Widawsky <ben@...dawsk.net>, Borislav Petkov <bp@...e.de>,
	Tatiana Al-Chueyr Martins <tatiana.alchueyr@...il.com>,
	Rob Landley <rob@...dley.net>, linux-doc@...r.kernel.org
Subject: Re: [PATCH v6 21/21] scripts/gdb: Add basic documentation

On Mon, Oct 28, 2013 at 09:58:58AM +0100, Jan Kiszka wrote:
> CC: Rob Landley <rob@...dley.net>
> CC: linux-doc@...r.kernel.org
> Signed-off-by: Jan Kiszka <jan.kiszka@...mens.com>
> ---
>  Documentation/gdb-kernel-debugging.txt | 159 +++++++++++++++++++++++++++++++++
>  1 file changed, 159 insertions(+)
>  create mode 100644 Documentation/gdb-kernel-debugging.txt
> 
> diff --git a/Documentation/gdb-kernel-debugging.txt b/Documentation/gdb-kernel-debugging.txt
> new file mode 100644
> index 0000000..944f2d2
> --- /dev/null
> +++ b/Documentation/gdb-kernel-debugging.txt
> @@ -0,0 +1,159 @@
> +Debugging kernel and modules via gdb
> +====================================
> +
> +The kernel debugger kgdb, hypervisors like QEMU or JTAG-based hardware
> +interfaces allow to debug the Linux kernel and its modules during runtime
> +using gdb. Gdb comes with a powerful scripting interface for python. The
> +kernel provides a collection of helper scripts that can simplify typical
> +kernel debugging steps. This is a short tutorial about how to enable and use
> +them. It focuses on QEMU/KVM virtual machines as target, but the examples can
> +be transferred to the other gdb stubs as well.
> +
> +
> +Requirements
> +------------
> +
> + o gdb 7.2+ (recommended: 7.4+) with python support enabled (typically true
> +   for distributions)
> +
> +
> +Setup
> +-----
> +
> + o Create a virtual Linux machine for QEMU/KVM (see www.linux-kvm.org and
> +   www.qemu.org for more details). For cross-development,
> +   http://landley.net/aboriginal/bin keeps a pool of machine images and
> +   toolchains that can be helpful to start from.
> +
> + o Build the kernel with CONFIG_DEBUG_INFO and CONFIG_KALLSYMS enabled, but
> +   leave CONFIG_DEBUG_INFO_REDUCED off.
> +
> + o Install that kernel on the guest.
> +
> +   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.
> +
> + o Enable the gdb stub of QEMU/KVM, either
> +    - at VM startup time by appending "-s" to the QEMU command line
> +   or
> +    - during runtime by issuing "gdbserver" from the QEMU monitor
> +      console
> +
> + o cd /path/to/linux-build
> +
> + o Start gdb: gdb vmlinux
> +
> +   Note: Some distros may restrict auto-loading of gdb scripts to known safe
> +   directories. In case gdb reports to refuse loading vmlinux-gdb.py, add
> +
> +    add-add-auto-load-safe-path /path/to/linux-build

Only one "add-" should be enough :)

Btw, gdb is nice enough to tell me:

$ gdb vmlinux
Reading symbols from /w/kernel/linux-2.6/vmlinux...done.
warning: File "/w/kernel/linux-2.6/scripts/gdb/vmlinux-gdb.py" auto-loading has been declined by your `auto-load safe-path' set to "$debugdir:$datadir/auto-load".
To enable execution of this file add
        add-auto-load-safe-path /w/kernel/linux-2.6/scripts/gdb/vmlinux-gdb.py
line to your configuration file "/home/boris/.gdbinit".
To completely disable this security protection add
        set auto-load safe-path /
line to your configuration file "/home/boris/.gdbinit".
For more information about this security protection see the
"Auto-loading safe path" section in the GDB manual.  E.g., run from the shell:
        info "(gdb)Auto-loading safe path"

Other than that, that's a very cool stuff and I'll make sure to be using
it the next time.

Acked-by: Borislav Petkov <bp@...e.de>

-- 
Regards/Gruss,
    Boris.

Sent from a fat crate under my desk. Formatting is fine.
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