docs: disable KASLR when debugging kernel

commit e604f1cb85367d2e5fd4cf253296d190996da81a upstream.

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>
Acked-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Cc: Natale Patriciello <natale.patriciello@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/Documentation/dev-tools/gdb-kernel-debugging.rst b/Documentation/dev-tools/gdb-kernel-debugging.rst
index 5e93c9bc6619da5d75a75179d858d60004e6f843..19df79286f000cac9ef12e1241b496dcce96dbe6 100644 (file)
--- a/Documentation/dev-tools/gdb-kernel-debugging.rst
+++ b/Documentation/dev-tools/gdb-kernel-debugging.rst
@@ -31,11 +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