linux-kernel - kdb - can't set breakpoint by default on x86

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Message-ID: <4E77DADF.6020608@am.sony.com>
Date:	Mon, 19 Sep 2011 17:14:23 -0700
From:	Tim Bird <tim.bird@...sony.com>
To:	"kgdb-bugreport@...ts.sourceforge.net" 
	<kgdb-bugreport@...ts.sourceforge.net>
CC:	linux kernel <linux-kernel@...r.kernel.org>
Subject: kdb - can't set breakpoint by default on x86

I recently tried using KDB on a 3.0.1 kernel, on an x86 target, using
a fairly-close-to-default kernel configuration.

I found that I could not set a breakpoint.
It sets inside KDB, but then fails to apply the breakpoint when
resuming to user space:

--------------------------------
/proc # echo g >sysrq-trigger
[2689816.806249] SysRq : DEBUG

Entering kdb (current=0xc7661d40, pid 511) on processor 0 due to Keyboard Entry

[0]kdb> bp sys_sync
Instruction(i) BP #0 at 0xc111ce70 (sys_sync)
    is enabled  addr at 00000000c111ce70, hardtype=0 installed=0

[0]kdb> g
_kdb_bp_install: failed to set breakpoint at 0xc111ce70
/proc # sync
/proc #
--------------------------------

I discovered that this was due to the text segment being marked
read-only, by mark_rodata_ro() in arch/x86/mm/init_32.c

This is controlled by CONFIG_DEBUG_RODATA.  With CONFIG_DEBUG_RODATA=y,
I get the following message on startup:

Write protecting the kernel read-only data: 6245

Also, breakpoints don't work.

Turning this off solves the problem, but seems counter-intuitive.
CONFIG_DEBUG_RODATA depends on CONFIG_DEBUG_KERNEL.

Should I:
1) turn off CONFIG_DEBUG_RODATA automatically when CONFIG_KGDB_KDB is set?
or
2) make CONFIG_DEBUG_RODATA depend on !KGDB_KDB?
or
3) something else more subtle?

Thanks,
 -- Tim

=============================
Tim Bird
Architecture Group Chair, CE Workgroup of the Linux Foundation
Senior Staff Engineer, Sony Network Entertainment
=============================

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