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

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Message-ID: <CAGn38=0kk1hxzMtZ9_P_kkyszE-4tXo28vNAYyYnQi=gRuJyBw@mail.gmail.com>
Date:	Tue, 20 Sep 2011 10:38:16 +0800
From:	Dongdong Deng <libfetion@...il.com>
To:	Tim Bird <tim.bird@...sony.com>
Cc:	"kgdb-bugreport@...ts.sourceforge.net" 
	<kgdb-bugreport@...ts.sourceforge.net>,
	linux kernel <linux-kernel@...r.kernel.org>
Subject: Re: kdb - can't set breakpoint by default on x86

On Tue, Sep 20, 2011 at 8:14 AM, Tim Bird <tim.bird@...sony.com> wrote:
> 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?


The CONFIG_DEBUG_RODATA  is the nightmare for new user of kgdb/kdb,
Most of kgdb/kdb user meet this problem when they first time using
without reading manual.

but as the CONFIG_DEBUG_RODATA is an security feature, most of distro
enable it.(ex: ubuntu)
and the distro enable kgdb/kdb debugger too,

if the KGDB/KDB mutual exclusion with CONFIG_DEBUG_RODATA,  the distro
just pick one...

BTW:  the kprobe have the same problem, it didn't handle with
CONFIG_DEBUG_RODATA too.

Dongdong
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