As it turns out, another way to work around the /dev/mem restriction
is with kprobes.
The devmem_is_allowed() function looks like this, and for
the purposes of using it by the crash utility, we'd like it
to return 1 always:
int devmem_is_allowed(unsigned long pagenr)
{
if (pagenr <= 256)
return 1;
if (!page_is_ram(pagenr))
return 1;
return 0;
}
I took the sample kretprobes.c file from Documentation/kprobes.txt
and set a kretprobe in devmem_is_allowed() that forces a return
value of 1:
static int ret_handler(struct kretprobe_instance *ri, struct pt_regs *regs)
{
regs->eax = 1;
return 0;
}
Here's the "kretprobes.c" module I used:
#include <linux/kernel.h>
#include <linux/module.h>
#include <linux/kprobes.h>
static const char *probed_func = "devmem_is_allowed";
/* Return-probe handler: force return value to be 1. */
static int ret_handler(struct kretprobe_instance *ri, struct pt_regs *regs)
{
regs->eax = 1;
return 0;
}
static struct kretprobe my_kretprobe = {
.handler = ret_handler,
/* Probe up to 20 instances concurrently. */
.maxactive = 20
};
static int __init kretprobe_init(void)
{
int ret;
my_kretprobe.kp.symbol_name = (char *)probed_func;
if ((ret = register_kretprobe(&my_kretprobe)) < 0) {
printk("register_kretprobe failed, returned %d\n", ret);
return -1;
}
printk("Planted return probe at %p\n", my_kretprobe.kp.addr);
return 0;
}
static void __exit kretprobe_exit(void)
{
unregister_kretprobe(&my_kretprobe);
printk("kretprobe unregistered\n");
/* nmissed > 0 suggests that maxactive was set too low. */
printk("Missed probing %d instances of %s\n",
my_kretprobe.nmissed, probed_func);
}
module_init(kretprobe_init)
module_exit(kretprobe_exit)
MODULE_LICENSE("GPL");
And then build it with the supplied Makefile snippet:
obj-m := kretprobes.o
KDIR := /lib/modules/$(shell uname -r)/build
PWD := $(shell pwd)
default:
$(MAKE) -C $(KDIR) SUBDIRS=$(PWD) modules
clean:
rm -f *.mod.c *.ko *.o
Load the module, and then while it's running, "crash /dev/mem" will
override its default usage of "/dev/crash" and just work.
This was on a RHEL5 kernel, but it should work for RHEL4 as well:
$ crash /dev/mem
crash 4.0-6.1
Copyright (C) 2002, 2003, 2004, 2005, 2006, 2007, 2008 Red Hat, Inc.
Copyright (C) 2004, 2005, 2006 IBM Corporation
Copyright (C) 1999-2006 Hewlett-Packard Co
Copyright (C) 2005, 2006 Fujitsu Limited
Copyright (C) 2006, 2007 VA Linux Systems Japan K.K.
Copyright (C) 2005 NEC Corporation
Copyright (C) 1999, 2002, 2007 Silicon Graphics, Inc.
Copyright (C) 1999, 2000, 2001, 2002 Mission Critical Linux, Inc.
This program is free software, covered by the GNU General Public License,
and you are welcome to change it and/or distribute copies of it under
certain conditions. Enter "help copying" to see the conditions.
This program has absolutely no warranty. Enter "help warranty" for details.
GNU gdb 6.1
Copyright 2004 Free Software Foundation, Inc.
GDB is free software, covered by the GNU General Public License, and you are
welcome to change it and/or distribute copies of it under certain conditions.
Type "show copying" to see the conditions.
There is absolutely no warranty for GDB. Type "show warranty" for details.
This GDB was configured as "i686-pc-linux-gnu"...
KERNEL: /usr/lib/debug/lib/modules/2.6.18-53.el5/vmlinux
DUMPFILE: /dev/mem
CPUS: 2
DATE: Thu Mar 6 14:43:06 2008
UPTIME: 23 days, 04:50:13
LOAD AVERAGE: 0.14, 0.20, 0.20
TASKS: 175
NODENAME: crash.boston.redhat.com
RELEASE: 2.6.18-53.el5
VERSION: #1 SMP Wed Oct 10 16:34:02 EDT 2007
MACHINE: i686 (1993 Mhz)
MEMORY: 511.5 MB
PID: 15518
COMMAND: "crash"
TASK: cb0ffaa0 [THREAD_INFO: d976c000]
CPU: 0
STATE: TASK_RUNNING (ACTIVE)
crash> p panic_on_oops
panic_on_oops = $2 = 1
crash> wr panic_on_oops 2
crash> p panic_on_oops
panic_on_oops = $3 = 2
crash>
Dave
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