Re: [Crash-utility] change the content of memory using crash on a live system -- use kprobes!

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 -- Crash-utility mailing list Crash-utility(a)redhat.com https://www.redhat.com/mailman/listinfo/crash-utility