Re: [Crash-utility] Crash fails with the error: "crash: read error: kernel virtual address:..."

----- "Adhiraj Joshi" <adhiraj(a)linsyssoft.com&gt; wrote: Your kernel is configured with CONFIG_STRICT_DEVMEM, which disallows the reading of /dev/mem above 1MB physical. That makes it useless for the crash utility: https://www.redhat.com/archives/crash-utility/2009-February/msg00003.html Fedora and RHEL kernels contain the /dev/crash driver, which gets automatically installed upon crash invocation on the live system. That driver, found in the kernel file "drivers/char/crash.c", is a replacement for the (restricted) /dev/mem driver. For your kernel, you've got 3 options: (1) Rebuild your kernel without the CONFIG_STRICT_DEVMEM restriction. (2) Port the Fedora /dev/crash driver (./drivers/char/crash.c) to your kernel. (3) Write a kretprobe module that tinkers with the return value of the kernel's devmem_is_allowed() function such that it always returns 1. And w/respect to each option: (1) Obviously this is the course of least resistance. (2) Porting the RHEL/Fedora /dev/crash driver is possible, but since page_is_ram() is not EXPORT_GPL()'d in 2.6.27 upstream kernels, then its usage by the driver would have to be removed. And if you were to make it EXPORT_GPL(), then it makes far more sense to just remove the CONFIG_STRICT_DEVMEM instead. (3) I'll append some information re: writing a kretprobe module, but like porting the /dev/crash driver, it may require rebuilding your kernel anyway. And if that's the case, just remove the CONFIG_STRICT_DEVMEM restriction. Also, you should upgrade from 4.0-7. There have been several fixes for 2.6.27 kernels since that version. Dave ---------------------------------------------------------------------------- Writing a kretprobe for devmem_is_allowed(): There are are currently three types of probes: kprobes, jprobes, and kretprobes (also called return probes), but for this purpose, a kretprobe is required because it allows the return value of the devmem_is_allowed() function to be modified such that it will always return 1. The kernel documentation contains both directions for building a kretprobe module and example module files for each kprobe type. Pre-2.6.25 kernel trees put sample cut-and-pastable module files contained within the "./Documentation/kprobes.txt" file itself. 2.6.25 and later kernel trees locate them in the "./samples/kprobes" directory. Note that pre-2.6.25 kernels must have been configured with both CONFIG_KPROBES, CONFIG_KALLSYMS and CONFIG_MODULES turned on. Additionally, 2.6.25 and later kernels also need CONFIG_KRETPROBES, In any case, if the target kernel configs preclude the installation of the module, then it probably makes more sense to rebuild the kernel with CONFIG_STRICT_DEVMEM turned off, and avoid this kprobe approach entirely. Take the "kretprobe-example.c" cut-and-pastable file from the the "./Documentation/kprobes.txt" file, or the standalone "./samples/kprobes/kretprobe_example.c" file, whichever is appropriate, and modify the kretprobe_init() and ret_handler() functions like so: static int __init kretprobe_init(void) { int ret; - my_kretprobe.kp.symbol_name = func_name; + my_kretprobe.kp.symbol_name = "devmem_is_allowed"; ret = register_kretprobe(&my_kretprobe); if (ret < 0) { printk(KERN_INFO "register_kretprobe failed, returned %d\n", ret); return -1; } printk(KERN_INFO "Planted return probe at %s: %p\n", my_kretprobe.kp.symbol_name, my_kretprobe.kp.addr); return 0; } and make the ret_handler() function to simply do this: static int ret_handler(struct kretprobe_instance *ri, struct pt_regs *regs) { regs->ax = 1; return 0; } Earlier kernel versions may have different pt_regs structure member names for the return register, i.e., "regs->eax" or "regs->rax" for the x86 and x86_64 architectures respectively. Build the kretprobe module as directed in ./Documentation/kprobes.txt" and insmod it, after which crash will run just fine with /dev/mem.