Re: [Crash-utility] Unable to change the content of memory using crash on a live system
Dheeraj Sangamkar wrote:
I use crash 4.0-3.9 on a live 2.6.9-55 kernel on i386/i686 as root.
crash> ls -l /dev/crash
crw------- 1 root root 10, 61 Mar 5 21:57 /dev/crash
crash> ls -l /dev/mem
crw-r----- 1 root kmem 1, 1 Mar 5 16:49 /dev/mem
crash> q
[root@linux17081 ~]# ls -l /dev/crash /dev/mem
ls: /dev/crash: No such file or directory
crw-r----- 1 root kmem 1, 1 Mar 5 16:49 /dev/mem
[root@linux17081 ~]# id
uid=0(root) gid=0(root)
groups=0(root),1(bin),2(daemon),3(sys),4(adm),6(disk),10(wheel)
So, the /dev/crash file has write permission for me. The
I am attempting to change the content of some memory.
crash> struct request_queue 0xf7b933f8
struct request_queue {
queue_head = {
<SNIP>
...
}
crash> struct -o request_queue | grep in_flight
[476] unsigned int in_flight;
crash> eval 0xf7b933f8 + 476
hexadecimal: f7b935d4
decimal: 4156110292 (-138857004)
octal: 36756232724
binary: 11110111101110010011010111010100
crash> rd f7b935d4
f7b935d4: fffffff1 ....
crash> wr f7b935d4 0
wr: cannot write to /dev/crash!
I get the error above even if I change the ownership of /dev/kmem to
root:root
crash> ls -l /dev/mem
crw-r----- 1 root root 1, 1 Mar 5 16:49 /dev/mem
Am I doing something wrong? How do I change the content of memory on a
live system using crash?
With Red Hat x86 and x86_64 kernels, you can't.
I feel your pain...
The crash utility traditionally has had the capability of writing
to /dev/mem, which can be a very useful, powerful (and dangerous)
tool for kernel debugging.
But Red Hat deemed the /dev/mem interface as a security hole,
and restricted the x86 and x86_64 /dev/mem drivers to just
the first 256 pages (1MB) of physical memory, making it useless
for the crash utility. They allowed me to create the /dev/crash
driver to replace it -- but it is effectively read-only because
the driver has no write file operations handler:
static struct file_operations crash_fops = {
owner: THIS_MODULE,
llseek: crash_llseek,
read: crash_read,
};
and so the kernel's vfs_write() returns EINVAL.
Changing the permission of /dev/mem won't help because it
isn't used by the crash utility when /dev/crash exists.
Sorry about that,
Dave