Re: [Crash-utility] Can't read stack contents from qemu dump
On 4.04.2018 17:48, Dave Anderson wrote:
----- Original Message -----
> Hello,
>
> I tried running crash-head (HEAD: 5d172b230cf4) against today's linus'
> master on a dump obtained via dump-guest-memory in qemu. And I got the
> following when the image is loaded:
>
> please wait... (determining panic task)
> bt: read error: kernel virtual address: fffffe0000007000 type: "stack
> contents"
>
> KERNEL: vmlinux
> DUMPFILE: memory-verbatim.img
> CPUS: 1
> DATE: Wed Apr 4 16:36:47 2018
> UPTIME: 00:27:48
> LOAD AVERAGE: 31.11, 17.80, 10.43
> TASKS: 145
> NODENAME: ubuntu-virtual
> RELEASE: 4.16.0-rc7-nbor
> VERSION: #570 SMP Wed Apr 4 16:03:44 EEST 2018
> MACHINE: x86_64 (3392 Mhz)
> MEMORY: 4 GB
> PANIC: ""
> PID: 0
> COMMAND: "swapper/0"
> TASK: ffffffff82016500 [THREAD_INFO: ffffffff82016500]
> CPU: 0
> STATE: TASK_RUNNING
> WARNING: panic task not found
>
> crash> bt
> PID: 0 TASK: ffffffff82016500 CPU: 0 COMMAND: "swapper/0"
> #0 [ffffffff82003dc8] __schedule at ffffffff817ea059
> bt: invalid RSP: ffffffff82003dc8 bt->stackbase/stacktop:
ffffffff82000000/ffffffff82002000 cpu: 0
>
>
> So the kernel has been compiled with : gcc (Ubuntu
> 5.4.0-6ubuntu1~16.04.9) 5.4.0 20160609 which has retpoline enabled.
>
> I have KASLR disabled: # CONFIG_RANDOMIZE_BASE is not set and the kernel
> is compiled with CONFIG_FRAME_POINTER=y .
>
> This scenario used to work around the 4.10 timeline. Am I doing
> something wrong or crash still needs time to work on the latest upstream
> kernel code?
Presumably the latter.
If you do a "task -R stack ffffffff82016500", I'm presuming that it
shows the stack base address is ffffffff82000000. And the looking at
the stackbase/stacktop values, the crash utility is presuming an 8K stack:
bt: invalid RSP: ffffffff82003dc8 bt->stackbase/stacktop:
ffffffff82000000/ffffffff82002000 cpu: 0
But the RSP is ffffffff82003dc8, which puts its beyond the 8K stack size,
so I'm presuming that the kernel is actually using 16K stacks. The most
recent kernel I have is 4.16.0-0.rc6.git3.1.fc29.x86_64, which uses 16K stacks.
This is correct, indeed the kernel size should be 16k. However...
Here is how the crash utility determines the stack size. The x86_64 stacksize
starts out with a default size of 2 pages, as set here in x86_64_init(PRE_SYMTAB):
case PRE_SYMTAB:
... [ cut ] ...
machdep->stacksize = machdep->pagesize * 2;
...
Then later on in task_init(), it gets resized as shown here, where
the STACKSIZE() macro is machdep->stacksize:
if (VALID_SIZE(task_union) && (SIZE(task_union) != STACKSIZE())) {
error(WARNING, "\nnon-standard stack size: %ld\n",
len = SIZE(task_union));
machdep->stacksize = len;
} else if (VALID_SIZE(thread_union) &&
((len = SIZE(thread_union)) != STACKSIZE()))
machdep->stacksize = len;
This is not resized at all, instead VALID_SIZE(thread_union) actually
fails, I've added the following else to the if statement there :
+ } else {
+ if (VALID_SIZE(thread_union)) {
+ error(WARNING, "WE ARE IN THE ELSE BRANCH: len: %llu
thread_union size: %llu STACKSIZE(): %llu\n",
+ len, SIZE(thread_union), STACKSIZE());
+ } else {
+ error(WARNING, "thread_union is invalid\n");
+ }
+ }
Also doing:
crash> struct thread_union
struct: invalid data structure reference: thread_union
So for some reason the thread_union cannot be found by gdb:
help -o | grep thread_union
thread_union: -1
The "task_union" no longer exists, and so it checks whether the
"thread_union" is larger than the default stacksize, and resets the
size appropriately.
On my 4.16.0-0.rc6.git3.1.fc29.x86_64 kernel, here is the thread_union:
crash> thread_union
union thread_union {
struct task_struct task;
unsigned long stack[2048];
}
SIZE: 16384
And so it gets reset:
crash> help -m | grep stacksize
stacksize: 16384
crash>
You can debug it from there. Let me know what you find.
Thanks,
Dave