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
>
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