Re: [PATCH v2] x86, kaslr: export offset in VMCOREINFO ELF notes

[Adrian Hunter <adrian.hunter@intel.com>]

On 27/01/14 17:25, Ingo Molnar wrote:
> 
> * Adrian Hunter <adrian.hunter@intel.com> wrote:
> 
>> On 25/01/14 09:47, Ingo Molnar wrote:
>>>
>>> * Kees Cook <keescook@chromium.org> wrote:
>>>
>>>> From: Eugene Surovegin <surovegin@google.com>
>>>>
>>>> Include kASLR offset in VMCOREINFO ELF notes to assist in debugging.
>>>>
>>>> Signed-off-by: Eugene Surovegin <surovegin@google.com>
>>>> Signed-off-by: Kees Cook <keescook@chromium.org>
>>>> ---
>>>> v2:
>>>>  - make sure "From:" got sent correctly
>>>> ---
>>>>  arch/x86/kernel/machine_kexec_64.c |    2 ++
>>>>  1 file changed, 2 insertions(+)
>>>>
>>>> diff --git a/arch/x86/kernel/machine_kexec_64.c b/arch/x86/kernel/machine_kexec_64.c
>>>> index 4eabc160696f..679cef0791cd 100644
>>>> --- a/arch/x86/kernel/machine_kexec_64.c
>>>> +++ b/arch/x86/kernel/machine_kexec_64.c
>>>> @@ -279,5 +279,7 @@ void arch_crash_save_vmcoreinfo(void)
>>>>  	VMCOREINFO_SYMBOL(node_data);
>>>>  	VMCOREINFO_LENGTH(node_data, MAX_NUMNODES);
>>>>  #endif
>>>> +	vmcoreinfo_append_str("KERNELOFFSET=%lx\n",
>>>> +			      (unsigned long)&_text - __START_KERNEL);
>>>>  }
>>>
>>> I've Cc:-ed Adrian Hunter, who has sent the following kaslr fixes for 
>>> perf yesterday:
>>>
>>>   http://lkml.org/lkml/2014/1/24/220
>>>
>>> Adrian, is this patch the right solution from the perf tooling 
>>> perspective?
>>
>> perf tools isn't a consumer of VMCOREINFO although I see VMCOREINFO 
>> already has _stext which would be enough for many purposes.
> 
> Yes - but let me explain where I'm coming from: I'd like the recent 
> KASLR related perf /proc/kcore based annotation bug to be fixed 
> properly.
> 
> Currently I'm not sure about the status of it. In your fixes 
> submission:
> 
>   Date: Fri, 24 Jan 2014 17:10:10 +0200
>   From: Adrian Hunter <adrian.hunter@intel.com>
>   Subject: [PATCH 0/8] perf tools: kaslr fixes
> 
> you mentioned the following:
> 
>     "- mustn't use kcore if the kernel has moved"
> 
> Does this that /proc/kcore annotation will not work if KASLR is 
> active?

No. In fact annotation works now with kcore.  Linus' problem was with vmlinux.

Kcore won't be used with kaslr in the case:
	1. record data with 'perf record'
	2. reboot
	3. use annotation on the previously recorded data

As I noted in the commit message, you can still use kcore if you made a copy
(with perf buildid-cache) at the time the data was recorded.

> 
> If yes then given that I expect most distros to turn on KASLR this 
> would essentially make /proc/kcore useless on a large set of Linux 
> systems. That would be suboptimal.
> 
> Thanks,
> 
> 	Ingo
> 
>