Re: [PATCH v3 0/6] implement KASLR for powerpc/fsl_booke/64

[Jason Yan <yanaijie@huawei.com>]

在 2020/3/2 16:47, Scott Wood 写道:
> On Mon, 2020-03-02 at 15:12 +0800, Jason Yan wrote:
>>
>> 在 2020/3/2 11:24, Scott Wood 写道:
>>> On Mon, 2020-03-02 at 10:17 +0800, Jason Yan wrote:
>>>>
>>>> 在 2020/3/1 6:54, Scott Wood 写道:
>>>>> On Sat, 2020-02-29 at 15:27 +0800, Jason Yan wrote:
>>>>>>
>>>>>> Turnning to %p may not be a good idea in this situation. So
>>>>>> for the REG logs printed when dumping stack, we can disable it when
>>>>>> KASLR is open. For the REG logs in other places like show_regs(),
>>>>>> only
>>>>>> privileged can trigger it, and they are not combind with a symbol,
>>>>>> so
>>>>>> I think it's ok to keep them.
>>>>>>
>>>>>> diff --git a/arch/powerpc/kernel/process.c
>>>>>> b/arch/powerpc/kernel/process.c
>>>>>> index fad50db9dcf2..659c51f0739a 100644
>>>>>> --- a/arch/powerpc/kernel/process.c
>>>>>> +++ b/arch/powerpc/kernel/process.c
>>>>>> @@ -2068,7 +2068,10 @@ void show_stack(struct task_struct *tsk,
>>>>>> unsigned
>>>>>> long *stack)
>>>>>>                     newsp = stack[0];
>>>>>>                     ip = stack[STACK_FRAME_LR_SAVE];
>>>>>>                     if (!firstframe || ip != lr) {
>>>>>> -                       printk("["REG"] ["REG"] %pS", sp, ip, (void
>>>>>> *)ip);
>>>>>> +                       if (IS_ENABLED(CONFIG_RANDOMIZE_BASE))
>>>>>> +                               printk("%pS", (void *)ip);
>>>>>> +                       else
>>>>>> +                               printk("["REG"] ["REG"] %pS", sp,
>>>>>> ip,
>>>>>> (void *)ip);
>>>>>
>>>>> This doesn't deal with "nokaslr" on the kernel command line.  It also
>>>>> doesn't
>>>>> seem like something that every callsite should have to opencode,
>>>>> versus
>>>>> having
>>>>> an appropriate format specifier behaves as I described above (and I
>>>>> still
>>>>> don't see why that format specifier should not be "%p").
>>>>>
>>>>
>>>> Actually I still do not understand why we should print the raw value
>>>> here. When KALLSYMS is enabled we have symbol name  and  offset like
>>>> put_cred_rcu+0x108/0x110, and when KALLSYMS is disabled we have the raw
>>>> address.
>>>
>>> I'm more concerned about the stack address for wading through a raw stack
>>> dump
>>> (to find function call arguments, etc).  The return address does help
>>> confirm
>>> that I'm on the right stack frame though, and also makes looking up a line
>>> number slightly easier than having to look up a symbol address and then
>>> add
>>> the offset (at least for non-module addresses).
>>>
>>> As a random aside, the mismatch between Linux printing a hex offset and
>>> GDB
>>> using decimal in disassembly is annoying...
>>>
>>
>> OK, I will send a RFC patch to add a new format specifier such as "%pk"
>> or change the exsiting "%pK" to print raw value of addresses when KASLR
>> is disabled and print hash value of addresses when KASLR is enabled.
>> Let's see what the printk guys would say :)
> 
> I'm not sure that a new format specifier is needed versus changing the
> behavior of "%p", and "%pK" definitely doesn't seem suitable given that it's
> intended to be more restricted than "%p" (see commit ef0010a30935de4).  The
> question is whether there is a legitimate reason to hash in the absence of
> kaslr.
> 

The problem is that if we change the behavior of "%p", we have to turn
all exsiting "%p" to "%pK". Hashing is still reasonable when there is no
kaslr because some architectures support randomize at build time such as 
arm64.


> -Scott
> 
> 
> 
> .
>