Re: [PATCH] printk: inject caller information into the body of message

[Dmitry Vyukov]

On Wed, Jun 20, 2018 at 1:37 PM, Fengguang Wu <fengguang.wu@xxxxxxxxx> wrote:
> On Wed, Jun 20, 2018 at 11:30:05AM +0200, Dmitry Vyukov wrote:
>>
>> On Wed, Jun 20, 2018 at 11:06 AM, Sergey Senozhatsky
>> <sergey.senozhatsky.work@xxxxxxxxx> wrote:
>>>
>>> Hi Dmitry,
>>>
>>> On (06/20/18 10:45), Dmitry Vyukov wrote:
>>>>
>>>> Hi Sergey,
>>>>
>>>> What are the visible differences between this patch and Tetsuo's
>>>> patch?
>>>
>>>
>>> I guess none, and looking at your requirements below I tend to agree
>>> that Tetsuo's approach is probably what you need at the end of the day.
>>>
>>>> The only thing that will matter for syzkaller parsing in the
>>>> end is the resulting text format as it appears on console. But you say
>>>> "I'm not pushing for this particular message format", so what exactly
>>>> do you want me to provide feedback on?
>>>> I guess we need to handle pr_cont properly whatever approach we take.
>>>
>>>
>>> Mostly, was wondering about if:
>>> a) you need pr_cont() handling
>>> b) you need printk_safe() handling
>>>
>>> The reasons I left those things behind:
>>>
>>> a) pr_cont() is officially hated. It was never supposed to be used
>>>    on SMP systems. So I wasn't sure if we need all that effort and
>>>    add tricky code to handle pr_cont(). Given that syzkaller is
>>>    probably the only user of that functionality.
>>
>>
>> Well, if I put my syzkaller hat on, then I don't care what exactly
>> happens in the kernel, the only thing I care is well-formed output on
>> console that can be parsed unambiguously in all cases.
>
>
> +1 for 0day kernel testing.
>
> I admit that goal may never be 100% achievable -- at least some serial
> console logs can sometimes become messy. So we'll have to write dmesg
> parsing code in defensive ways.
>
> But some unnecessary pr_cont() broken-up messages can obviously be
> avoided. For example,
>
> arch/x86/mm/fault.c:
>
>         printk(KERN_ALERT "BUG: unable to handle kernel ");
>         if (address < PAGE_SIZE)
>                 printk(KERN_CONT "NULL pointer dereference");
>         else
>                 printk(KERN_CONT "paging request");
>
> I've actually proposed to remove the above KERN_CONT, unfortunately the
> patch was silently ignored.


I've just cooked this change too, but do you mind reviving your patch?

It actually makes the code even shorter, which is nice:

--- a/arch/x86/mm/fault.c
+++ b/arch/x86/mm/fault.c
@@ -671,13 +671,9 @@ show_fault_oops(struct pt_regs *regs, unsigned
long error_code,
                        printk(smep_warning, from_kuid(&init_user_ns,
current_uid()));
        }

-       printk(KERN_ALERT "BUG: unable to handle kernel ");
-       if (address < PAGE_SIZE)
-               printk(KERN_CONT "NULL pointer dereference");
-       else
-               printk(KERN_CONT "paging request");
-
-       printk(KERN_CONT " at %px\n", (void *) address);
+       printk(KERN_ALERT "BUG: unable to handle kernel %s at %px\n",
+               (address < PAGE_SIZE ? "NULL pointer dereference" :
+               "paging request"), (void *) address);

        dump_pagetable(address);
 }