Messages in this thread | | | Date | Wed, 20 Jun 2018 20:48:18 +0800 | From | Fengguang Wu <> | Subject | Re: [PATCH] printk: inject caller information into the body of message |
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On Wed, Jun 20, 2018 at 02:45:25PM +0200, Dmitry Vyukov wrote: >On Wed, Jun 20, 2018 at 2:41 PM, Fengguang Wu <fengguang.wu@intel.com> wrote: >> On Wed, Jun 20, 2018 at 02:31:51PM +0200, Dmitry Vyukov wrote: >>> >>> On Wed, Jun 20, 2018 at 1:37 PM, Fengguang Wu <fengguang.wu@intel.com> >>> 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@gmail.com> 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? >> >> >> Yes, sure. My version is more dumb. Since I'm not sure if it's OK to >> do string formatting at this critical point. Let's see how others >> think about the 2 approaches. I'm fine as long as our problem is fixed. :) > >It already does string formatting for address. And I think we also >need to get rid of KERN_CONT for address while we are here.
Ah yes, sorry I overlooked the next KERN_CONT..
> >> diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c >> index 9a84a0d08727..c7b068c6b010 100644 >> --- a/arch/x86/mm/fault.c >> +++ b/arch/x86/mm/fault.c >> @@ -671,11 +671,10 @@ 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"); >> + printk(KERN_ALERT "BUG: unable to handle kernel NULL pointer >> dereference"); >> else >> - printk(KERN_CONT "paging request"); >> + printk(KERN_ALERT "BUG: unable to handle kernel paging >> request"); >> >> >> printk(KERN_CONT " at %px\n", (void *) address); >> >>> 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); >>> } >>> >> >
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