Messages in this thread | | | Date | Wed, 20 Jun 2018 19:37:31 +0800 | From | Fengguang Wu <> | Subject | Re: [PATCH] printk: inject caller information into the body of message |
| |
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.
>From this point of view I guess pr_cont is actually syzkaller's worst >enemy. If pr_const is officially hated, and it causes corrupted crash >reports, then we can resolve it by just getting rid of more pr_cont's. >So potentially we do not need any support for pr_cont in this patch. >However, we also need to be practical and if there are tons of >pr_cont's then we need some intermediate support of them, just because >we won't be able to get rid of all of them overnight. > >But even if we attach context to pr_cont, it still causes problems for >crash parsing, because today we see: > >BUG: unable to handle >... 10 lines ... >kernel >... 10 lines ... >paging request >... 10 lines ... >at ADDR > >Which is not too friendly for parsing regardless of contexts.
We met exactly the same issue and ended up with special handling in https://github.com/intel/lkp-tests/blob/master/lib/dmesg.rb:
/(BUG: unable to handle kernel)/, /(BUG: unable to handle kernel) NULL pointer dereference/, /(BUG: unable to handle kernel) paging request/,
>So I am leaning towards to getting rid of pr_cont's as the solution to >the problem.
+1 for reducing unnecessary pr_cont() uses.
Thanks, Fengguang
| |