Re: [PATCH] ARC: prevent showing irrelevant exception info in signal message

From: Vineet Gupta
Date: Thu Jul 05 2018 - 17:26:23 EST

On 07/03/2018 03:57 AM, Eugeniy Paltsev wrote:
> On Mon, 2018-07-02 at 10:57 -0700, Vineet Gupta wrote:
>> +CC Al
>> On 06/29/2018 12:39 PM, Eugeniy Paltsev wrote:
>>> We process signals in the end of syscall/exception handler.
>>> It the signal is fatal we print register's content using
>>> show_regs function. show_regs() also prints information about
>>> last exception happened.
>>> In case of multicore system we can catch the situation when we
>>> will print wrong information about exception. See the example:
>>> ______________________________
>>> CPU-0: started to handle page fault
>>> CPU-1: sent signal to process, which is executed on CPU-0
>>> CPU-0: ended page fault handle. Started to process signal before
>>> returnig to userspace. Process signal, which is send from
>>> CPU-0. As th signal is fatal we call show_regs().
>>> show_regs() will show information about last exception
>>> which is *page fault* (instead of "trap" which is used for
>>> signals and happened on CPU-0)
>>> So we will get message like this:
>>> /home/waitpid02
>>> potentially unexpected fatal signal 8.
>>> Path: /home/waitpid02
>>> CPU: 0 PID: 100 Comm: waitpid02 Not tainted 4.10.0-rc4 #2
>>> task: 9f11c200 task.stack: 9f3ae000
>>> [ECR ]: 0x00050200 => Invalid Write @ 0x00000000 by insn @ 0x000123ec
>>> [EFA ]: 0x00000000
>>> [BLINK ]: 0x123ea
>>> [ERET ]: 0x123ec
>>> @off 0x123ec in [/home/waitpid02]
>>> VMA: 0x00010000 to 0x00016000
>>> [STAT32]: 0x80080882 : IE U
>>> BTA: 0x000123ea SP: 0x5ffd3db0 FP: 0x00000000
>>> LPS: 0x20031684 LPE: 0x2003169a LPC: 0x00000006
>>> [-----other-info-----]
>>> This message is confusing because it show information about page fault
>>> ( [ECR ]: 0x00050200 => Invalid Write ) which is absolutely irrelevant
>>> to signal.
>> Agreed this is misleading. @Al, is there a way to identify process termination
>> from signals because it did something wrong vs. say unhandled signal. For former,
>> we want to dump additional info in show_regs() such as PC / Fault addres etc and
>> not in other scenario.
>>> This situation was reproduced with waitpid02 LTP test.
>>> _____________________________
>>> So remove printing information about exceptions from show_regs()
>>> to avoid confusing messages. Print information about exceptions
>>> only in required places instead of show_regs()
>>> Now we don't print information about exceptions if signal is simply
>>> send by another userspace app. So in case of waitpid02 we will print
>>> next message:
>>> _____________________________
>>> ./waitpid02
>>> potentially unexpected fatal signal 8.
>>> [STAT32]: 0x80080082 : IE U
>>> BTA: 0x20000fc4 SP: 0x5ff8bd64 FP: 0x00000000
>>> LPS: 0x200524a0 LPE: 0x200524b6 LPC: 0x00000006
>>> [-----other-info-----]
>>> _____________________________
>> The prints I'm seeing now, for a segv from NULL pointer access is even more
>> confusing !
>> There's a mixup of prints....
>> -------------------->8--------------------
>> Path: /segv
>> CPU: 0 PID: 70 Comm: segv Not tainted 4.17.0+ #412
>> [ECR ]: 0x00050200 => Invalid Write @ 0x00000000 by insn @ 0x000103ac
>> [EFA ]: 0x00000000
>> [BLINK ]: 0x20047bb0
>> [ERET ]: 0x103ac
>> @off 0x103ac in [/segv]
>> VMA: 0x00010000 to 0x00012000
>> potentially unexpected fatal signal 11.
>> [STAT32]: 0x80080882 : IE U
>> BTA: 0x00010398 SP: 0x5fc95e1c FP: 0x5fc95e20
>> LPS: 0x20039ffc LPE: 0x2003a000 LPC: 0x00000000
>> r00: 0x00000001 r01: 0x5fc95e94 r02: 0x00000000
>> r03: 0x00000064 r04: 0x80808080 r05: 0x2f2f2f2f
>> ...
>> -------------------->8--------------------
>> and for the process killed by signal 8, we get below.
>> -------------------->8--------------------
>> [ARCLinux]# kill -8 71
>> [ARCLinux]# potentially unexpected fatal signal 8.
>> [STAT32]: 0x80080882 : IE U
>> BTA: 0x20020660 SP: 0x5fbcddec FP: 0x5fbcde1c
>> LPS: 0x20039ffc LPE: 0x2003a000 LPC: 0x00000000
>> r00: 0xfffffdfc r01: 0x5fbcddf0 r02: 0x00000000
>> r03: 0x00000008 r04: 0x80808080 r05: 0x2f2f2f2f
>> r06: 0x7a2f5f4a r07: 0x00000000 r08: 0x00000065
>> ...
>> [1]+ Floating point exception ./sleep
>> -------------------->8--------------------
>> I'm not sure whats the improvement here vs. the status quo.
> Why do you think this is confusing?
> The main change is that we don't print exception registers for signal based kill.

For the pure signal based termination, what is the point of printing the rest of
registers. If you say "it is to give a feel of what the userspace was doing at the
time...." then we are lacking the most crucial piece which is the PC at the time
(i.e. ERET placeholder).

> Moreover, new behavior is more like x86-64 behavior. See the example:

For a null pointer based termination, we now have a ugly looking "potentially
unexpected..." print in the middle of reg file dump, that is not what x86 does.
Anyways, that print It is undesirable, but not a deal breaker. The issue is point
above, can we remedy it.

BTW in your original patch, for a null pointer access, the printing code now
allocates 2 pages, in each of show_xxx routines and the one in show_regs() is now
pointless, as it is not used there at all there - so please fix that as well.