[BUG?]3.0-rc4+ftrace+kprobe: set kprobe at instruction 'stwu' lead to system crash/freeze

[Yong Zhang]

On Fri, Jul 1, 2011 at 6:03 PM, tiejun.chen <tiejun.chen at windriver.com> wrote:
>> root at unknown:/root> insmod kprobe_example.ko func=show_interrupts
>> Planted kprobe at c009be18
>> root at unknown:/root> cat /proc/interrupts
>> pre_handler: p->addr = 0xc009be18, nip = 0xc009be18, msr = 0x29000
>> post_handler: p->addr = 0xc009be18, msr = 0x29000,boostable = 1
>> Oops: Exception in kernel mode, sig: 11 [#1]
>> PREEMPT MPC8536 DS
>> Modules linked in: kprobe_example
>> NIP: df159e74 LR: c0106f40 CTR: c009be18
>> REGS: df159d90 TRAP: 0700   Not tainted  (3.0.0-rc4-00001-ge8ffcca-dirty)
>> MSR: 00029000 <EE,ME,CE>  CR: 20202688  XER: 00000000
>> TASK = dfaa5340[613] 'cat' THREAD: df158000
>> GPR00: fffff000 df159e40 dfaa5340 df024a00 df159e78 00000000 df159f20 00000001
>> GPR08: c10060d0 c009be18 00029000 df159e70 00000000 1001ca74 1ffb5f00 100a01cc
>> GPR16: 00000000 00000000 00000000 00000000 df024a28 df159f20 00000000 dfbff080
>> GPR24: 10016000 00001000 df159f20 df159e78 dfbff080 df159e78 df024a00 df159e70
>> NIP [df159e74] 0xdf159e74
>> LR [c0106f40] seq_read+0x2a4/0x568
>> Call Trace:
>> [df159e40] [00029000] 0x29000 (unreliable)
>> [df159e74] [00000000]   (null)
>> Instruction dump:
>> XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX
>> XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX
>> ---[ end trace 60026bfc1fe79aed ]---
>> Segmentation fault
>
> Maybe I can understand this problem.
>
> When we kprobe these operations such as store-and-update-word for SP(r1),
>
> stwu r1, -A(r1)
>
> The program exception is triggered then PPC always allocate an exception frame
> as shown as the follows:
>
> old r1 --------
>         ...
>         nip
>         gpr[2]~gpr[31]
>         gpr[1] <--------- old r1 is stored here.
>         gpr[0]
>       -------- <-- pr_regs @offset 16 bytes
>       padding
>       STACK_FRAME_REGS_MARKER
>       LR
>       back chain
> new r1 --------
>
> Here emulate_step() is called to emulate 'stwu'. Actually this is equivalent to
> 1> update pr_regs->gpr[1] = mem(old r1 + (-A))
> 2> 'stw <old r1>, mem<(old r1 + (-A)) >
>
> You should notice the stack based on new r1 would be covered with mem<old r1
> +(-A)>. So after this, the kernel exit from post_krpobe, something would be
> broken. This should depend on sizeof(-A).
>
> For kprobe show_interrupts, you can see pregs->nip is re-written violently so
> kernel issued.

Yeah, my debug also show this, so this is the root cause.
Thanks for your explanation.

>
> But sometimes we may only re-write some violate registers the kernel still
> alive. And so this is just why the kernel works well for some kprobed point
> after you change some kernel options/toolchains.
>
> If I'm correct its difficult to kprobe these stwu sp operation since the
> sizeof(-A) is undermined for the kernel. So we have to implement in-depend
> interrupt stack like PPC64.

Hmmm, a dedicated exception stack will smooth the concern IMHO,
Ben, Kuma?

Thanks,
Yong


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