[BUG kretprobes] kretprobe triggers General Protection Faults

[BUG kretprobes] kretprobe triggers General Protection Faults

[Mathieu Desnoyers]

Hi,

I had a bug report[1] from a user trying to add a kretprobe on the system
call entry code path:

arch/x86/kernel/entry_64.S:

ffffffff813dffe2 <system_call_fastpath+0x16>:
        cmpl $__NR_syscall_max,%eax
#endif
        ja badsys
        movq %r10,%rcx
        call *sys_call_table(,%rax,8)  # XXX:    rip relative
        movq %rax,RAX-ARGOFFSET(%rsp)   <--- return address pointing here

And all hell breaks loose (various types of faults, machine reboots,
applications exit randomly, etc.). I understand that this code path
is not marked as unsafe against kprobes, and I tested that a kprobes
indeed works fine there. However, kretprobes probably presumes a function
stack layout that is just not valid for the syscall entry routine.

Any thoughts on how kretprobes should handle this ?

Thanks,

Mathieu

[1] http://bugs.lttng.org/issues/687

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com

Re: [BUG kretprobes] kretprobe triggers General Protection Faults

[Masami Hiramatsu]

Hi Mathieu,

(2014/02/26 4:46), Mathieu Desnoyers wrote:
> Hi,
> 
> I had a bug report[1] from a user trying to add a kretprobe on the system
> call entry code path:
> 
> arch/x86/kernel/entry_64.S:
> 
> ffffffff813dffe2 <system_call_fastpath+0x16>:
>         cmpl $__NR_syscall_max,%eax
> #endif
>         ja badsys
>         movq %r10,%rcx
>         call *sys_call_table(,%rax,8)  # XXX:    rip relative
>         movq %rax,RAX-ARGOFFSET(%rsp)   <--- return address pointing here

Hm, I guess you put kretprobes on the functions on the sys_call_table,
right?

> And all hell breaks loose (various types of faults, machine reboots,
> applications exit randomly, etc.). I understand that this code path
> is not marked as unsafe against kprobes, and I tested that a kprobes
> indeed works fine there. However, kretprobes probably presumes a function
> stack layout that is just not valid for the syscall entry routine.

All the syscall entry functions caused this issue? or some
specific function(s) ?
And could you tell me the kernel version you used?

> Any thoughts on how kretprobes should handle this ?

I'll try to reproduce it in kvm environment.

Thank you!

-- 
Masami HIRAMATSU
IT Management Research Dept. Linux Technology Center
Hitachi, Ltd., Yokohama Research Laboratory
E-mail: masami.hiramatsu.pt@hitachi.com