[BUG kretprobes] kretprobe triggers General Protection Faults

Mathieu Desnoyers
2014-02-25T19:46:00Z

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.