Re: [PATCH] bpf: whitelist syscalls for error injection

[Howard McLauchlan <hmclauchlan@fb.com>]

On 03/18/2018 07:13 PM, Andy Lutomirski wrote:
> On Sun, Mar 18, 2018 at 6:47 AM, Dominik Brodowski
> <linux@dominikbrodowski.net> wrote:
>> On Fri, Mar 16, 2018 at 03:55:04PM -0700, Howard McLauchlan wrote:
>>> On 03/13/2018 04:56 PM, Andy Lutomirski wrote:
>>>> On Tue, Mar 13, 2018 at 11:16 PM, Howard McLauchlan <hmclauchlan@fb.com> wrote:
>>>>> Error injection is a useful mechanism to fail arbitrary kernel
>>>>> functions. However, it is often hard to guarantee an error propagates
>>>>> appropriately to user space programs. By injecting into syscalls, we can
>>>>> return arbitrary values to user space directly; this increases
>>>>> flexibility and robustness in testing, allowing us to test user space
>>>>> error paths effectively.
>>>>
>>>> Temporary NAK IMO.  Specifically:
>>>>
>>>>> diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h
>>>>> index a78186d826d7..e8c6d63ace78 100644
>>>>> --- a/include/linux/syscalls.h
>>>>> +++ b/include/linux/syscalls.h
>>>>> @@ -191,6 +191,8 @@ static inline int is_syscall_trace_event(struct trace_event_call *tp_event)
>>>>>
>>>>>  #define SYSCALL_DEFINE0(sname)                                 \
>>>>>         SYSCALL_METADATA(_##sname, 0);                          \
>>>>> +       asmlinkage long sys_##sname(void);                      \
>>>>> +       ALLOW_ERROR_INJECTION(sys_##sname, ERRNO);              \
>>>>
>>>> sys_xyz() is not just the syscall itself; it's also a helper that's
>>>> used for entirely silly reasons by various bits of kernel code for
>>>> quite a few syscalls.  Fortunately, Dominik has patches to fix that,
>>>> and Linus is even considering pulling them for 4.16.  This patch will
>>>> most likely conflict with the final result of Dominik's series.
>>>>
>>>> Can you and Dominik coordinate a bit to get this patch or its
>>>> equivalent landed on top of Dominik's work?  It might make sense for
>>>> Dominik to just add this patch to his series so it can land with the
>>>> rest of it.  Dominik, Ingo, what do you think?
>>>>
>>>> --Andy
>>>>
>>>
>>> Dominik,
>>>
>>> This patch applies cleanly on top of your patch series. Is there anything you'd need from me to get this in on top of your work?
>>
>> Howard,
>>
>> would this form part of the kernel<->userspace interface and therefore needs
>> to be kept stable? If so, this patch should wait until the arch-specific
>> syscall calling convention is agreed upon.
>>
>> Moreover, the patches I sent out already do not cover all syscalls yet.
>> Until all in-kernel users of sys_*() are gone (or at least outside arch/),
>> I'd prefer to postpone this patch.
>>
> 
> I was assuming that this ALLOW_ERROR_INJECTION thing is *not*
> considered stable ABI.  We should be free to change the way that the
> syscall entry code calls syscalls whenever we like.
> 
> If you want a stable syscall error injection mechanism, make it work
> like seccomp instead, please.
> 

This is not supposed to be considered stable. It's for debug purposes only and 
would normally be configured off.

Howard