Subject: [PATCH v8 2/6] arm64: ptrace: allow tracer to skip a system call

From: [email-masked] (AKASHI Takahiro)
Date: Thu, 20 Nov 2014 14:52:34 +0900

On 11/20/2014 02:13 PM, AKASHI Takahiro wrote:
> On 11/20/2014 04:06 AM, Will Deacon wrote:
>> On Wed, Nov 19, 2014 at 08:46:19AM +0000, AKASHI Takahiro wrote:
>>> On 11/18/2014 11:04 PM, Will Deacon wrote:
>>>> On Tue, Nov 18, 2014 at 01:10:34AM +0000, AKASHI Takahiro wrote:
>>>>>
>>>>> +    if (((int)regs->syscallno == -1) && (orig_syscallno == -1)) {
>>>>> +        /*
>>>>> +         * user-issued syscall(-1):
>>>>> +         * RESTRICTION: We always return ENOSYS whatever value is
>>>>> +         *   stored in x0 (a return value) at this point.
>>>>> +         * Normally, with ptrace off, syscall(-1) returns -ENOSYS.
>>>>> +         * With ptrace on, however, if a tracer didn't pay any
>>>>> +         * attention to user-issued syscall(-1) and just let it go
>>>>> +         * without a hack here, it would return a value in x0 as in
>>>>> +         * other system call cases. This means that this system call
>>>>> +         * might succeed and see any bogus return value.
>>>>> +         * This should be definitely avoided.
>>>>> +         */
>>>>> +        regs->regs[0] = -ENOSYS;
>>>>> +    }
>>>>
>>>> I'm still really uncomfortable with this, and it doesn't seem to match what
>>>> arch/arm/ does either.
>>>
>>> Yeah, I know but
>>> as I mentioned before, syscall(-1) will be signaled on arm, and so we don't
>>> have to care about a return value :)
>>
>> What does x86 do?
>
> On x86, syscall(-1) returns -ENOSYS if not traced, and we can change a return
> value if traced.
>
>>>> Doesn't it also prevent a tracer from skipping syscall(-1)?
>>>
>>> Syscall(-1) will return -ENOSYS whether or not a syscallno is explicitly
>>> replaced with -1 by a tracer, and, in this sense, it is *skipped*.
>>
>> Ok, but now userspace sees -ENOSYS for a skipped system call in that case,
>> whereas it would usually see whatever the trace put in x0, right?
>
> Yes.
> If you don't really like this behavior, how about this patch instead of my [2/6] patch?
>
> diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
> index 726b910..1ef57d0 100644
> --- a/arch/arm64/kernel/entry.S
> +++ b/arch/arm64/kernel/entry.S
> @@ -668,8 +668,15 @@ ENDPROC(el0_svc)
>           * switches, and waiting for our parent to respond.
>           */
>   __sys_trace:
> +       cmp     w8, #-1                         // default errno for invalid

I needed to correct the code here:
w8 should be w26, thinking of compat syscalls.

> +       b.ne    1f                              // system call
> +       mov     x0, #-ENOSYS
> +       str     x0, [sp, #S_X0]
> +1:

and this part might better be generalized like the following:

__sys_trace:
	cmp	w26, w25	// cannot use x26 and x25 here
	b.hs	1f		// scno > sc_nr || scno < 0
	b	2f
1:
	mov	x0, #-ENOSYS
	str	x0, [sp, #S_X0]
2:

If you will be comfortable, I will submit a new patch soon.

-Takahiro AKASHI

>          mov     x0, sp
>          bl      syscall_trace_enter
> +       cmp     w0, #-1                         // skip the syscall?
> +       b.eq    __sys_trace_return_skipped
>          adr     lr, __sys_trace_return          // return address
>          uxtw    scno, w0                        // syscall number (possibly new)
>          mov     x1, sp                          // pointer to regs
> @@ -684,6 +691,7 @@ __sys_trace:
>
>   __sys_trace_return:
>          str     x0, [sp]                        // save returned x0
> +__sys_trace_return_skipped:
>          mov     x0, sp
>          bl      syscall_trace_exit
>          b       ret_to_user
>
> With this change, I believe, syscall(-1) returns -ENOSYS by default whether traced
> or not, and still you can change a return value when tracing.
> (But a drawback here is that a tracer will see -ENOSYS in x0 even at syscall entry
> for syscall(-1).)
>
>
> -Takahiro AKASHI
>
>
>
>> Will
>>