[PATCH v8 2/6] arm64: ptrace: allow tracer to skip a system call
takahiro.akashi at linaro.org
Mon Nov 24 23:42:10 PST 2014
On 11/21/2014 04:17 AM, Will Deacon wrote:
> On Thu, Nov 20, 2014 at 05:13:04AM +0000, 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:
>>>> 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?
>> 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.
>> + cmp w8, #-1 // default errno for invalid
>> + b.ne 1f // system call
>> + mov x0, #-ENOSYS
>> + str x0, [sp, #S_X0]
>> 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:
>> str x0, [sp] // save returned x0
>> 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).)
> But it's exactly these drawbacks that I'm objected to. syscall(-1) shouldn't
> be treated any differently to syscall(42) with respect to restarting,
> exactly like x86.
Can you elaborate a bit more as to "restarting?"
We can't make any assumption about the number of arguments taken by *invalid* syscall(-1)
and so changing a value in x0 (or any other registers) doesn't make any difference.
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