Re: [PATCH v8 2/6] arm64: ptrace: allow tracer to skip a system call
From: Will Deacon
Date: Thu Nov 20 2014 - 14:17:46 EST
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.
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