Messages in this thread Patch in this message | | | Date | Thu, 20 Nov 2014 19:17:38 +0000 | From | Will Deacon <> | Subject | Re: [PATCH v8 2/6] arm64: ptrace: allow tracer to skip a system call |
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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. > */ > __sys_trace: > + cmp w8, #-1 // default errno for invalid > + b.ne 1f // system call > + mov x0, #-ENOSYS > + str x0, [sp, #S_X0] > +1: > 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).)
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.
Will
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