On Tue, Nov 25, 2014 at 07:42:10AM +0000, AKASHI Takahiro wrote: > 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. > >> */ > >> __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. > > Can you elaborate a bit more as to "restarting?"
Sorry, I meant skipping. There was another thread about syscall restarting at the same time I wrote that, so my mind was elsewhere! > 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. > () Ok, that's a fair point. Will -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
