On 11/30, Benjamin Herrenschmidt wrote: > > On Sun, 2009-11-29 at 22:07 +0100, Oleg Nesterov wrote: > > > > If I change the test-case to use NEWVAL == 1000 (or any other value > > greater than NR_syscalls), then the tracee sees ENOSYS and this is > > correct too. > > > > But I do not see how it is possible to change the retcode on powerpc. > > Unlike x86, powepc doesn't set -ENOSYS "in advance", before doing > > do_syscall_trace_enter() logic. This means that if the tracer "cancels" > > syscall, r3 will be overwritten by syscall_enosys. > > > > This probably means the kernel should be fixed too, but I am not > > brave enough to change the asm which I can't understand ;) > > Yes, the asm should be changed. I suppose we could check if the result > of do_syscall_trace_enter is negative, and if it is, branch to the exit > path using r3 as the error code. Would that be ok ? > > Something like this: > > --- a/arch/powerpc/kernel/entry_64.S > +++ b/arch/powerpc/kernel/entry_64.S > @@ -240,6 +240,9 @@ syscall_dotrace: > bl .save_nvgprs > addi r3,r1,STACK_FRAME_OVERHEAD > bl .do_syscall_trace_enter > + cmpdi cr0,r3,0 > + blt syscall_exit > +
Yes, but this doesn't allow to a) cancel this syscall and b) make it return a non-negative result to the tracee. Perhaps poweprc should set pt_regs->result = -ENOSYS before calling do_syscall_trace_enter() like x86 does ? (in this case syscall_exit() shouldn't change RESULT(r1)). This way the tracer can change both pt_regs->result and gpr[0] independently. Oleg.