Messages in this thread Patch in this message | | | Subject | Re: powerpc: syscall_dotrace() && retcode (Was: powerpc: fork && stepping) | From | Benjamin Herrenschmidt <> | Date | Mon, 30 Nov 2009 10:15:01 +1100 |
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On Sun, 2009-11-29 at 22:07 +0100, Oleg Nesterov wrote: > On 11/28, Ananth N Mavinakayanahalli wrote: > > > > syscall-reset is the only failure I see on > > powerpc: > > > > errno 14 (Bad address) > > syscall-reset: syscall-reset.c:95: main: Assertion `(*__errno_location > > ()) == 38' failed. > > unexpected child status 67f > > FAIL: syscall-reset > > (to remind, it also fails without utrace) > > Once again, I know nothing about powerc, perhaps I misread the code, > but I believe this test-case is just wrong on powerpc and should be > fixed. > > On powerpc, syscall_get_nr() returns regs->gpr[0], this means this > register is used to pass the syscall number.
Correct.
> This matches do_syscall_trace_enter(), it returns regs->gpr[0] as a > (possibly changed by tracer) syscall nr. > > arch/powerpc/kernel/entry_64.S does > > syscall_dotrace: > > bl .do_syscall_trace_enter > mr r0,r3 // I guess, r3 = r0 ?
r3 is the return value from a function so this replaces the syscall number > ... > b syscall_dotrace_cont > > syscall_dotrace_cont: > > syscall_dotrace_cont: > > cmpldi 0,r0,NR_syscalls > bge- syscall_enosys > > syscall_enosys: > > li r3,-ENOSYS > b syscall_exit > > > Now return to the test-case, syscall-reset.c. The tracee does > l = syscall (-23, 1, 2, 3) and stops. > > The tracer does > > #define RETREG offsetof(struct pt_regs, gpr[0]) > #define NEWVAL ((long) ENOTTY) > > l = ptrace(PTRACE_PEEKUSER, child, RETREG, 0l); > > l == -23, this is correct, note syscall(-23) above. > > l = ptrace(PTRACE_POKEUSER, child, RETREG, NEWVAL); > > And expects the tracee will see NEWVAL==ENOTTY after return from > the systame call. > > Of course this can't happen. We changed the syscall number, the > new value is ENOTTY == 25 == __NR_stime, sys_stime() correctly > returns -EFAULT. > > ----------------------------------------------------------------- > > 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:
diff --git a/arch/powerpc/kernel/entry_32.S b/arch/powerpc/kernel/entry_32.S index 1175a85..7a88c88 100644 --- a/arch/powerpc/kernel/entry_32.S +++ b/arch/powerpc/kernel/entry_32.S @@ -419,6 +419,9 @@ syscall_dotrace: stw r0,_TRAP(r1) addi r3,r1,STACK_FRAME_OVERHEAD bl do_syscall_trace_enter + cmpwi cr0,r3,0 + blt ret_from_syscall + /* * Restore argument registers possibly just changed. * We use the return value of do_syscall_trace_enter diff --git a/arch/powerpc/kernel/entry_64.S b/arch/powerpc/kernel/entry_64.S index 9763267..ec709a7 100644 --- 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 + /* * Restore argument registers possibly just changed. * We use the return value of do_syscall_trace_enter
Cheers, Ben.
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