Messages in this thread | | | Date | Sun, 29 Nov 2009 22:07:16 +0100 | From | Oleg Nesterov <> | Subject | powerpc: syscall_dotrace() && retcode (Was: powerpc: fork && stepping) |
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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.
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 ? ... 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.
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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 ;)
Oleg.
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