Re: [RFC PATCH] ptrace: add PTRACE_GET_SYSCALL_INFO request

Oleg Nesterov
2018-11-07T11:21:10Z

On 11/07, Elvira Khabirova wrote:
>
> In short, if a 64-bit task performs a syscall through int 0x80, its tracer
> has no reliable means to find out that the syscall was, in fact,
> a compat syscall, and misidentifies it.
> * Syscall-enter-stop and syscall-exit-stop look the same for the tracer.

Yes, this was discussed many times...

So perhaps it makes sense to encode compat/is_enter in ->ptrace_message,
debugger can use PTRACE_GETEVENTMSG to get this info.

> Secondly, ptracers also have to support a lot of arch-specific code for
> obtaining information about the tracee. For some architectures, this
> requires a ptrace(PTRACE_PEEKUSER, ...) invocation for every syscall
> argument and return value.

I am not sure about this change... I won't really argue, but imo this
needs a separate patch.

> +#define PT_IN_SYSCALL_STOP 0x00000004 /* task is in a syscall-stop */
...
> -static inline int ptrace_report_syscall(struct pt_regs *regs)
> +static inline int ptrace_report_syscall(struct pt_regs *regs,
> + unsigned long message)
> {
> int ptrace = current->ptrace;
>
> if (!(ptrace & PT_PTRACED))
> return 0;
> + current->ptrace |= PT_IN_SYSCALL_STOP;
>
> + current->ptrace_message = message;
> ptrace_notify(SIGTRAP | ((ptrace & PT_TRACESYSGOOD) ? 0x80 : 0));
>
> /*
> @@ -76,6 +79,7 @@ static inline int ptrace_report_syscall(struct pt_regs *regs)
> current->exit_code = 0;
> }
>
> + current->ptrace &= ~PT_IN_SYSCALL_STOP;
> return fatal_signal_pending(current);
...

> + case PTRACE_GET_SYSCALL_INFO:
> + if (child->ptrace & PT_IN_SYSCALL_STOP)
> + ret = ptrace_get_syscall(child, datavp);
> + break;

Why? If debugger uses PTRACE_O_TRACESYSGOOD it can know if the tracee reported
syscall entry/exit or not. PTRACE_GET_SYSCALL_INFO is pointless if not, but
nothing bad can happen.

Oleg.