Re: [PATCH v4 1/2] ptrace: save the type of syscall-stop in ptrace_message

[Oleg Nesterov]

On 11/28, Dmitry V. Levin wrote:
>
> On Wed, Nov 28, 2018 at 02:49:14PM +0100, Oleg Nesterov wrote:
> > On 11/28, Dmitry V. Levin wrote:
> > >
> > > +/*
> > > + * These values are stored in task->ptrace_message by tracehook_report_syscall_*
> > > + * to describe current syscall-stop.
> > > + *
> > > + * Values for these constants are chosen so that they do not appear
> > > + * in task->ptrace_message by other means.
> > > + */
> > > +#define PTRACE_EVENTMSG_SYSCALL_ENTRY	0x80000000U
> > > +#define PTRACE_EVENTMSG_SYSCALL_EXIT	0x90000000U
> > 
> > Again, I do not really understand the comment... Why should we care about
> > "do not appear in task->ptrace_message by other means" ?
> > 
> > 2/2 should detect ptrace_report_syscall() case correctly, so we can use any
> > numbers, say, 1 and 2?
> > 
> > If debugger does PTRACE_GETEVENTMSG it should know how to interpet the value
> > anyway after wait(status).
> 
> Given that without this patch the value returned by PTRACE_GETEVENTMSG
> during syscall stop is undefined, we need two different ptrace_message
> values that cannot be set by other ptrace events to enable reliable
> identification of syscall-enter-stop and syscall-exit-stop in userspace:
> if we make PTRACE_GETEVENTMSG return 0 or any other value routinely set by
> other ptrace events, it would be hard for userspace to find out whether
> the kernel implements new semantics or not.

Hmm, why? Debugger can just do ptrace(PTRACE_GET_SYSCALL_INFO, NULL), if it
returns EIO then it is not implemented?

Oleg.