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

[Dmitry V. Levin]

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.


-- 
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