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. -- ldv
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