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.
