Messages in this thread | | | Date | Wed, 28 Nov 2018 18:23:46 +0300 | From | "Dmitry V. Levin" <> | Subject | Re: [PATCH v4 1/2] ptrace: save the type of syscall-stop in ptrace_message |
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On Wed, Nov 28, 2018 at 03:20:06PM +0100, Oleg Nesterov wrote: > 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?
The debugger that uses PTRACE_GET_SYSCALL_INFO does not need to call PTRACE_GETEVENTMSG for syscall stops. My concern here is the PTRACE_GETEVENTMSG interface itself. If we use ptrace_message to implement PTRACE_GET_SYSCALL_INFO and expose PTRACE_EVENTMSG_SYSCALL_{ENTRY,EXIT} for regular PTRACE_GETEVENTMSG users, it should have clear semantics.
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