On Wed, Nov 28, 2018 at 06:23:46PM +0300, Dmitry V. Levin wrote: > 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. Since our implementation of PTRACE_GET_SYSCALL_INFO uses ptrace_message to distinguish syscall-enter-stop from syscall-exit-stop, we could choose one of the following approaches: 1. Do not document the values saved into ptrace_message during syscall stops (and exposed via PTRACE_GETEVENTMSG) as a part of ptrace API, leaving the value returned by PTRACE_GETEVENTMSG during syscall stops as undefined. 2. Document these values chosen to avoid collisions with ptrace_message values set by other ptrace events so that PTRACE_GETEVENTMSG users can easily tell whether this new semantics is supported by the kernel or not. The first approach was implemented in v2 of this series: the constants were PT_SYSCALL_IS_{ENTERING,EXITING} defined in include/linux/ptrace.h. The second approach was implemented in v3: the constants are PTRACE_EVENTMSG_SYSCALL_{ENTRY,EXIT} defined in include/uapi/linux/ptrace.h, they are also going to be documented in ptrace(2) man page. Since the use of ptrace_message is exposed to PTRACE_GETEVENTMSG users anyway, I do not see any reason to choose the first approach over the second. -- ldv
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