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


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