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Date:   Sun, 25 Nov 2018 05:10:59 +0300
From:   "Dmitry V. Levin" <ldv@altlinux.org>
To:     Joey Pabalinas <joeypabalinas@gmail.com>
Cc:     Elvira Khabirova <lineprinter@altlinux.org>,
        Oleg Nesterov <oleg@redhat.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Ingo Molnar <mingo@redhat.com>,
        Andy Lutomirski <luto@kernel.org>,
        Eugene Syromyatnikov <esyr@redhat.com>,
        linux-api@vger.kernel.org, linux-kernel@vger.kernel.org,
        strace-devel@lists.strace.io
Subject: Re: [PATCH RESEND v3 1/3] ptrace: pass type of a syscall-stop in
 ptrace_message
Message-ID: <20181125021059.GA1190@altlinux.org>
References: <20181125022150.46258a20@akathisia>
 <20181125022227.55e8caf7@akathisia>
 <20181125015402.glcaw3kghcu4pr22@gmail.com>
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On Sat, Nov 24, 2018 at 03:54:02PM -1000, Joey Pabalinas wrote:
> On Sun, Nov 25, 2018 at 02:22:27AM +0100, Elvira Khabirova wrote:
> > Define two constants, PTRACE_EVENTMSG_SYSCALL_ENTRY and
> > PTRACE_EVENTMSG_SYSCALL_EXIT, and place them in ptrace_message
> > for the duration of syscall-stops.
> > This way ptracers can distinguish syscall-enter-stops
> > from syscall-exit-stops using PTRACE_GETEVENTMSG request.
>=20
> Is there an advantage to using two constants instead of a single
> sys_exit bit (set/unset for syscall-exit-stop/syscall-enter-stop)?

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.


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