linux-kernel - Re: [PATCH RESEND v3 1/3] ptrace: pass type of a syscall-stop in ptrace

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Message-ID: <20181125021059.GA1190@altlinux.org>
Date:   Sun, 25 Nov 2018 05:10:59 +0300
From:   "Dmitry V. Levin" <ldv@...linux.org>
To:     Joey Pabalinas <joeypabalinas@...il.com>
Cc:     Elvira Khabirova <lineprinter@...linux.org>,
        Oleg Nesterov <oleg@...hat.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...hat.com>,
        Andy Lutomirski <luto@...nel.org>,
        Eugene Syromyatnikov <esyr@...hat.com>,
        linux-api@...r.kernel.org, linux-kernel@...r.kernel.org,
        strace-devel@...ts.strace.io
Subject: Re: [PATCH RESEND v3 1/3] ptrace: pass type of a syscall-stop in
 ptrace_message

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.
> 
> 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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