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

["Dmitry V. Levin" <ldv-u2l5PoMzF/Vg9hUCZPvPmw@public.gmane.org>]

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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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